THE COMPLETE COSMOS Chapter 1: The Sun Birth, life and death of the Sun. Interior dynamics, exterior fireworks. Sunspots, corona, solar wind - latest on our local star. Outline Triggered by the death of a giant star, the birth of the Sun and planets. To the Sun's core, where energy is produced by nuclear fusion. The journey of this energy from the core to the solar surface - and across space to Earth. The Sun as the ultimate source of energy for our planet.

Layer by layer, the Sun's structure: first, the yellow photosphere with its sunspots, next the fiery red chromosphere with its prominences, and finally the million-degree corona, the Sun's outer atmosphere. The key influence of the Sun's magnetic field. Much of the information in this section is from the spacecraft SOHO, the Solar Heliospheric

Observatory. At the end of the chapter, the death of the Sun. Initially, it becomes a bloated red giant engulfing Mercury, Venus and Earth. Then, the Sun puffs off its outer layers to reveal the white hot core which collapses to a white dwarf, finally cooling to become a dark relic of a star.

Sub-chapters A Star Is Born • Our view of the Sun - now and in the past. The Sun as a star. A basic description of its functions. The mechanics of the Sun's birth and the formation of the planets. A brief tour of the Solar System. • How the Sun is the source of most of the Earth's energy resources, particularly fossil fuels.

Core to Surface • Internal composition of the Sun. How mass is transformed into energy in the Sun's core and how this energy travels to the Earth. • How the solar spectrum reveals the chemical composition of the Sun. • The temperature and overall physical appearance of the photosphere.

The Magnetic Sun • Sunspots and the Sun's magnetic field. The 11-year solar cycle and magnetic reversals. • Differential rotation of the Sun.

Solar Eruptions • The chromosphere, prominences and flares. • The overall physical appearance of the chromosphere. • Prominences - arcs of gas looping around magnetic field lines. • Violent flares blasting ripples across the face of the Sun. • Description of the corona, the Sun's outer atmosphere, visible during a total eclipse. • Great ejections from the corona. Reasons for the corona's varying appearance and activity.

Observations by the spacecraft SOHO. The solar wind, a stream of electrified particles continuously emitted by the Sun.

Secrets of the Sun • The mystery of why the corona is so hot. • Investigations of the Sun by SOHO. SOHO's revelations of the Sun internal behavior.

Death of the Sun • Explanation of why the Sun will one day expire. The various stages of its death and the effects on the inner planets.

Background Nuclear Fusion The Sun’s energy is generated by nuclear fusion. At very high temperatures, in the hearts of stars like our Sun, the nuclei of small atoms are fused together to make the nuclei of larger ones. Deep inside the Sun’s core, a “fusion reactor” has been in continuous operation since firing up some five billion years ago. But what is nuclear fusion?

It works like this: Two hydrogen nuclei combine to release a flash of energy and a positron - a positively-charged electron - and a strange particle called a neutrino. A third hydrogen nucleus joins the combined pair. Instantly, there’s another flash of energy. Like magic the trio has become helium-three. Then, by fusing again with an identical trio, they become - in a flash - helium-four. They emit the two extra hydrogen nuclei - and more energy.

The mass of the helium-four nucleus is 0.7 percent less than the combined mass of the four component hydrogen nuclei from which it is assembled. It is this small percentage loss of mass that is converted into energy. Once formed, the helium-four nuclei remain stable because the temperature within the Sun’s core is currently too low for the next stage of thermonuclear fusion, involving carbon nuclei, to take place.

In this way, within the core of the Sun, 600-million tons of hydrogen are converted into 596 million tons of helium every second. As a by-product, the “missing” four-million tons of mass is turned into energy every single second. It is this non-stop process that makes our Sun a star - and will keep it blazing for the next five billion years.

Albert Einstein first came up with the idea that mass is a form of energy. His celebrated equation E = mc2 describes mathematically this conversion of mass to energy. The amount of energy (E) released by the conversion of a mass (m) is equal to m multiplied by the speed of light (c) squared. Since c is a large number, a very large amount of energy can be released by the conversion of quite small quantities of mass.

The Sun as an Energy Source The Sun is the ultimate source of most of the Earth's energy resources. Without the Sun's energy reaching Earth through space, there would be no plant or animal life on our planet. It is the Sun's energy, stored by plants and tiny organisms that lived on the Earth millions of years ago, which is released when we burn fossil fuels such as coal, oil and natural gas. Solar power is an example of a renewable energy source; unlike fossil fuels it will not run out. The Sun's energy, in the form of sunlight, may be harnessed directly to heat water, produce electricity by using solar cells, or by employing mirrors to focus the Sun's rays in a solar furnace. Fossil fuels, however, can only be burnt once and they are not recyclable. The gases produced from burning fossil fuels are responsible, in part, for the pollution of the Earth's atmosphere.

The energy of most alternative energy sources, such as wind and wave power, also comes from the Sun. As the Sun warms the Earth, it creates winds. The wind's kinetic energy can be converted into electrical energy by a windmill or wind turbine. Some of the wind's energy disturbs the surface of the sea and creates waves. Wave-power can also be used to generate electricity.

The Solar Cycle In 1843, the German astronomer Heinrich Schwabe discovered that the number of sunspots visible on the Sun's face periodically varies. At a maximum in the cycle, over 100 sunspots may be present, but towards solar minimum the number falls considerably. During this period there may be several weeks with no spots visible at all. On average, the sunspot cycle lasts 11 years, but there are considerable variations. Since records began, the length of individual cycles has varied from approximately seven to14 years. In 1893, at the Royal Greenwich Observatory, in England, E.W Maunder concluded from his study of old solar records that between 1645 and 1715 sunspots virtually disappeared – a period known as the "Maunder Minimum." This period coincided with a marked cooling of the Earth's climate. The polar ice sheets and glaciers advanced farther than at any time since the last ice age. In London, winter "Frost Fairs" were celebrated on the frozen River Thames. The period from 1645 to 1715 was called "The Little Ice Age." At the end of each 11-year cycle, when few or no spots are visible, the Sun's magnetic field reverses its polarity. A duration of 22 years, therefore, elapses before the Sun returns to its original magnetic pattern. Roughly midway between magnetic reversals, the Sun is at magnetic maximum: this is when the greatest number of sunspots are evident.

Links for Further Information A comprehensive index of site providing images of the Sun. http://sec.noaa.gov/solar_sites.html/

SOHO mission homepage, including picture gallery, resources for teachers, latest images and recent news. http://sohoww.nascom.nasa.gov/

Educational site aimed at school students containing general topics of interest about the Sun. http://solar-center.stanford.edu/

Selection of soft X-ray images of the Sun acquired by the YOHKOH satellite. http://www.lmsal.com/SXT/homepage.html/

General introduction to solar astronomy from a set of lecture notes, including a general description of the Sun and discussion of solar phenomena. http://www-solar.dcs.st-andrews.ac.uk/"alan/sun_course/Introduction/Main_menu/

Very recent full-disc hydrogen alpha images of the Sun - up to three per day – acquired at the Culgoora Solar Observatory in Australia. http://www.ips.oz.au/culgoora/index.html/

Daily solar images - by high resolution and full-disc - and movies of solar phenomena acquired at Big Bear Solar Observatory. http://www.bbso.njit.edu/ Current and archive pictures of full-disc solar images acquired at The National Solar Observatory, Sacramento Peak. http://www.sunspot.noao.edu/

Questions and Activities for the Curious 1. Calculate the Sun's diameter (1,392,000 kilometers), in terms of the Earth's diameter, which is 12,756 kilometers. Find the volume of both the Sun and the Earth, and then give the Sun's volume in terms of Earth's.

2. Investigate at least three ways in which the Sun directly affects Earth.

3. A spectroscope is an instrument used by astronomers to examine the Sun's spectrum. What information can be deduced by using such instrument?

4. The gas helium was identified in the spectrum of the Sun before being found on Earth. Research what other elements have been identified in the Sun's spectrum.

5. What visible features occur on the Sun's surface?

6. Between 1645 and 1715 there were virtually no sunspots visible on the Sun. The corona may have been absent and displays of the aurora were also lacking. The period is known as 'The Little Ice Age' because the climate was cooler at this time. Research what effects were noted during this period.

7. How can we tell that the Sun rotates? What is peculiar about its rotation?

8. What region of the Sun is seen only during a total eclipse? How hot is the region, and why can it not be seen every day?

Complete Cosmos Chapter 24: Big Bang, Big Crunch Theory of the Big Bang. From that cataclysmic explosion, the Universe continues to expand. But will it stop and reverse?

Outline How did the Universe begin? How might it end? The Big Bang theory explains how the cosmos fired up - but not why. The story begins with a cataclysmic explosion that spawns matter, space and time. From that initial expansion, atoms form from protons, neutrons and electrons. Then come the first chemical elements - hydrogen, helium, and a little lithium.

Expansion continues, temperatures drop. After 300,000 years, the Universe becomes transparent. Light and other radiation speed across space. Today, particle accelerators on Earth simulate conditions in the Big Bang - testing the theory.

As the Universe expands, Big Bang radiation slowly cools. In 1965, this remnant "glow" - the cosmic microwave background - is confirmed. Nearly 30 years later, the Cosmic Background Explorer spacecraft, maps the temperature of the cosmos in minute detail. COBE finds evidence of a primitive structure - the precursor of galaxy formation. Galaxies in clusters and supercluster are held together by gravitational forces. Evidence of their evolution is provided by supercomputer simulation. But still, the early history of individual galaxies remains unexplained. Later, there is galactic cannibalism as larger galaxies consume smaller ones.

The age of the Universe is a problem. Different methods give different answers. Some stars appear to be up to 15 billion years old. Yet certain measurements of the expansion rate of the Universe show it at only 13 billion years old. The satellite Hipparcos settles matters by precisely measuring the distance of stars. It finds some are farther away than previously thought. This changes the age estimates and solves the conundrum - both the puzzling stars and the Universe are younger than previously thought. The Universe continues to expand - but for how long? Will it go on forever? Or will it stop and reverse - ending in a Big Crunch?

Sub-chapters The Big Bang • The origin of the Universe and those crucial questions: how did it happen, why did it happen, and how will it end? To this day, much remains a mystery.

The Veil Lifts • The Big Bang theory in detail - from a blindingly hot beginning, energy spontaneously generates matter and anti-matter; matter dominates; a seething array of sub-atomic particles. • The formation of protons, neutrons, electrons and the first atoms. Three minutes after the Big Bang, the first elements - hydrogen, helium, and a trace of lithium – are created. • As expansion continues, temperatures drop. After 300,000 years the Universe becomes transparent. Light speeds unimpeded across the ever expanding cosmos. • On Earth, understanding conditions in the Big Bang by using a particle accelerator - an atom-smasher - to simulate the early Universe. • The fleeting production of sub-atomic particles, energy converted into different forms. • In 1965, a microwave antenna picks up a cosmic hiss - recognized as the left-over heat from the fireball of the Big Bang.

Primitive Structure • The cosmic microwave background - how original ultraviolet radiation has shifted to the microwave part of the spectrum. • COBE - the Cosmic Background Explorer satellite which detects minute temperature variations in the microwave background - the first primitive structures in the Universe. • Development of large-scale structure as the Universe expands - how mutual gravitational attraction draws galaxies into clusters and groups.

Formation of Galaxies • The formation of galaxies remains a puzzle. Supercomputers simulate complex interactions as galaxies collide. • And as galaxies merge, gravity orchestrates a chaotic dance. The violent evolution of a galaxy supercluster. • Galaxy mergers. Such interactions are the norm - and continue today. A current hypothesis is that galaxies grow from smaller aggregations of matter - larger galaxies consuming smaller ones in merger after merger.

Age of the Universe • The age of the Universe depends on the speed of its expansion. If it's slow, the Big Bang happened some 15 billion years ago. If expansion is rapid, the Universe is younger - between 10 and 13 billion years old. • Some measurements put the age of the Universe at only 11 billion years, yet the most ancient stars are thought to be 15 billion years old. But how can that be? Stars cannot be older than the Universe. • The satellite Hipparcos helps determine the age of the Universe by accurately measuring the distance of stars. • Some stars are more distant than previously thought - changing the scale of the Universe, and hence estimates of its age. It is younger than previously thought. The conundrum appears to be resolved. • Explanation of parallax, a measurement method used by Hipparcos.

The Big Crunch • Its continued expansion will eventually lead to a dispersed, bleak and lonely Universe • A dramatic alternative is The Big Crunch - where expansion stops and reverses.

Background Cosmology Cosmology is the study of the origin and evolution of the Universe. Cosmology tries to explain the current structure of the Universe, how it was in the past and what might happen in the distant future. Attempts to answer these questions have inspired many models and theories - a lot of speculation based on comparatively little observational material.

That is why it is so difficult to discriminate between different cosmological ideas – the most informative parts of the Universe are farthest away. Objects in such regions are extremely faint and their exact nature unknown. It is hard to tell how similar they are to more familiar celestial objects closer to us.

Currently, cosmology favors the theory of the Big Bang. In an instant, from a speck smaller than an atom, the Universe was created. It happened between 15 and 20 thousand million years ago. From a superdense and unbelievably hot beginning, the Universe expanded - an expansion that continues today.

For 21st century cosmologists, the most important problems are to determine the rate at which the Universe is expanding and how it has expanded in the past. And most fascinating of all, will the Universe continue to expand forever? Or, depending on its total amount of matter and energy, will it collapse back on itself in a Big Crunch?

Another huge cosmological problem is hidden mass. There appeared to be evidence that vast quantities of dark matter lurked in the Universe, possibly 100 times as much as visible mass. But, again, recent work suggests there may be nothing there at all. Cosmology is an infant science.

The Cosmic Background Radiation Important evidence for the Big Bang theory was a discovery in 1965 by Arno Penzias and Robert Wilson. They detected a weak signal coming in from every part of the Universe. Dubbed "cosmic background microwave radiation", it could only be explained as the relic of a primeval explosion - what else but the Big Bang?

Some 300,000 years after the Big Bang - when matter and radiation had cooled to few thousand degrees - space became transparent and the original radiation spread out into the expanding Universe. Since then, the Universe has expanded a thousand fold. The original radiation has been diluted, redshifted and cooled to a temperature just 2.736 degrees above absolute zero.|

But why is cosmic background microwave radiation so uniform across the sky? It puzzles astronomers because the radiation comes from every direction - from parts of the Universe that have never been in contact with each other. Cosmologists call it the "horizon problem". One explanation is that the early Universe expanded so fast that radiation from the Big Bang became "inflationary", smoothly spreading and stretching everywhere. The jury is still out.

The Cosmic Background Explorer (COBE) The Cosmic Background Explorer (COBE) was the first US space mission devoted to cosmology. Launched by Delta rocket into circular Earth orbit, COBE traveled over both poles at a height of 900 kilometers. The date: November 19, 1989 COBE weighed 2.27 tons and carried three instruments. They were designed to map the cosmic microwave background radiation with extraordinary sensitivity. COBE was also to search for radiation released by the earliest galaxies soon after their birth. The overall objective was to answer basic questions about the Big Bang and how clusters of galaxies came about.

Astronomers generally agree that the Universe originated in an explosion between ten and 20 thousand million years ago. Everything - space, time and matter - came into existence at the same moment. The Universe started to expand and has been doing so ever since. It also cooled quickly from its original very high temperature. Today, the remnant of the Big Bang is detected as a weak background of cosmic microwave radiation coming from all directions - at a temperature of 2.736 degrees above absolute zero.

The problem for COBE was that the background radiation seemed to be astonishingly smooth. This suggested that the original expanding Universe had also been smooth and uniform. Yet how could our present Universe - which is far from smooth - have evolved from such a beginning? How could clusters and superdusters of galaxies have formed? COBE's mission was to search for minute differences in temperature across the background radiation - difference that would indicate a degree of non-uniformity.

To the relief of theorists, it was announced - in April 1992 - that COBE had made a very important discovery. The spacecraft had detected variations of just ten millionths of a degree - one part in 100,000 - in the temperature of the background radiation. These "ripples" appeared to be the largest and oldest structures in the Universe. They were the "seeds" that evolved into the galaxies and large-scale structures we see today.

The Helium Problem The amount of helium in the Universe is further support for the Big Bang theory. Measurements show that helium is the second most plentiful element. Such an abundance could not have come from stars alone - where nuclear fusion converts hydrogen into helium. Scientists propose, therefore, that most helium was created in the Big Bang. A few minutes after the Big Bang, the Universe would have consisted almost entirely of hydrogen nuclei - single protons - and helium nuclei. Their ration, by mass, would have been 70-75 percent hydrogen to 30-25 percent helium. The fact that hydrogen and helium exist in just these proportions in the Universe today strongly supports the theory. In 1990, using the Hubble Space Telescope, scientists observed the spectral signature of helium in the light of the distant quasar UM675, 12 billion light years distant. It confirmed the profusion of helium in the early Universe.

Links for Further Information A site explaining the main pillars of standard cosmology: http://www.amtp.cam.ac.uk/user/gr/public/bb_pillars.html

A site with links to key topics on cosmology. http://easyweb.easynet.co.uk/~zac/chapter9.htm

A page explaining the theory of the Big Crunch. http://www.caltech. edu/~goodstein/crunch.html

A page discussing the future of the Universe - with information about the Big Bang and Big Crunch. http://windows.ivv.nasa.gov/the_universe/Future.html

Questions and Activities for the Curious 1. How does the Steady State theory - popular in the 1950s and 1960s - differ from the Big Bang theory?

2. What is cosmic background radiation and why is to so important?

3. Describe some significant discoveries of the COBE satellite between 1989 and 1992.

4. How did galaxies probably form in the early Universe?

5. Why is possible to estimate the age of the Universe if we know how fast it is currently expanding.

6. How did the Hipparcos satellite help solve the problem of some ancient stars seeming to be older than the Universe?

7. Describe the method of parallax used to measure the distances to the nearest stars.

8. Explain what is meant by the Big Crunch and how it might come about.

THE COMPLETE COSMOS Chapter 17: Aurorae and Eclipses Celestial shows. How the solar wind conjures an aurora. Lunar and solar eclipses explained. A recent eclipse of the Sun.

The Sun continuously emits a stream of electrified particles known as the solar wind. Earth is normally protected from these particles by its magnetosphere, a great magnetic "bubble". Occasionally the solar wind increases to "storm-force" as a result of gigantic eruptions on the Sun. On Earth, such bursts cause geomagnetic storms, blacking out cities and knocking out satellites. Between these major storms, electrified particles often leak through Earth's magnetic defenses, around the poles, causing beautiful displays in the night sky known as aurorae - the aurora borealis in the northern hemisphere and the aurora australis in the southern.

Another form of celestial spectacular is an eclipse - either an eclipse of the Sun or an eclipse of the Moon. Eclipses are caused by special alignments of the Sun, Earth and Moon. The mechanisms of solar and lunar eclipses, always difficult to explain, are carefully and graphically illustrated. From the Caribbean island of Curacao, coverage of the 1998 total eclipse of the Sun. Observers travel thousand of kilometers to catch the show. Tension mounts during build-up phases when the Sun is only partially obscured. Then comes the splendor of totality - with dramatic footage of prominences and the solar corona - and, finally, the spectacular diamond ring when totality is over.

Solar Wind * Introducing the Sun as the source of all our energy, emitting heat and many forms of radiation. * The solar wind, the continuous stream of electrified particles emitted by the Sun. * How great eruptions on the Sun can boost the solar wind to "storm-force". * The effects on Earth of the resulting geomagnetic storm, disabling power lines, sending satellites into a spin and disrupting communications. * Animation reveals how the Earth's magnetic field is squeezed as the material ejected from the Sun strikes, causing a geomagnetic storm.

Lights of the Aurora * How the normal steady stream of solar wind particles is funneled down towards the Earth only around the poles. * Formation of the auroral ovals - one around the north magnetic pole, another around the south pole. * The mechanism of the aurora. How atmospheric atoms are excited to fluoresce by collisions with electrified particles spiraling down magnetic field lines. * The majesty and beauty of an auroral display.

How Eclipses Happen * Eclipses occur because sunlight is blocked. * The mechanics of the Moon's orbit around the Earth, the inclination of its orbit and why we don't see an eclipse every lunar month - explained with computer graphics. * The "nodes" of the Moon's orbit and their significance. * Geometry of an eclipse - both lunar and solar. How the Earth's atmosphere causes the Moon to turn red during a lunar eclipse. Eclipses of the Sun * Mechanism of solar eclipses in detail. The ellipticity of the Moon's orbit, and how the Moon's shadow cone sometimes doesn't reach Earth - resulting in an annular eclipse. * How a total solar eclipse occurs when the shadow cone does reach Earth, and how that shadow sweeps across the Earth's surface. The speed of the shadow.

The Real Thing * February, 1998. Enthusiasts from around the world gather on the island of Curacao, in the Caribbean, to witness a total eclipse of the Sun. * First contact, as the Moon bites into the solar disk. The partial phases. * Using special filters to view the eclipse in safety. Projecting crescents of the Sun. * Clouds threaten the show, tension rises. But clear skies return as totality approaches. * The full glory of totality, showing the corona and prominences, with the planets Mercury and Jupiter visible. * The Moon exactly covers the solar disk - 400 times smaller, yet 400 times closer. An incredible coincidence of Nature!

Background The Solar Wind and Earth's Magnetosphere The continuous outwards motion of the gas in the Sun's corona gives rise to the solar wind. Even though the total outflow of material from the Sun is about one million tons per second, this mass- loss has a negligible influence on the Sun's evolution. The high temperatures of the corona (1-2 million degrees) ensure that the constituents of the solar wind (mainly electrons, protons and alpha-particles) are ionized, i.e. electrically charged.

This stream of electrified atomic particles normally flows outwards from the Sun at speeds of 300-400 kilometers per second. On occasions, however, solar flares or massive ejections of material from the Sun can boost the solar wind to "storm-force", reaching speeds up to 800-1,000 kilometers per second. At these velocities, a burst of electrified particles from the Sun may take only about 48 hours to travel the distance to Earth.

The Earth's magnetic field causes a bubble in the solar wind. This cavity, called the magnetosphere, partially shields Earth from the high-speed electrically-charged particles of the solar wind. Such particles are often energetic enough to damage living cells and are, therefore, potentially harmful to us.

The magnetic field protects us because when charged particles encounter it, they are deflected by its magnetic force into a spiraling motion around the field lines. This slows the particles of the solar wind, causing them to flow around the Earth in much the same way as water in a stream is diverted around a partially-submerged rock. Thus we are spared the full impact of the Sun's charged particles.

The Aurora and Earth's Radiation Belts As the solar wind particles stream past the Earth, they generate electric currents in our upper atmosphere. These currents cause electrified particles - electrons and protons - to spiral down the Earth's magnetic field lines. The particles rain on the upper atmosphere in two oval-shaped regions around the Earth's north and south magnetic poles. When the particles collide with gases in the upper atmosphere, atoms of oxygen and nitrogen light up like the gases in a fluorescent tube. The result is the beautiful shimmering display called the aurora borealis (Northern Lights) or aurora australis (Southern Lights), depending upon the hemisphere from which the phenomenon is being observed. The exact process by which the aurora forms is still controversial. The beautiful streamers of an auroral display are shaped by the Earth's magnetic field, much as sprinkled iron filings outline the field of an ordinary bar magnet.

Deep inside Earth's magnetosphere, our planet's magnetic field is strong enough to trap charged particles which have managed to leak through the magnetopause, the outer boundary of the Earth's magnetic domain. These particles are held captive in two huge, doughnut shaped rings called the Van Allen radiation belts. These belts were discovered in 1958 during the flight of the first successful US artificial satellite, Explorer 1. The inner Van Allen belt contains mainly protons and extends from 2,000-5,000 kilometers above the Earth. The outer Van Allen belt contains mainly electrons. It is about 6,000 kilometers thick and centered at an altitude of 16,000 kilometers. The particles trapped in the Van Allen belts are energetic enough to penetrate spacecraft and to be a hazard to astronauts, damaging tissue. Astronauts, therefore, try to avoid passing through the belts or to get through them as quickly as possible.

The following sections are reproduced, with permission, from the book "THE WEST COUNTRY ECLIPSE" 11 AUGUST 1999", by the British astronomer Patrick Moore, ISBN 0-9531716-1-2.

Eclipses of the Moon Like all non-luminous bodies, the Earth casts a shadow in space, and if the Moon passes into this cone of shadow - which can only occur at Full Moon - its supply of direct sunlight is cut off, producing a lunar eclipse. The Full Moon turns a dim, often coppery-red color, before passing out of the shadow again. In general it does not vanish completely, because some of the Sun's rays are bent or refracted on to the Moon by way of the layer of atmosphere surrounding the Earth, but there are times when the eclipsed Moon is hard to trace with the naked eye. Everything depends upon conditions in the Earth's upper air through which the refracted sunlight has to pass.

Lunar eclipses may be either total or partial, and as seen from any particular location on Earth are more common than eclipses of the Sun - because when a lunar eclipse happens it can be seen from any place from which the Moon is above the horizon at the time. This is not true of solar eclipses, as we shall see below.

Eclipses of the Sun Eclipses of the Sun are of three types: total, annular and partial. All are interesting, but for sheer grandeur total eclipses are unrivalled. Only then can the deep red chromosphere, the "flame-like" prominences and the pearly corona be seen with the naked eye.

Total eclipses occur when the Sun, the Moon and the Earth are exactly lined up, so that the Moon's shadow reaches the surface of the Earth. But the shadow is only just long enough to do this, and totality can be seen from only a very restricted area of the Earth's surface which explains why from any particular location, eclipses of the Sun are much less common than those of the Moon. The width of the track of totality can never be more than 272 kilometers, and is usually less. For example, the path of totality during the 11 August 1999 total eclipse is only just over 100 kilometers wide as it crosses Cornwall and Devon. To either side of the main cone of the shadow, the Sun is only partly hidden. Moreover, totality is brief. From any one site it can never last for longer than seven minutes 31 seconds, and so far as I know there has never been an observation of an eclipse as protracted as this. The record appears to be held by the 1955 totality as seen from the Philippine Islands, which lasted for seven minutes eight seconds. The last English total eclipse, that of 29 June, 1927, lasted for a mere 24 seconds, and the width of the track of totality was only 52 kilometers.

There is, of course, one way to overcome this problem. To prolong totality as long as possible, the eclipse should take place near the equator where the Earth's rotational velocity reaches a maximum value of nearly 1,700 km per hour from west to east. This cancels out some of the motion of the Moon's shadow which travels at about 3,400 km per hour from east to west. Some eclipses are not total from anywhere on Earth. Such, for instance, was the eclipse of 12 October, 1996; as seen from London just over 60 per cent of the solar disk was hidden..

The third type of eclipse - the annular - occurs because the Moon's distance from the Earth varies appreciably; its orbit, like those of virtually all Solar System bodies, is appreciably eccentric. The distance ranges from 356,400 km at its closest (perigee) out to 406,700 km at its furthest (apogee), giving a mean center to center distance of 384,400 km. This means that the apparent diameter changes, from 29 arcminutes 22 seconds at apogee to as much as 33 arcminutes 31 seconds at perigee. The mean apparent diameter of the Sun as seen from Earth is 32 arcminutes. It follows that when the Moon is at or near apogee, its disk is too small to cover that of the Sun, and if the alignment is perfect we see a ring of sunlight left showing around the dark disk of the Moon. This explains the name; annulus is Latin for 'ring'.

The maximum possible duration of the annular phase of an eclipse is 12 minutes and 24 seconds, but most are much shorter. So far as we are concerned, the next British annular eclipse will fall on 31 May 2003, as seen from the very north of Scotland; from Aberdeen and Perth the eclipse will he only partial. Occasionally there are eclipses which are annular along most of the central track, but total at the mid-point. This happened on 3 October 1986, which was mainly annular but was total for about a tenth of a second as seen from the middle of the Atlantic Ocean. Note, incidentally, that since the average length of the Moon's cone of shadow is less that the mean distance between the Moon and the Earth, annular eclipses are more frequent than total eclipses in the ratio of 5 to 4.

The Glory of a Total Solar Eclipse First Contact: The moment when the Moon's disk begins to move across the brilliant face of the Sun, and a tiny notch appears at the Sun's limb. Of course, the exact moment can be predicted with great accuracy, but it takes a few seconds for the notch to become noticeable.

To view the partial phases of the eclipse you MUST have proper eye protection. Viewing the Sun through ordinary sun-glasses, even dark ones, is certainly asking for trouble; these give no protection whatsoever, and neither do exposed photographic film, photographic filters, crossed polarisers, gelatin filters, compact disks, or smoked glass. Please DO NOT be tempted to use any of these.

The partial phases of the eclipse may be observed safely through a welder's glass rated at number 14 or higher, or a pair of aluminized mylar 'spectacles'. Mylar is a very tough plastic film, and solar filters are made by coating it with a thin layer of aluminium. DO make sure that any filters you use carry the "CE" mark, and check them very carefully for any damage. DO NOT use filters if they are scuffed, scratched or have holes in them, and DO NOT use any filter if you are not certain that it is approved and safe, or if you have any other doubts about it. Always hold the filter firmly over both your eyes BEFORE looking up at the Sun, and do not remove it until AFTER looking away. DO NOT look at the Sun through any optical instrument, e.g. telescope, binoculars or camera, even if you are wearing special filters.

Gradually the Moon passes on to the face of the Sun. For a surprisingly long time there is no perceptible diminution in light or fall in temperature, but when the Sun is more than half covered these effects start to become evident. If there are any sunspots (as there probably will be in August 1999) compare them with the darkness of the Moon; the lunar disk will be seen to be much the blacker. By the time that the Sun is nearly half covered, anyone standing near a tree or bush will be able to see tiny crescent-shaped images on the ground around them. Gaps in the foliage act as 'pinhole cameras' and focus the images of the crescent Sun.

Shadow Bands: These curious, narrow wavy bands of light and dark are purely atmospheric phenomena. They are sometimes seen moving across the ground just before totality (and just after), but not always; conditions have to be exactly right.

Lunar Shadow: As totality approaches, the whole scene changes with amazing rapidity. The temperature falls, the sky darkens, and the shadow of the Moon can be seen rushing across the landscape - or, better, seascape.

Diamond Ring: Just before the last sliver of the Sun's brilliant disk disappears, we see the effect termed the Diamond Ring - a brilliant point which lasts for an all-too-brief period. It is usually better seen at the end of totality.

Baily's Beads: The Moon's limb is not smooth; there are high mountains and deep valleys. Moments before totality, the sunlight comes to us through the lunar valleys on the limb, and the result is a series of bright points of light. They were described in detail during the 1836 eclipse by the English astronomer, Francis Baily, and are named after him. In fact the 1836 eclipse was not total, but annular, so that during a 'short annular', when the Moon's disk is almost large enough to cover the Sun, Baily's Beads are quite conspicuous. They are not likely to be seen during the next British annular, that of 31 May 2003, because the Moon will be near apogee, and will cover no more than 94 per cent of the Sun.

The Sun is wholly hidden, and totality has begun. The pearly corona and 'flame-like' prominences flash into view; the sky is dark, and almost at once bright stars and planets can be seen. There is an abrupt fall in temperature; birds, understandably confused, start to roost, and some types of flowers to close. There is often a strange, somewhat eerie calm. The corona is not always of the same shape. Near sunspot minimum it sends out wings' and streamers, while near maximum it is more symmetrical - though of course no hard and fast rules can he laid down.

Mid-totality: Again no two eclipses are alike, but all in all it is fair to say that the corona gives out about as much light as the full moon. This means that direct viewing, even with a telescope, is safe. Pause to look round the sky; any bright planets will shine forth, together with bright stars, though if there is any trace of haze or thin cloud it is likely that only Venus and Jupiter, if favorably placed, will be obvious. Third contact: Totality ends as suddenly as it had begun.

The Diamond Ring: This is one of the most glorious moments of the entire eclipse, but it lasts for so short a time. In a few seconds the Sun's brilliant disk starts to reappear; the corona fades from view, and the Diamond Ring is lost. This is certainly the most dangerous moment for the careless observer. The slightest sliver of the Sun's brilliant disk is as dangerous as the uneclipsed Sun itself - and so if you have been viewing directly, make sure you take your eye away from the 'danger-zone' in time. Remember, too, that an SLR camera acts in the same way as a telescope or binocular lens. Partial phase: Gradually the Moon moves off the face of the Sun. The sky quickly brightens: birds leave their roosts and flowers open; the Earth seems to 'wake up'. With luck you may see the effects of the receding shadow of the Moon in the form of a curved dark, sometimes purplish patch covering part of the sky.

Fourth contact: The Moon finally moves off the Sun, and everything is hack to normal; the eclipse is well and truly over. In fact, it usually happens that few people wait to see fourth contact; they are too busy packing up their equipment and comparing notes. Such is the sequence of events for a total eclipse seen under ideal conditions. Sadly, this does not often happen. If there is any cloud around, the outer corona will be lost and no stars or planets will be seen. It may happen that the sky is partly cloudy, in which case all that can be done is to hope for the best.

(End of extracts from Patrick Moore's book "THE WESTCOUNTRY ECLIPSE: 11 AUGUST 1999".)

Links for Further Information A page with information about Sun-Earth interactions, and auroral observations. http://snowcat.polar.rm.cnr.it/artico/sun.html

Data and other links relating to solar-terrestrial environmental research. http://shnet1.stelab.nagoya-u.ac.jp/omosaic/goin95/subg96/subg4.html and http://shnet1.stelab.nagoya-u.ac.jp/omosaic/goin95/goin95.html

Details of lunar eclipses, with geometry, applications, astronomy, mathematics, text and graphics. http://www.ntplx.net/~mawdsley/m4apluna.htm

NASA's and Goddard Space Flight Center's very comprehensive eclipse home page, including images, eclipse alerts, data, photography, and links to many other sites relating to solar and lunar eclipses. A must for would-be eclipse-watchers! http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

Questions and Activities for the Curious 1. If the Earth rotated more slowly would you expect it to have a stronger or weaker magnetic field? Give your reasons.

2. What is the solar wind, and why does its speed vary?

3. Describe some of the effects that solar flares and massive solar ejections can have on the Earth.

4. How is the aurora related to the Earth's magnetic field?

5. Why aren't there eclipses every month?

6. Describe an annular eclipse of the Sun and explain how it occurs.

7. With diagrams, show the relative positions of the Sun, Earth and Moon at the time of a total solar eclipse and a total lunar eclipse.

8. Briefly describe some of the phenomena you would expect to see during a total eclipse of the Sun.

THE COMPLETE COSMOS Chapter 15: Where Next? A spaceport in Earth-orbit, the colonization of the Moon and Mars, the taming of Mars - plus an elevator into space!

Outline A futuristic shuttle soars into orbit and docks with a spaceport. Is this the weekend-break of the future? Would such a station be the staging post for onward journeys to the Moon and beyond? A glimpse of cities-of-the-future on the Moon and Mars. That's the dream. The reality is the International Space Station (ISS) currently under construction in Earth- orbit. ISS is scheduled for completion in 2004. As assembly gains momentum, so do test flights of X-33, the half-size prototype of Venturestar, the next generation of Space Shuttle.

Preview of a self-sustaining human colony on Mars - laboratories, factories, offices and homes connected by airtight corridors. The possibility of terraforming Mars - giving it breathable air. But the job would take 100,000 years.

To travel beyond the Solar System, we would need new sorts of craft with ion or nuclear propulsion systems. But even at one-tenth the speed of light, a journey to the nearest star would take over 40 years.

Some futurists believe we could colonize Venus. Currently, it's a hellish hot-house. Could it be tamed by seeding the Venusian clouds with green algae? Hundreds of thousands of years later, could Venus look like Earth - with oceans, a cooler climate, even life? For space guru Arthur C. Clarke, "Where Next" is closer to home. He visualizes Earth girdled by a gigantic space wheel supported by four huge towers rising from the equator. Within, people would ride into geostationary orbit - and back again.

Sub-chapters Airport to Spaceport • A shuttle craft-of-the-future ferries weekenders to a "leisure wheel" in Earth orbit. • Airport to spaceport in one smooth hop. Such a spaceport would be a staging post for onward journeys to the Moon. • A transporter pod touches down at a Moon-base. Beyond is a lunar city. • Next stop, a self-contained Martian city - modeled on the lunar cities and serviced from the Moon.

International Space Station • From science fiction to science fact. The International Space Station (ISS) under construction in Earth orbit. Scheduled for completion in 2004, it takes over where Mir leaves off. • Robots will carry out checks and assist the multi-national crews. The main objective: testing of the long-term effects of living in space.

A New Shuttle • Initially ISS crews will travel aloft on the old Space Shuttle - but a new generation of shuttle craft is planned. • Test flights of a half-size prototype, the X-33 - forerunner of Venturestar, an entirely reusable shuttle that will lift cargo into orbit at one tenth of today's costs.

Cities on Mars • The late 21st century. Thanks to spaceports like ISS, Mars has been colonized. A solar- powered, self-sustaining city of laboratories, factories, offices and homes - interconnected by airtight corridors. • The possibility of turning Mars into a planet like Earth - but it would take 100,000 years to "terraform" the Red Planet.

Journeys to the Stars • Problems of traveling to the outermost planets and beyond. We are held back by our propulsion systems and limited fuel capacity. • Solar power and solar sailing are not the solutions. • Ion or nuclear propulsion might provide the thrust to travel to the stars. But even at one- tenth the speed of light, journeys to the nearest stars would take decades. • For the time being such voyages are science-fiction.

Planetary Engineering • Possibilities of colonizing Venus, perhaps our next target after the Moon and Mars. Seeding the planet's poisonous clouds with green algae might eventually produce an Earth-like world with a breathable atmosphere, oceans, and a cooler climate - a place where life could take hold. • But is such planetary engineering desirable? • Another vision of the future - the notion of space guru Arthur C. Clarke. Four towers - each housing a space elevator - stretching upwards from the equator to an encircling "halo" in geostationary orbit 36,000 kilometers above the Earth - the spokes of a gigantic space wheel with our planet as the hub.

Background The International Space Station (ISS) Construction of the ISS began on November 20, 1998, with the launch of Russia's 24-tonne Zarya "Sunrise" module on a Russian Proton rocket. The module comprises ISS's "Functional Cargo Block".

A few days later, America's Space Shuttle "Endeavour" deployed ISS's "Unity Node" and two "Pressurized Mating Adaptors" (PMAs). PMA-1 connects the Unity Node and Functional Cargo Block, while PMA-2 provides a dock for Shuttle.

It will take 44 launches and numerous supply missions to complete the $100 billion space station. When finished, hopefully in 2004, ISS will have an end-to-end wingspan of 108 meters. The station will be 88 meters long and 44 meters tall.

ISS will weigh 400 tons and be assembled in orbit from almost 100 separate components. The work will involve more than 1,700 hours of spacewalks - twice as many as the hours jointly accumulated by Russian and US astronauts in the first 37 years of manned space flight. The total structure will cover 4000 square meters, the size of two football pitches.

The station will accommodate six to seven astronauts and scientists. Both living and working space will be pressurized and have roughly the same cabin volume as two Boeing 747 jumbo jets, approximately 1,300 cubic meters. ISS will orbit at an average altitude of 350 kilometers above the Earth, at an inclination of 51.6 degrees to the equator. If all goes well, ISS should be manned from 2001, when a multi-national crew of three will begin its stay aboard the station. The trio will use robotic arms to maneuver components into place. The astronauts will make spacewalks to connect the complex power, computing and utility connections.

Thirteen nations are contributing practical resources to ISS, along with scientific and technical expertise. Astronauts and scientists will be chosen from those nations. The first laboratory module is to be called "Destiny". It will be central to investigations in life sciences, earth sciences, space science, microgravity and engineering, as well to the research and development of space products.

Venturestar - the Future Shuttle Venturestar is being developed to replace the US Space Shuttle. It is hoped that Venturestar will deliver a wide range of payloads more reliably and less expensively than today's launch vehicles. As well as a full ISS replacement crew, this "future-shuttle" should to ferry up to 20 tons of cargo to Earth orbit.

The new craft is a fully reusable single-stage-to-orbit vehicle. That means Venturestar will not have to jettison fuel tanks or rocket boosters along its flight path as Shuttle does today. Venturestar will operate more like an airplane, undergoing inspection, refueling and reloading between flights. By reusing the entire vehicle, the operating expenses should be just one-tenth of today's costs.

Before Venturestar goes into production, a demonstrator vehicle is to prove the concept. Known simply as X-33, the model will be one-half the size, one-ninth the weight and a one-quarter the cost of the full-size Venturestar. The X-33 will not reach orbit or carry payloads. It will merely fly sub-orbital trajectories specifically designed to test the thermal protection system, aerodynamics and flight capability of Venturestar. Propulsion is one of the main differences between the Space Shuttle and Venturestar. The linear aerospike engine was initially designed and tested in the 1970s for use on Shuttle. As it transpired, engineers opted for another design - the "Space Shuttle Main Engine". But NASA returned to the aerospike engine in 1995 when the concept for the X-33 was proposed.

Venturestar's aerospike engine will be lighter then today's conventional rocket engines. The aerospike automatically adjusts to changing atmospheric conditions, enhancing its efficiency as the vehicle climbs into orbit. In addition, with multiple combustion chambers on each engine, the aerospike should be more "fail-safe" than conventional rocket engines with their single combustion chambers.

Colonizing the Moon and Mars Will humans be living on the Moon or even Mars in the 21st century? It is now a real possibility. Teams of scientists are working hard to realize the dream. Their first step is to establish large, permanently occupied space stations in Earth orbit. And who knows? We could have space cities, as first suggested by the Russian rocket pioneer Konstantin Tsiolkovskii. An orbiting metropolis might be home to more than 1,000 people. They would produce electricity from solar power. Waste products and water would be recycled to reduce supply needs. Ferries would bring regular cargoes of raw materials and shuttle passengers to and from Earth.

Moon colonies would be the next step. The lunar surface is firm enough to support buildings. Disadvantages are a lack of atmosphere, bitterly cold nights and scorching days. Initially, a lot of supplies would have to come from Earth - which is very expensive. Minerals, however, could be mined on the Moon. They would be vital in constructing lunar bases, as well as supplying factories in Earth orbit and for use on Earth itself. Additionally, if lunar water is confirmed on the Moon, bound up in rocks at the north and south poles, then this would be an invaluable resource for future lunar exploitation.

Work on a lunar base could start in 2005 or even earlier. This "frontier town" would probably have a number of inflatable domes. Some would be used as living quarters, others as workshops. Inside too, plants would be cultivated for food and amenity. By 2025, there could well be lunar colonies of several hundred people.

After the Moon, Mars will almost certainly be the next world colonized in the 21st century. The Moon will act as a supply base, a staging post, for the Red Planet. And just like the Moon, agriculture would be developed under pressurized Martian domes. By the end of the century, Mars could have a population of several thousand - humans as the first real Martians!

The colonists would soon adapt to the planet's weaker gravity. But a trip home would be quite uncomfortable - with Earth's gravity so much stronger. Inside the Martian domes life would have its restrictions. Outside would be worse. Colonists would have to wear spacesuits. One day, perhaps, Mars would be "terraformed" - providing an atmosphere like that on Earth. Only then would humans be able to walk unprotected on the surface of Mars.

Links for Further Information Background and current status on the International Space Station. http://station.nasa.gov/reference/status/index.html

Special reports from NASA and background information on Mir - plus a weekly status report link. http://shuttle-mir.nasa.gov/shuttle-mir/specrpts/

A site about the project and construction of the International Space Station http://space.miningco.com/msub20.htm

A page about VentureStar with graphical images of the spacecraft. http://www.venturestar.com/index.html

Questions and Activities for the Curious 1. As the cost of launching payloads is reduced, so outer space will be liable to exploitation. What problems may ensue?

2. Imagine a colony on the Moon. What sort of work would go on there?

3. Describe the various stages in the construction of the International Space Station.

4. Think of an appropriate name for the International Space Station. Give reasons for your choice. 5. Imagine you are a travel agent specializing in space destinations. Devise an advertisement to promote a holiday on Mars.

6. There are plans to terraform Mars - to turn it into another Earth. It would take 100,000 years. Is such planetary engineering desirable? Would it be worth the effort?

7. Since colonies on the Moon and Mars are likely to be international, how would they be governed and by whom?

8. Traveling at one tenth the speed of light, a return trip to the nearest star system beyond the Sun would take 85 years. How might humans make such a journey?

THE COMPLETE COSMOS Chapter 3: Venus Beneath the clouds - planetary hell. A poisonous, crushing atmosphere, searing heat, volcanoes and a runaway greenhouse effect. Why?

Outline Venus is a planet of lowlands, shaped by volcanoes and shrouded in dense, poisonous clouds. The brightest planet in our night sky, Venus is beautifully observed in the evening twilight sky - a factor determined by Venus's orbit of the Sun. Observations of a transit of Venus in 1769 were used to refine our knowledge of distances within the Solar System.

Radar signals from Earth, which pierced the clouds, revealed surface features for the first time - plus the planet's very slow backwards rotation. Russia's Venera probes showed the planet's hostile nature - early spacecraft were crushed. Later, radar mapping uncovered a surface of plains, pockmarked by volcanoes. Venus has only two continents, in contrast to Earth's seven.

In the 1990s, images from the Magellan spacecraft yielded recognizable landscapes – after computer processing. Gravity mapping provided more information about the interior of Venus. The chapter concludes by suggesting that Venus, possibly, could once have been Earth-like. But then, alas, the Venusian oceans evaporated and the atmosphere thickened, leaving the baked, hostile Venus of today.

Sub-chapters An Unlikely Twin • A tour of Venus, swooping towards the planet, down through the clouds, over plains and volcanoes, finally traveling back through the toxic skies. • Comparisons with Earth, highlighting their similar size, and their differences – the searing temperatures of Venus, its crushing atmosphere and poisonous clouds. • How Venus may once have looked - more like Earth.

Venus Unveiled • Venus viewed from Earth - why Venus is often fairly low in the dusk or dawn twilight sky. • The orbits of Venus and Mercury, as viewed from Earth. • Transits of Venus across the solar face. How observations of a transit by England's Captain James Cook in 1769 led to a more accurate measure of the distance between Earth and the Sun.

Mapping Venus by Radar • How radar provides the first information about the surface of Venus, and reveals its slow backwards rotation. • Early spacecraft discover an atmosphere 90 times denser than Earth's, clouds of sulphuric acid, and very high surface temperatures. • Venus as a world of extensive lowland plains, with few uplands and only two continents.

Venusian Volcanism • 1990: America's Magellan mapper arrives at Venus. How computer animation transformed its radar imagery, enabling us to "view" the surface and "fly" the planet. • How Magellan's radar scans the planet, strip by strip. • A tour of the surface of Venus: impact craters, volcanoes with lava flows, evidence for volcanic activity over billions of years.

Inside Venus • How gravity mapping reveals the structure and composition beneath the surface. • Inside, Venus is rather like Earth, but without a molten outer core, so today there is no magnetic field.

Pressure Cooker • Venus orbiting within the habitable zone of the Solar System, but closer to the Sun than the Earth. • How Venus may once have resembled Earth. Then, billions of years ago, the oceans evaporate. • Carbon dioxide is released from the oceans, the atmosphere thickens, and heat is trapped near the surface. • A runaway greenhouse effect creates the hostile world we see today.

Background The Greenhouse Effect on Venus The great amount of the gas carbon dioxide in the dense cloudy atmosphere of Venus has led to what is called a "runaway greenhouse effect". Although the thick cloud layers keep out up to 80% of the Sun's rays, some visible sunlight does reach the surface, where it is absorbed by the rocks. This sunlight, however, is re-radiated as infrared radiation, and the carbon dioxide clouds do not let this through, trapping the heat near the planet's surface.

The effect is very similar to what happens in a greenhouse in the summer. The greenhouse glass lets through visible sunlight, but does not allow the infrared radiation, re-radiated by the plants and soil, to escape. Heat is trapped inside. Temperatures soar - hence the term "greenhouse effect".

The runaway greenhouse effect on Venus has caused the planet's surface temperature to rise to nearly 480 degrees Celsius. Venus is, therefore, the hottest planet, even though Mercury is closer to the Sun.

Measuring Distances in the Solar System One way to calculate the average distance between the Earth and the Sun is for a number of people in different parts of the world to observe and measure a transit of the planet Venus. A transit is when the planet passes exactly between Earth and the Sun, appearing in silhouette as a black dot crossing the brilliant face of the Sun.

The British explorer Captain James Cook led one of many expeditions to observe the transit of Venus in 1769. His group studied the event from the island of Tahiti in the Pacific Ocean. Calculations made from Cook's observations of the transit - and many others elsewhere - enabled astronomers to determine the average distance between Earth and the Sun. This measurement is known as the "astronomical unit", and is approximately 150 million kilometers. Knowing the astronomical unit enabled astronomers to calculate the relative dimensions of the entire Solar System. Today, the length of the astronomical unit can be determined far more accurately by other methods.

Radar and the Strange Rotation of Venus Radar astronomy involves transmitting a beam of radio waves that are then bounced off a target - for example, the surface of Venus - and picked up by a radio receiver, usually the transmitter working in reverse. A great advantage of radar for investigating Venus is that radio waves can penetrate the dense clouds to reach the planet's surface very easily.

When astronomers first made radar contact with Venus, they were surprised. They discovered that Venus spins on its axis more slowly than any other planet. It turns once on its axis in 243 Earth days, and, even more strangely, Venus spins from east to west. Most of the other planets, including Earth, rotate from west to east. In the course of a day on Venus, the planet moves a considerable distance along its orbit around the Sun. For this reason its "day" - measured from one sunrise to the next (unfortunately not visible through the dense clouds) - is far less than its period of rotation. This "day" on Venus lasts 117 Earth days.

Links for Further Information Venera Mission to Venus site, containing basic description of Venus, information on personnel, launch/orbital information, and images from the Venera missions. http://nssdc.gsfc.nasa.gov/planetary/venera.html

Comprehensive Magellan page. Includes mission data, the geology of Venus, information on the Magellan craft, its radar system, science personnel and images. http://pds.jpl.nasa.gov/mveg/guide.html

A good Venus site with history of the planet, size and structure, missions to the planet, its atmosphere and retrograde rotation, plus other links to Venus. http://seds.lpl.arizona.edu/nineplanets/nineplanets/venus.html

Impressive site for Venus images and movies. http://seds.lpl.arizona.edu/nineplanets/nineplanets/pxvenus.html

Another good Venus site, featuring significant dates for Venus, images of Venus, surface information, Mariner 2 background and general information. http://www.acim.usl.edu/SOLAR/venus/venus.html

The Magellan spacecraft home page, including links to numerous images, technical data, and animations. http://www.jpl.nasa.gov/magellan/

Questions and Activities for the Curious 1. As viewed from Earth, the apparent brightness of Venus changes as it moves along its orbit. Describe the main factors which determine the variations in the planet's brightness as seen from Earth.

2. Captain Cook led an expedition to Tahiti to observe the transit of Venus in 1769. Imagine that you were a member of this expedition, and describe your experiences on the voyage.

3. What two main reasons explain the lack of large numbers of impact craters on the surface of Venus?

4. How might the activity of the volcanoes on Venus affect the atmosphere and conditions on the planet's surface?

5. Research information on the greenhouse effect. Why it is important for the future of the Earth's atmosphere.

6. Why is the greenhouse effect relevant to the reason why dogs should not be locked in a car, with all the windows closed, in hot weather?

7. Scientists believe there were once oceans on the surface of Venus. How would you account for the disappearance of all this water?

8. Venus is often referred to as Earth's twin sister. What properties do the two planets have in common? In what ways are they dissimilar?

THE COMPLETE COSMOS Chapter 12: Uranus and Neptune The outer giants. Uranus - a crazy tilt and a chaotic moon called Miranda. Neptune - tempests and a moon spurting geysers.

Outline Seventh planet from the Sun, Uranus is a gas giant far larger than Earth - but modest in size compared to Jupiter. Uranus has an 84-year orbit, a day of 17 hours 48 minutes, a strangely tilted axis, and a magnetic field that is offset by 60 degrees to the rotation axis.

That Uranus has rings was discovered in 1977, when a star was observed winking several times before and after it passed behind the planet. In 1986, Voyager 2 made the first fly-by, but saw very little in the way of clouds and weather. Later, the Hubble Space Telescope revealed three hazy layers of atmosphere. A description of probable internal structure of Uranus.

Uranus has 17 moons - Miranda, Ariel, Umbriel, Oberon and Titania being the most important. Miranda, which has a chaotic surface and ice cliffs 15 kilometers high, may be the rough reassembly of a moon shattered in an ancient collision.

By observing the movement of Uranus, astronomers discovered that it was being perturbed by another, more distant planet. They correctly predicted its position - the planet now known as Neptune. Very slightly smaller than Uranus, Neptune has a 165-year orbit and a day of 16 hours seven minutes.

Compared to the almost featureless blur of Uranus, Neptune has plenty of clouds and weather. At the equator, winds roar at 2,000 kilometers an hour. The planet's atmosphere and internal structure are described.

Neptune also has rings and eight known moons. The largest moon, Triton, is icy and extremely cold at -238 degrees Celsius. Triton has geysers that shoot blackened nitrogen eight kilometers high. It may not always have been a moon - but a planetismal captured by Neptune.

Sub-chapters Strange Tilted Uranus • Spectacular fly-around of Uranus - its system of rings revealing the planet's strangely tilted axis. Close-up of the strange moon Miranda, the rough diamond of the Solar System. • The remoteness of Uranus, 19 times further from the Sun than Earth. • Discovery of the rings of Uranus when a star is seen to wink several times before and after it passes behind the planet. They emphasize the severe tilt of the planet - all of 98 degrees. Uranus orbits on its side. • Size with both Earth and Jupiter. Uranus is far larger than Earth but modest compared to Jupiter.

Magnetic Field and Atmosphere • In 1986, the Voyager spacecraft shows Uranus as an apparently featureless ball. • With infra-red, the Hubble Space Telescope reveals three hazy layers of atmosphere and hints of weather. • The magnetic field through Uranus is demonstrated by a moving arrow. The planet's magnetic axis is shown to be strangely tilted by 60 degrees to the rotational axis. On Earth this tilt is only 11 degrees. • Composition of the atmosphere of Uranus - mainly hydrogen, with helium and a touch of methane. • The planet's internal structure - layer by layer - down to the rocky core. The magnetic field is probably generated within the mantle of ices.

Weird Moons • Introduction to the most unusual moon of Uranus - Miranda - a chaotic world less than 500 kilometers across. • A massive impact shatters Miranda early in its life. Miranda reassembles, but some of the debris forms the rings of Uranus. • Miranda's ice cliffs, up to 15 kilometers high. • Close-ups of another four of the 17 known moons of Uranus: • Ariel, dull compared with Miranda, but twice the size, with canyons, faults and frost. • Umbriel, a little bigger than Ariel, but with an ancient surface pock-marked by craters. • Titania, the largest moon of Uranus, with a canyon comparable to the East African Rift Valley. • Finally Oberon, the outermost large moon. • The length of the day on Uranus, and the length of its year - 84 Earth years.

Neptune: Outermost Giant • Fly-out from Uranus until to Neptune - last of the gas giant planets. • The remoteness of Neptune, 4,500 million kilometers from the Sun, and length of its year - 165 Earth years. • Problems in predicting the position of Uranus lead to the discovery of Neptune. • Something is perturbing Uranus, pulling the planet out of position. Using these disturbances, astronomers discover a new planet - now called Neptune - in 1846. • Neptune compared in size with Uranus - very slightly larger - and with both Earth and Jupiter. Like Uranus, Neptune is far bigger than Earth but modest compared to Jupiter.

Turbulent Atmosphere • After the blandness of Uranus, Voyager finds plenty of weather on Neptune, driven by heat from within the planet. • Neptune's clouds, storms, and winds which roar at up to 2,000 kilometers an hour. • The Great Dark Spot, a tempest the size of Earth, seen by Voyager, but shown to have disappeared by the Hubble Space Telescope seven years later. • Length of the day on Neptune, composition of the planet's atmosphere – mainly hydrogen, with helium and a touch of methane. Internal structure - layer by layer – down to the rocky core.

Rings and Geysers • Neptune's two main rings are seen in close-up - plus their particles of icy dust. • The planet's eight moons, six of them discovered by Voyager, but focusing on the largest, Triton, which has the coldest known surface. • Erupting geysers on Triton spurting blackened nitrogen eight kilometers high. • Possibility that Triton is a captured planetesimal, and perhaps similar to Pluto, the tiny planet beyond.

Background Moons of Uranus The five largest moons of Uranus were known before the Voyager 2 spacecraft encountered the planet in 1986. In order of decreasing distance from the planet, the five are Oberon, Titania, Umbriel, Ariel and Miranda. The names, borrowed from English literature (the works of William Shakespeare and Alexander Pope), were suggested by John Herschel, son of the astronomer William Herschel. The names broke with the tradition of using names from Greek or Roman mythology.

Ten new, smaller Uranian moons were discovered by Voyager 2, all of them closer to the planet than Miranda. Their names (in order of decreasing distance from the planet) are Puck, Belinda, Cressida, Portia, Rosalind, Desdemona, Juliet, Bianca, Ophelia, and Cordelia. The naming followed the precedent set by John Herschel. More recently, two new more distant moons (as yet unnamed) have been discovered, bringing the total to 17. The two largest moons, Oberon and Titania, are each less than half the diameter of Earth's Moon. Both moons have average densities between 1.6 and 1.7 times that of water, surprisingly high compared to Saturn's icy moons. The pair are a mixture of ice and rock, but scientists had anticipated the bodies would be icier and, therefore, have lower densities.

Titania, the largest of the Uranian moons, has plenty of craters, many with bright rays. There are also complex rift valleys and fault lines hundreds of kilometers long. Smooth sections indicate where volcanic resurfacing has taken place. Titania's surface fractures may have been caused by the expansion of frozen water beneath the surface. Oberon, however, shows no signs of internal activity since visible rift valleys and fault lines are virtually absent from its heavily cratered surface.

Umbriel and Ariel are roughly three-quarters the size of Oberon and Titania. Umbriel is the darkest of the large Uranian moons. It has a reflectivity of just 19 per cent. Huge craters pock- mark its surface. Unlike the other large moons, Umbriel has a dark, relatively bland surface with a lack of bright, younger ray craters. The surface is probably extremely ancient. In contrast, the surface of Ariel, the brightest of the Uranian moons, has spectacularly deep rift valleys, together with broad, smooth valleys. Its surface certainly appeared "younger" than those of Oberon and Titania.

The smallest of the large Uranian moons, Miranda, has been described as the most bizarre body in the Solar System. It has a remarkable variety of surface terrain, with rolling, heavily cratered plains next to three huge, 200-300 kilometer oval-to-trapezoidal regions known as "coronae". They have networks of concentric canyons but the regions are less cratered than the plains. Miranda has grooves, craters, valleys and ice cliffs towering up to 15 kilometers. There is also a corona region shaped like a chevron and another nicknamed "the racetrack". Various theories offer explanations of Miranda's odd features. One is that at some time in the past the moon was impacted, shattered and haphazardly fell together again under the influence of its own gravity.

Strange Magnetic Fields Five days before its closest approach to the planet in January, 1986, Voyager 2 found that Uranus has a magnetic field. The extreme tilt of its rotation axis - 98 degrees - was already known, so that Uranus orbits on its side. The great surprise was the discovery that the planet's magnetic axis - an imaginary line joining its north and south magnetic poles - is tilted by 59 degrees with respect to the planet's rotation axis. This is by far the greatest offset of any of the planet. For example, Earth's magnetic axis is tilted by just over 11 degrees to its rotation axis, and Saturn has the smallest tilt of all, zero degrees. The inclination of the Uranian magnetic axis causes the magnetic field to wobble back and forth as the planet rotates. Another strange feature is that the magnetic field is not centered at the very core of Uranus, but is shifted by nearly one third of the planet's radius from its center. On Earth, this displacement is only eight hundredths of the planet's radius. The probably reason is that on Uranus, the magnetic field is generated within an electrically- conducting layer deep inside, but not at the core.

When Voyager 2 arrived at Neptune in 1989, it found a magnetic field almost as strange as that of Uranus. The tilt of Neptune's rotation axis is only about 29 degrees - far less than Uranus, and more like that of the Earth. Neptune's magnetic axis, however, is inclined by 47 degrees to the planet's rotation axis - only slightly less than Uranus. Even more remarkably, the magnetic axis is shifted by 55 per cent of the planet's radius from its center. Clearly, Neptune's magnetic field must again be generated within an electrically-conducting layer deep within, but located some way from the planet's core. Once again, the magnetic field wobbles back and forth as the planet rotates.

Triton - Neptune's Remarkable Moon Five hours after Voyager 2 had swooped above the north pole of Neptune in August, 1989, the craft made a close pass of the planet's largest moon, Triton. What surprises! Triton turned out to be smaller than anticipated, with a diameter of just over 2,700 kilometers - considerably smaller than our Moon.

Triton was also more reflective and much colder than expected, with a surface temperature of - 238 degrees Celsius. Its density is about twice that of water. The moon seems to be composed of about one-third ice and two-thirds rock. It has a very tenuous atmosphere consisting mainly of nitrogen, with traces of methane and carbon monoxide. The surface pressure is just 14 microbars - 70,000 times less than Earth's air at sea level. Winds at the surface blow at around 20 kilometers an hour, westward.

Triton's surface is greatly varied. It's coated with water ice - so cold it must be as hard as steel at such low temperatures - overlaid by methane and nitrogen ices. Triton is remarkably flat - with few normal craters and no mountains or deep valleys. The coloring of parts of the surface is surprisingly vivid. A large pinkish ice cap covers the south pole - resembling strawberry ice- cream in color and thought to consist of nitrogen snow and ice.

It is in this region that astronomers have identified nitrogen ice geysers. Presumably, Triton has a layer of liquid nitrogen under pressure a few tens of meters below the surface. Finding its way upwards through cracks in the crust, the material explodes through the surface as a shower of ice and gas. It spurts at up to 150 meters per second, rising to a height of several kilometers. It then blows downwind producing dark 80 kilometer streaks.

North of the pink polar ice cap, are two regions of equatorial terrain, with a well defined edge. The eastern region is partly covered by relatively smooth terrain, but also with hummocky and knobbly areas. There are low-walled plains as flat and smooth as frozen lakes. The western equatorial region is criss-crossed by fissures with central ridges. It resembles a melon skin - hence the name "cantaloupe terrain".

Triton, without doubt, is a remarkable world. Southern midsummer falls in about 2006, so major changes in the pinkish ice cap may be expected. So too, in the northern polar region which at the time of the Voyager encounter was plunged into winter. Unfortunately, no new space probes to Neptune are planned in the near future. Links for Further Information A page about the Voyager spacecraft and its mission, including images acquired at both Uranus and Neptune. http://www.star.le.ac.uk/edu/solar/voyager.html

Overall information on Uranus, with key facts, images and animations. http://www.star.le.ac.uk/edu/solar/uranus.html

Overall information about Neptune, with key facts, images and animations. http://www.star.le.ac.uk/edu/solar/neptune.html

All that we have learned so far about Uranus, presented in a simple and easy to understand way. http://www.crystalinks.com/uranus.html

A page with many links leading to various sites relating to the planet Neptune. http://www.astro.ku.dk/~lars_c/tbp/solar/eng/neptune.htm

Questions and Activities for the Curious 1. If Uranus and Neptune are both represented by the size of tennis balls, how big would Jupiter be on the same scale? How big would the Sun be, and how far would Uranus and Neptune be from the Sun using this scale?

2. Describe the technique that was used to discover the rings of Uranus in 1977.

3. Discuss the similarities and differences between the five largest moons of Uranus.

4. The moons of Uranus are named after characters in plays by William Shakespeare and Alexander Pope. For each of the moons, identify the play from which the name comes. If two new moons were discovered, suggest names for these moons, and say why you chose them.

5. Research and explain why the planet Uranus was originally called "Georgium Sidus" by the man who discovered it, William Herschel.

6. Both John Couch Adams, in England, and Urbain-Jean-Joseph Le Verrier, in France, predicted the existence of an eighth planet beyond Uranus. Research what you can about this astronomical detective story.

7. Compare the visible appearance, atmospheres and internal structures of the planets Uranus and Neptune.

8. Describe the axial tilts and magnetic fields of Uranus and Neptune, and the similarities and differences between them.

THE COMPLETE COSMOS Chapter 12: Space Frontier Human space exploration - from Yuri Gagarin's first orbit of Earth, to the race for the Moon, and the Apollo landings.

Outline The evolution of rockets - from ancient China to the early 20th century. Robert Goddard's pioneering work on liquid-fuelled rockets and the development of German military rockets during World War Two.

The initially successful Soviet space program. While the US stumbles, the Soviets put the Sputnik 1 satellite and several dogs into orbit. America succeeds in 1961 as Ham, the chimpanzee, circles the Earth. The same year, Yuri Gagarin becomes the first man in space. Two years later, Valentina Tereshkova is the first woman. In 1965, Alexei Leonov is the first cosmonaut to walk in space.

The three stages of America's race for the Moon. Phase 1 - Mercury: Alan Shepherd's sub-orbital hop and John Glenn's flight into orbit. Phase 2 - Gemini: Edward White is the first American to spacewalk and Gemini craft achieve orbital docking with a rocket upper-stage. Phase 3 - Apollo: three astronauts die during launchpad training and Apollo 8 loops around the Moon. In 1969, Neil Armstrong, Buzz Aldrin and Michael Collins journey to the Moon aboard Apollo 11. The first manned lunar landing.

Despite an explosion aboard Apollo 13 in 1970, James Lovell, Jack Swigert and Fred Haise limp safely back to Earth. Recap of the six Moon landings - including the final mission, Apollo 17, the first to carry a scientist. Cancellation of future lunar landings and, in 1975, the first link-up in Earth orbit between a Soviet and American spacecraft.

Sub-chapters Dawn of the Space Age • Invention of the rocket in ancient China. • Physicist Robert Goddard's work on the liquid-fuelled rocket, a prototype for space. • World War Two - the Germans develop powerful military rockets under the leadership of Werner von Braun. • The Soviets are the first in space. Their Sputnik 1 satellite is launched in October 1957. • One month later, a dog, called Laika, travels into space aboard Sputnik 2. • America's space program is initially unsuccessful. Its Vanguard project results in five out of six rockets exploding during lift-off. • In 1961 - an American success. Ham, the chimpanzee, orbits Earth.

First Men in Space • April 12, 1961: Yuri Gagarin becomes the first man in space on the spacecraft Vostok1. • In 1963, cotton mill worker Valentina Tereshkova becomes the first woman in space. • Alexei Leonov is the first person to leave his spacecraft and "spacewalk". • President John F. Kennedy pledges to land a man on the Moon before the end of the 1960s. There are three phases. • Phase 1 - the Mercury project. Alan Shepherd's sub-orbital hop in a Mercury capsule. • February 20,1962: John Glenn is the first American into space aboard Friendship 7. • After three Earth orbits, a loose heat shield makes re-entry a rocky ride, but Glenn returns safely to Earth.

Preparing for the Moon • Phase 2 - the Gemini project. Edward White spacewalks in June 1965. • Two Gemini craft rendezvous in space, and later Gemini craft successfully dock with a rocket upper stage. • Phase 3 - the Apollo project. On January 27, 1967, a fire in the Apollo 1 capsule kills Virgil Grissom, Edward White and Roger Chaffee during launchpad training. • In December 1968, Apollo 8 loops around the Moon.

Apollo 11 • Early 1969: extensive tests on lunar spacesuits and landing modules. • July 20, 1969: Armstrong and Aldrin make the historic first lunar landing while Collins stays in the command module circling the Moon.

Near Disaster ! • 1970 - an explosion aboard Apollo 13 cripples the craft. • After limping around the Moon, using the lunar module as a "life-raft", astronauts Jim Lovell, Jack Swigert and Fred Haise are safely returned to Earth.

From Competition to Cooperation • The final lunar landing, Apollo 17, is the first to carry a scientist, Harrison "Jack" Schmidt. He is able to cover relatively long distances in the lunar rover. • Subsequent Moon landings are cancelled after interest wanes. • July 17, 1975: an Apollo vehicle and a Soyuz craft link-up in Earth orbit. The launch of Soyuz is the first televised Soviet rocket launch.

Background Rocket Propulsion Unlike an aircraft, a rocket can move through space without the help of air. Like a firework, a rocket works on the principle of reaction. A typical firework consists of a hollow tube filled with gunpowder and sealed at one end. When the gunpowder is ignited, it burns very quickly. This produces a lot of hot gas that gushes from the open end. It is the action of the gas pushing in one direction that thrusts the nose of the rocket in the other. With sufficient thrust the firework lifts into the air. When the gunpowder is exhausted, the firework falls back to Earth. Gunpowder cannot be used as fuel for a space rocket because it burns too quickly.

Gunpowder is a solid fuel and a firework is a solid-fuelled rocket. Most space-rockets, however, use liquid fuels. Two liquids - such as hydrogen and oxygen (cooled to very low temperatures) - are carried in separate tanks. The liquids are pumped through pipes into a chamber. When they mix they react violently together. This produces enormous quantities of hot gases that rush out of the nozzle at the base of the rocket, propelling it upwards.

Why are liquid fuels better than solid-fuels? Firstly, because liquid fuels burn at a steady rate for much longer than solid fuels, thus producing more thrust. Secondly, liquid-fuelled rockets are safer than solid-fuelled vehicles. Once lit, a solid-fuelled rocket will burn until all the fuel is exhausted. But in a liquid-fuelled rocket, the mixing of the two liquid fuels is more easily and safely controlled.

The thrust from one rocket is insufficient for it to escape the pull of Earth's gravity. This is overcome by using two or more rockets. They are placed one on top of the other - and called a multi-stage rocket. The huge first stage provides enough thrust to lift the whole rocket off the ground. With its fuel is spent, the first stage is jettisoned and falls back to Earth. As a result, the on-going vehicle is spared unnecessary weight. The next stage, or stages, fire up and provide the thrust required to take the rest of the rocket into space.

Three stages were used for the giant American Saturn 5 rocket that launched astronauts to the Moon. The Russians have a different method - using a central basic rocket with pairs of strap-on boosters at the side.

To the Moon and Back In 1961, President John F. Kennedy set American scientists the challenge of landing men on the Moon before 1970. The project was called Apollo. Altogether, 12 astronauts were successfully landed on the lunar surface, the first in July 1969, and the last in December 1972.

To get three astronauts per mission to the Moon required a very large and a very powerful rocket. These are the vital statistics of the giant Saturn 5 rocket that did the job. It stood over 110 meters tall on the launch pad - as high as a 40-storey tower block. All that came back to Earth was the tiny cone-shaped Command Module (CM) containing the crew. On top of the Saturn 5 rocket was the Apollo spacecraft, 25 meters tall and consisting of several components. The Spacecraft Lunar Module Adaptor (SLMA) linked the Apollo spacecraft to the rocket during lift-off. The SLMA surrounded and protected the Lunar Module (LM). The CM was attached to the Service Module (SM), a cylinder 7.5 meters across with its own rocket motor and fuel tanks. Together they were called CSM. After launch, the Apollo spacecraft and the third stage of the rocket entered Earth orbit. Then the motors of the third stage were fired, propelling the astronauts toward the Moon. Next, the CSM and the SLMA separated and the CSM was turned around to dock with the LM. The CSM and LM then headed towards the Moon.

Once in lunar orbit, one astronaut stayed inside the CSM, while the other two entered the LM for the descent to the Moon. The LM had two parts - one was the descent stage and the other the ascent stage. The main engine on the descent stage took the LM gently to the lunar surface. The LM landed on four legs, each with a large foot-pad. After a brief and cautious exploration, the two astronauts returned to the LM. Fitted with its own engine, the ascent stage of the LM blasted off, using the descent stage as a launch pad. The ascent of the LM took it back to the CSM, waiting in orbit above. The CSM and LM then docked and the astronauts crawled back through the tunnel linking the two spacecraft. The LM was then released to crash into the Moon.

The three astronauts returned to Earth in the CSM. Shortly before re-entry, the SM dropped away and burned up. The CM was protected by its heat shield from the fiery heat of re-entry caused by air resistance. Parachutes opened and the CM splashed down gently in the ocean. Waiting ships retrieved the CM and the crew.

Soviet "Firsts" in Space The Soviets launched the Space Age by sending the world's first artificial satellite, Sputnik 1, into Earth orbit on October 4 1957. The early years of space exploration were dominated by the former Soviet Union. The Americans were left far behind. Less than a month after Sputnik 1, on November 3, 1957, came the launch of Sputnik 2. A dog called Laika, became the first space passenger. She perished - but two other dogs, Belka and Strelka, were subsequently returned safely to Earth. At last, on February 1, 1959, the United States got into space with the Explorer 1 satellite. By January, 1961, the US had successfully sent several monkeys and a chimpanzee called Ham into orbit.

The Soviets forged ahead again by putting the first man into space. On April 12, 1961, Yuri Gagarin hurtled into Earth-orbit aboard Vostok 1. Then, on August 6, 1961, Gherman Titov became the first person to spend a full day in space aboard Vostok 2 . He was also first to experience space sickness. On February 20, 1962, more than ten months after the Soviets, the Americans launched their first man into space, John Glenn. But the Soviets pulled ahead again when, on August 11 and 12, 1962, they achieved the first space rendezvous when Vostoks 3 and 4 approached to within a few kilometers of each other.

Yet another Soviet first followed on June 16, 1963, with the first woman in space. Valentina Tereshkova flew aboard Vostok 6, rendezvousing with Vostok 5, again at a distance of a few kilometers. With Voskhod 1, the Soviets launched the first three-man spacecraft on October 12, 1964. They also had the first spacewalker, Alexei Leonov. On March 18, 1965, he took an orbital stroll outside his craft, Voskhod 2. After this, the Soviets seemed to lose some of their impetus.

Meanwhile, the US made strides with its two-man Gemini program. Successes included the first American spacewalk (or extra-vehicular activity) by astronaut Edward White and a series of orbital rendezvous and docking maneuvers. The Apollo program, which would eventually send men to the Moon, experienced early disaster when the three-man crew of Apollo 1 was killed in a fire on the launch-pad. As a consequence, there were many safety improvements. The Soviets were also unfortunate when the parachutes on the Soyuz 1 capsule failed to open. The craft crash- landed, killing lone cosmonaut Vladimir Komarov.

But the Soviet program to put men the Moon never really took off. After the successful American landings, the Soviets significantly changed emphasis. While the US concentrated on the development of Space Shuttle, the Soviets launched a space station, Salyut 1, in April 1971 - another first. Nearly two years later, the American followed with their own space station, Skylab. The Soviets launched a whole series of manned space stations, culminating in the highly successful Mir, aboard which numerous space endurance records were set.

Links for Further Information Detailed page on the history of NASA - from the Soviet launch of Sputnik 1 to the Apollo 11 lunar landing. Contains images and extensive links. http://www.cob.montevallo.edu/student/SettleSL/NASA.htm

Overview of Soyuz missions, including key events, personnel and launch dates. Links to Vostok, Voskhod and Salyut pages. http://nauts.com/histpace/vehiclesNT/histsoyuzNT.html

Excellent page covering all the Apollo missions. Includes various aspects of all the flights plus images. http://www.ksc.nasa.gov/mirrors/images/html/apollo.htm

Good page covering the Gemini missions. Presented in the same format as the Apollo page. http://www.ksc.nasa.gov/mirrors/images/html/gemini.htm

The history of rocketry from China to Robert Goddard. Informative page charting the evolution of the rocket through history. http://www.namesinspace.com/history/june.june.html

Brief history of Werner von Braun's role in the development of American rockets. http://www.spacevoyages.com/visions2.html

Questions and Activities for the Curious 1. Research the German V2 rocket developed in World War Two. Explain why it was a prototype for space travel.

2. Investigate the early "animal astronauts" with particular reference to the dogs, monkeys and chimpanzees.

3. Imagine you are Yuri Gagarin and are about to become the first man in space. Describe your hopes and fears in the hours leading up to blast-off.

4. Summarize the many successes of the early Soviet space program between 1957 and 1965.

5. Imagine you are a journalist. Write a short news report for your paper about the landing of the first men on the Moon.

6. Describe the difficulties faced by the crew of Apollo 13 after an explosion crippled their spacecraft. How were the problems overcome and how was the crew rescued?

7. Write a brief description of each of the Apollo missions, beginning with Apollo 7 and ending with the flight of Apollo 18 - the Apollo-Soyuz link-up.

8. Do you think that the rivalry between the Americans and Soviets during the "Space Race" was beneficial or detrimental to progress in space exploration?

THE COMPLETE COSMOS Program 8: Saturn The many rings and moons of this exotic gas giant. Preview of a landing on Titan, a moon like primitive Earth.

After Jupiter, Saturn is the largest planet in the Solar System. Nearly ten times farther from the Sun than Earth, Saturn has a ten-and-quarter-hour day and its orbit of the Sun takes almost 30 years.

Although 750 times larger than Earth, Saturn is so buoyant that it would float in water. An explanation of Saturn's composition, atmospheric features, internal structure, magnetic field and aurorae. Saturn's rings - although spanning the distance between the Earth and Moon - are less than a kilometer thick. Viewed edge-on from Earth, they all but disappear. Comprising billions of moonlets, from specks of dust to icy boulders the size of trucks, the rings may well be pulverized space rocks. Voyager space probes reveal the complexity of the rings - ringlets, divisions, mysterious spokes and shepherd moons.

A tour of some of Saturn's 18 known moons. Most exotic is the largest Saturnian satellite, Titan. Discovered in 1655 by the Dutch astronomer Huygens, Titan is like a frozen infant Earth, its surface entirely shrouded by smog. In 2004, the Cassini spacecraft will reach Saturn, deploying a lander to investigate Titan's atmosphere and surface. Will there be oceans of liquid hydrocarbons? Is Titan a laboratory for life?

Program Segments Ringed Planet • Lift-off for Cassini, a one-billion-dollar space mission to explore the ringed planet. • Saturn's position in the Solar System - an animated journey from Earth, past Mars and Jupiter. • Saturn's orbit, daily rotation, and size comparison with Jupiter. • A buoyant gas giant that would float in water. • Saturn swallowing Earth nearly 750 times.

Atmosphere & Magnetic Field • Saturn's composition - 94 per cent hydrogen, the rest mainly helium. • Belts of weather, obscured by haze. Winds roaring at 1,500 kilometers per hour. • Saturn's atmosphere, layer-by-layer - and internal structure, down to the rocky core. • A magnetic field - like Jupiter's, generated in a layer of metallic hydrogen. • Auroral haloes at the poles, created by the solar wind.

Rings in Close-Up • Fly-through of the rings from afar. In close-up, they're revealed as billions of rocks of widely different sizes. • Saturn and its system of rings that would fit between Earth and the orbit of the Moon. • From Earth, our view of the rings changes as Saturn orbits the Sun - because of the planet's axial tilt. Sometimes the rings appear wide open. At other times, they're edge-on and virtually disappear because they are so thin. • The enormous complexity of the rings as revealed by the Voyager spacecraft. • The Cassini Division.

Small Icy Moons • Shepherd moons marshalling the ring particles - and a theory of ring formation. • Voyager timelapse of Saturn and its moons. A selection of the most diverse: • Tiny Mimas, made of ice and rock, scarred by a huge impact. • Enceladus, nearly twice the size of Mimas - icy fountains may spurt as Enceladus is flexed by the pull off its neighbors.

Larger Moons • Tethys, with a great canyon scouring its icy surface. • Dione, one hemisphere brighter than the other - a world of craters and troughs. • Rhea, larger still, and pocked by numerous impacts. • Iapetus, one hemisphere snow white, the other coal black. • Tiny Hyperion, shaped like a hamburger.

Hail Cassini! • Titan, Saturn's biggest moon, larger than the planet Mercury. Carbon-based compounds may be present, but Titan is hidden beneath an impenetrable veil of smog. • The Cassini craft due to arrive in 2004 - deploying a lander named Huygens that descends through the Titan atmosphere. • Touchdown of Huygens - maybe a splashdown if there are oceans of hydrocarbons there. • Data is flashed back to Earth via Cassini's orbiter.

Background Saturn's Weather and Winds In many ways, Saturn looks Jupiter. There are alternating bands of dark and light clouds - called belts and zones, respectively. These bands lack the colorful contrast of Jupiter's. After computer enhancement, however, images acquired by the Voyager spacecraft revealed features in the clouds such as storm systems. Voyager also spotted ovals of white, brown and red. White clouds are composed of ammonia particles but scientists are baffled why other clouds are brown, blue and red. They may be colored by other chemicals, organic compounds, or complex reactions in the atmosphere.

The different appearances of Jupiter and Saturn are linked to their different masses. Saturn is less than one-third as massive as Jupiter. Jupiter's powerful "surface" gravity - the result of the planet's much greater mass - compresses its main cloud layers to a depth of just 75 kilometers. Saturn's relatively weaker gravity causes less compression, so its main cloud layers increase to a depth of nearly 300 kilometers. Consequently, the colors of Saturn’s cloud belts are less dramatic than those on Jupiter because they are partly obscured by the deep haze layers lying above.

By following various features in the cloud belts of Saturn, scientists have analyzed the planet's winds. Like Jupiter, Saturn has counterflowing westward-moving and eastward moving currents - causing strongly zonal weather systems. At Saturn's equator, the cloud top winds reach 500 meters per second (1,800 kilometers per hour), which makes them, at this level, approximately two-thirds the speed of sound! Saturn, like Jupiter, has an internal heat source. Both planets radiating more energy than they receive from the Sun.

The Rings of Saturn Saturn is perhaps best known for its magnificent system of rings. Astronomers have discovered that all four gas giants (Jupiter, Saturn, Uranus and Neptune) have ring systems. Saturn's are by far the brightest and best known.

The bright rings are 273,000 kilometers in diameter - rather less than the distance between Earth and the Moon. They rings, however, are remarkably thin - probably only a few hundred meters deep and, in some places, no more than 30 meters. How did they form?

Either by the break-up of one of Saturn's moons which strayed too close to the planet and was torn apart by tidal forces. Or by the failure of small fragments of icy material to accrete into a moon-sized body.

Three major rings are visible from Earth. The outer A ring is separated from the brighter B ring by the 4,000-kilometre wide Cassini Division. The division, surprisingly, is not empty but simply has a lower density of fragments. Another division is in the outer half of the A ring. Named the Encke Division, it is only about 270 kilometers wide. Within the B ring is the faint, semi- transparent C ring, also known as the Crepe Ring.

When the Voyager spacecraft flew past Saturn, they showed that the three major rings were subdivided into thousands of ringlets - with many, many gaps. The Voyagers also discovered two more rings - G and E - much farther from Saturn. They were quite tenuous and extended towards the planet's moon system. Pioneer 11 detected the very narrow F ring just beyond the outer edge of the A ring.

The rings are not solid, rigid sheets of matter. They consist of myriads of tiny moonlets, ranging in size from tiny dust grains to icy boulders tens of meters across, all moving along their own individual orbits around Saturn. Observations have shown that frozen water is definitely present in the ring particles. At temperatures of between -180 and -200 degrees Celsius, this water ice is in no danger of evaporating.

Shepherd Moons The Voyagers showed that the particles in Saturn's F ring are confined to a well defined, narrow band, no more than 100 kilometers in width. Voyager cameras also revealed that two tiny satellites, each measuring about 50 kilometers across, orbit Saturn on either side of the F ring. It is the gravitational influence of these two moons which keeps the ring particles in position. The inner satellite, Prometheus, moves around Saturn at a slightly faster speed than icy particles at the inner edge of F ring. As this satellite overtakes the ring particles, its gravitational pull tends to speed them up, nudging them into orbits a little farther from Saturn, and back into the narrow central region of the F ring.

The outer satellite, Pandora, moves around Saturn at a somewhat slower speed than particles at the outer edge of the F ring, so as they pass by the satellite they experience a tiny gravitational tug that tends to slow them down. This causes them to move into orbits a little closer to Saturn, and back into the main part of the F ring. The combined effect of the two moons confines the F ring particles into the narrow, well-defined band that we observe. For obvious reasons, the two tiny satellites, Prometheus and Pandora, are called "shepherd moons".

Another shepherd satellite, Atlas, orbits Saturn just beyond the A ring's outer edge. It is the influence of this moon that is responsible for the sharp border to the ring. Towards the outer edge of ring A, particles overtake the slower moving Atlas. In doing so, they experience a gravitational tug that slows them down very slightly, preventing them from wandering into orbits farther from Saturn.

Links for Further Information: Cassini Home Page. Information on Saturn, its rings and moons, the spacecraft, the mission, facts and images and a downloadable teachers' guide. http://www.jpl.nasa.gov/cassini/

Information on Saturn, internal and external features, atmosphere and magnetosphere, moons and rings, facts and myths, missions, news and images. http://www.windows.umich.edu/cgi-bin/ tour.cgi?link=/saturn/saturn.html

Animation images of Saturn, its features, moons and rings, plus text. Cassini information and images. http://www.jpl.nasa.gov/cassini/Images/slides/slidetop.html

Exploring the planets - Saturn. View from Earth, atmosphere, magnetosphere, rings, moons, future exploration, past missions, images and links. http://ceps.nasm.edu:2020/ETP/SATURN/etpsaturn.html

Saturn's ring-plane crossing of 1995 -1996. Images of Saturn's rings as viewed from Earth. Links to Saturn's rings, moons, science background and images. http://newproducts.jpl.nasa.gov/saturn/

Questions and Activities for the Curious: 1. If Earth were the size of tennis ball, how big would Saturn be on the same scale? What would be the diameter of its rings? How big would the Sun be, and how would Saturn be from the Sun on this scale?

2. If there were an ocean big enough, Saturn would float - whereas Jupiter would sink. Explain why this is so.

3. Why do features in Saturn's atmosphere appear to be much fainter and "washed out" compared to the features in Jupiter's atmosphere?

4. Discuss the various layers that make up Saturn's atmosphere and its internal structure. Where is its magnetic field produced?

5. Imagine you are in a spacecraft skimming across Saturn's rings and heading towards the planet. Describe what you would see.

6. During the almost 30 years that it takes Saturn to orbit the Sun, the appearance of its rings as seen from Earth changes. Describe these changes and why they happen.

7. Research and report on the similarities and differences between the known moons of Saturn. Explain how "shepherd moons" work.

8. Describe the appearance of Saturn's largest moon Titan, the composition of its atmosphere, and the probable conditions on its surface.

THE COMPLETE COSMOS Chapter 14: Robots Our scouts in the Solar System. Probes that trail-blaze on Mars, plunge into Jupiter, and land on Saturn's moon Titan.

Outline Montage of the robot probes whose mission is to explore the Solar System. One such is Mars Surveyor Orbiter - programmed to use aero-braking to alter orbit around the Red Planet. Mission objectives.

Past missions - the heroic journeys and discoveries of Voyagers 1 and 2. They looked at Jupiter, Saturn, Uranus and Neptune. The odyssey of the Ulysses probe, gravity assisted by Jupiter, over the poles of the Sun. How Earth helped slingshot the Galileo craft to Jupiter. Galileo's tour of Jupiter and its moons. A descent into the Jovian atmosphere. The triumph of the Mars Pathfinder mission - the landing on an ancient Martian flood plain. Pathfinder movies of its Sojourner rover. How Pathfinder imaged Mars in 3-D. Missions that go wrong. The explosion, shortly after lift-off, of the Ariane 5 rocket - a maiden flight that destroyed the four robot probes aboard. Russia's Mars '96 spacecraft that never got beyond Earth-orbit. The craft was supposed to fire penetrators into the Martian soil and analyze the sub-surface.

The giant radio dishes of NASA's Deep Space Network which pick up the transmissions of robot explorers and send commands to guide them through space.

Sub-chapters Lift-Off for Mars • An array of robot probes, which have visited, or are due to visit the other planets of the Solar System. • Launch, on a Delta rocket, of Mars Surveyor Orbiter - shedding its nose cone and propulsion units, unfurling its solar array. • Slipping into an elliptical orbit, the spacecraft has to achieve a close, circular orbit to conduct its survey of the Red Planet. To do so, it must use its solar array as an "aerobrake" in the upper atmosphere.

Mars Surveyors • Mission objectives of Mars Surveyor, part of a ten-year program.

The Voyagers • Voyagers 1 and 2, both launched in 1977. They pass Jupiter in 1979 - with Voyager 1 arriving at Saturn in 1980 and Voyager 2 in 1981. • Saturn and its rings imaged by the Voyagers - including spectacular time-lapse of a Saturn approach. Saturn is the climax for Voyager 1 and it exits the Solar System. • Voyager 2 continues on to Uranus, collecting data on both the planet and its craggy little moon, Miranda. Images are converted into an animated flight over Miranda's surface. • 1989: Voyager 2 finally arrives at Neptune. Time-lapse of the south pole, an arc over the north pole. Then Voyager 2 heads for interstellar space.

To the Sun and Jupiter • 1990: An ingenious route to the Sun by the spaceprobe Ulysses. By traveling first to Jupiter and using it as a gravity-assist to catapult downwards and backwards, Ulysses achieves the trajectory required to view and image both poles of the Sun. • 1989: Again using gravity assist, the Galileo spacecraft swings by Venus once and Earth twice, to gather sufficient energy for the journey to Jupiter. • Galileo parachutes a probe through Jupiter's clouds. Before it is crushed by pressure, the probe gathers fascinating information on atmospheric composition and conditions. • Galileo loops round Jupiter and its moons.

Mars Pathfinder • July 4, 1997: Mars Pathfinder touches down on Mars. • A Martian sunrise and Pathfinder is activated. It transmits to Earth 16,500 images and two-and-a-half-billion bits of information. • Pathfinder's stereo imaging camera equipment. Light enters the lens and passes through three filters to produce a color image. This occurs again with a second lens. • Two images of the same scene are produced. When merged, Mars is seen in 3-D for the first time. • Pathfinder time-lapse of the Sojourner rover negotiating the Martian terrain and colliding with the large rock dubbed Yogi.

Space Mishaps • Ariane 5 explodes shortly after lift-off. Aboard, four robot probes are destroyed - the Cluster mission that should have investigated the solar wind. • Russia's Mars '96 spacecraft carries penetrators designed to fire into the Martian soil to examine the sub-surface. Rocket failure means the craft never leaves Earth orbit. • On Earth, giant radio dishes listen to the transmissions of journeying robots. In return, commands are beamed through space to tell the robots what to do. • Robots allow us to see worlds we will never experience at first hand.

Background Gaining Speed by Gravity Assist In1961, Michael Minovitch, a 25-year-old graduate in mathematics, was hired as a summer employee at NASA's Jet Propulsion Laboratory (JPL). Minovitch wondered if a planet's gravity could be used to provide a "kick" to a passing spacecraft. In doing so, he created a revolution in the design of interplanetary space missions.

Minovitch's notion was that a carefully aimed spacecraft could pick up momentum from the "gravity-assist" of one planet in order to travel on to a second planet. Indeed, a further boost could be obtained from the second planet to take the craft to a third – and so on.

The only energy required would be to launch the craft from Earth to the first planet. All subsequent planets were, so to speak, a free ride. As an added bonus, due to the gains in speed, the travel times to each of the planets beyond the first would be significantly reduced.

To achieve gravity-assist, is a precise business. As it closes in, a spacecraft will pass either the "trailing" or "leading" hemisphere of a planet. Such a close encounter causes two effects. Firstly, the spacecraft's path is bent. Secondly, the spacecraft either gains or loses energy - i.e. speed.

Bending occurs regardless of whether the spacecraft passes the leading or the trailing hemisphere. The direction of bending is determined by scientists choosing the appropriate hemisphere. The amount of bending is controlled by how closely the craft approaches the planet. The bending of the flight path occurs both with respect to the planet and with respect to the Sun. There is no net change in speed, however, with respect to the planet. The spacecraft is in continual free-fall with respect to the planet. Its final speed – far after approach - is exactly the same as its initial speed - far before approach – with respect to the planet.

But, with respect to the Sun, it's a different story. The spacecraft's velocity relative to the Sun is always equal to the spacecraft's velocity relative to the assisting planet, together with the planet's velocity relative to the Sun.

From the point of view of the Sun, imagine an outward-bound spacecraft passing the trailing edge of a planet. As it approaches the planet, the craft's velocity is less than when it leaves. In other words, there is a net increase in the speed of the outward-bound spacecraft - and a net slowing down of the planet. Energy has been transferred from the planet to the spacecraft.

On the other hand, if an outward-bound spacecraft swings by the leading edge of a planet - from the point of view of the Sun - the roles are reversed. The spacecraft slows down and the planet speeds up. Energy has been transferred from the spacecraft to the planet.

These principles also apply to gravity-assist from the big moons of the Solar System – a trick brilliantly exploited by the Galileo spacecraft in its orbital tour of Jupiter's satellite system.

When they flew by Jupiter, Saturn and Uranus, the Voyager 1 and Voyager 2 spacecraft picked up speed from the trailing hemispheres of the planets. The Voyages gained speed at the expense of the planets. In precise terms, when Voyager 1 passed Jupiter, the craft gained 16 kilometers per second relative to the Sun - and the planet lost one centimeter per 30 billion years relative to the Sun, causing Jupiter's orbital period to shrink by nearly one nanosecond.

Early Uses of Gravity-Assist The first application of gravity-assist was in Mariner 10's mission to Mercury via Venus. Mariner 10 launched from Earth in 1973 and traveled directly to Venus. In February 1974, Venus bent and boosted Mariner's trajectory to Mercury. Then, in March/April 1974, as it swung by Mercury, Mariner received a another gravity-assist. This enabled the craft to encounter Mercury a second time - in September 1974. And guess what? Mercury delivered another gravity-assist that allowed a third and final Mercury encounter in March 1975.

The Pioneer 11 mission was the second application of gravity-assist. Pioneer was originally intended to encounter only Jupiter - in 1974 - as a trailblazer for the subsequent Voyager 1 and Voyager 2 missions. As it turned out, there was an opportunity for Pioneer to get a gravity-assist from Jupiter for an onward trip to Saturn. Pioneer's "bend" was almost 180 degrees - causing the spacecraft to travel all the way back across the inner Solar System to pass closely by Saturn five years later, in 1979!

The Grand Tour of the Outer Planets Michael Minovitch (see above) realized that the powerful gravity of Jupiter was the key to outer planet exploration. As the largest planet, Jupiter had the strongest gravity field. It was suddenly possible, Minovitch believed, to explore Saturn, Uranus, Neptune and Pluto by using the gravity- assist of Jupiter to slingshot spacecraft speedily to the outer planets. Minovitch identified windows of opportunity between 1962 and 1966 and between 1976 until at least 1980. He suggested a 1976 launch for a "Grand Tour" of the outer planets.

In 1965, Gary Flandro - then at NASA's Jet Propulsion Laboratory (JPL) - designed a set of Grand Tour trajectories using the gravity-assist concept. He included an example of an Earth- Jupiter-Saturn-Uranus-Neptune mission. Flandro pointed out that these planets align themselves for such a mission only once every 176 years or so. The next set of Earth-launch opportunities would occur in 1976, 1977 and 1978. Thus came the impetus for what ultimately became the Voyager Project, including Voyager 2's Grand Tour of the outer planets . Between 1974 and 1976, scientists at JPL evaluated the merits of over ten thousand Voyager spacecraft trajectories. The objective was to maximize the knowledge that could be gleaned from the Jovian and Saturnian systems. Of primary interest were Jupiter's moon Io and Saturn's moon Titan. The Voyager 1 and 2 trajectories must, therefore, have at least one close approach to each of the moons. Additionally, the best trajectories had to have the largest number of close fly-bys of the remaining Jovian and Saturnian moons.

When the Voyagers launched, their actual trajectories included two gravity swing-bys at Jupiter, two at Saturn, one at Uranus and one at Neptune.

Communicating with Spacecraft The antennae of NASA's Deep Space Network (DSN) are the link between unmanned spacecraft - robot explorers - and space scientists on Earth. DSN tracks the craft, DSN transmits or uplinks commands and information to the craft, and DSN receives or downlinks data from the craft. From NASA's Jet Propulsion Laboratory in California, commands to robots are routed via the Ground Communication Facility (GCF) to the appropriate DSN Deep Space Communications Complex (DSCC) for transmission to the spacecraft.

GCF uses a combination of communication satellites and conventional surface and undersea circuits to link JPL and the DSCCs. Three DSCCs - at Goldstone in California, Canberra in Australia and Madrid in Spain - are located at widely separated longitudes to provide continuous tracking of interplanetary craft as the Earth rotates. Each site is similarly equipped.

The Goldstone DSCC is in the heart of the Mojave Desert. It has three main antennae – a 34- metre diameter antenna which can both transmit and receive, a 70-metre antenna that can both transmit and receive, and a 34-metre diameter antenna which can only receive. More than one antenna can be used simultaneously to increase the strength of the signal coming from a spacecraft.

The Canberra DSCC is at Tidbinbilla, New South Wales. It has three main antennae – a 34-metre transmit/receive station, a 70-metre transmit/receive station, and a 34-metre receive-only station. The Madrid DSCC is at Robledo. It also has three main antennae - a 34-metre transmit/receive antenna, a 34-metre receive-only station, and a 70-metre transmit/receive antenna. Voice communication - in fact, a continuous phone call – is maintained between all three DSCCs and the Network Operations Control center at JPL. Despite its DSCC capability, NASA sometimes requires enhanced power. At the time of its Neptune encounter, the signals from Voyager 2 were so feeble that NASA needed giant ears to catch the spacecraft's news. When they reached Earth, Voyager's signals were a mere 0.0000000000000001 watts. A digital wrist watch operates at a power 20 billion times greater !

So in the US, NASA had to hook up with the Very Large Array in New Mexico - 27 dishes each measuring 27 meters, part of the National Radio Astronomy Observatory. And because Voyager's closest approaches to Neptune and Triton occurred when Australia was best positioned to hear the craft, the 64-metre Parkes Radio Observatory was roped in to help. Yet more tracking was provided by Japan's 64-metre radio observatory antenna at Usuda, Japan.

As Voyager 2 passed Neptune, signals from Earth - traveling at the speed of light – took over four hours to reach the craft. It took another four hours for Voyager's acknowledgement of their receipt to bounce back to Earth. Such a time lag greatly complicates spacecraft operations. Imagine steering a ship where navigational data and the views from the bridge are more than four hours old.

Links for Further Information Mars Surveyor Orbiter page - mission overview, objectives, launch phases, aero-braking and images. http://mars.jpl.nasa.gov/msp98/orbiter/mission.html

Mars Surveyor Lander page - mission overview, objectives, launch phases, landing scenario and images. http://mars.jpl.nasa.gov/msp98/lander/mission.html

Voyager home page - extensive images, daily and weekly reports, missions summaries for each planet. http://vraptor.jpl.nasa.gov/voyager/voyager.html

Ulysses mission page. Extensive information - the spacecraft, its trajectory, the Jupiter fly-by, mission overview, comet-watch program, scientific results and images. http://stardust.jpl.nasa.gov/comets/ulysses.html

Galileo home page. Full information on mission objectives, scientific firsts, results, press releases and images. http://nssdc.gsfc.nasa.gov/planetary/galileo.html

Mars Pathfinder page. Comprehensive image gallery with accompanying text - plus links to other images, animations and the Mars sunset movie. http://www.brandx.net/dbajot/mpf/

Questions and Activities for the Curious 1. What are the benefits of using robots, as opposed to humans, for space exploration?

2. Describe the highlights of the Voyager 1 and 2 missions to the outer planets.

3. Explain the principles of "gravity-assist" with an example of its use.

4. Outline the circuitous six-year flight of the Galileo spacecraft from Earth to Jupiter, identifying the various bodies it passed en route.

5. What characteristics are important in designing a robot vehicle to move about on the rock- strewn surface of Mars?

6. How might robots, developed for tasks in outer space, be used for our benefit here on Earth? 7. What are the problems of communicating with deep space probes?

8. Pluto, the most distant planet, is the only one yet to be explored by spacecraft. What problems might be encountered on such a mission? THE COMPLETE COSMOS Chapter 10: Realm of the Comets

Comets and where they originate - the Oort Cloud and the Kuiper Belt. Perhaps tiny Pluto isn't a planet at all.

Outline The painstaking technique used by Clyde Tombaugh to discover the outermost planet Pluto in 1930. Vital statistics of Pluto and its companion moon Charon - their combined mass (less than one-fifth of our Moon), icy composition, surface features and distance from the Sun.

But is Pluto a planet at all? Or did it originate in the Kuiper Belt which lies just beyond? Kuiper abounds with similar icy bodies - planetesimals and comets. More distant still is the Oort Cloud, another vast storehouse of comets, extending perhaps one-third of the way to the next nearest star.

The structure and nature of comets, including their icy nuclei. Magnificent two-color comet tails - yellow for dust and blue for gas. The behavior of comets, their highly elongated orbits around the Sun, and the perturbing effects of the Sun and Jupiter. Sun plungers, Sun-grazers and Shoemaker- Levy 9 which was torn apart by Jupiter's gravity. One by one, the fragments carpet-bombed the planet, creating shockwaves the size of Earth.

Comet Halley, the most famous comet of all - its appearances in historical times and the epic fly- by of the comet's nucleus in 1986 by the spaceprobe Giotto. How Halley may have been captured long ago from the Kuiper Belt and locked into its present orbit. Comet Hale-Bopp, the great cometary visitor of 1997. The Stardust mission that flies through the coma of Comet Wild Two, collecting samples of dust for dispatch to Earth.

Sub-chapters Pluto and Charon • The Lowell Observatory, Flagstaff, Arizona. An elderly Clyde Tombaugh demonstrates the equipment he used in 1930 to discover Pluto. • Pluto, the most remote planet, and its moon Charon. Orbiting 40 times farther from the Sun than Earth, they are frozen worlds, composed of water and methane, their combined mass less than one-fifth that of our Moon. • The Kuiper Belt, a disk of icy fragments just beyond the planets. Perhaps Pluto originated here. Pluto may not be a true planet.

Icy Vagabonds • Farther out is the Oort Cloud, a sphere of freezing debris encircling the Solar System, stretching one-third of the distance to the next nearest star. • Pluto's compositional resemblance to the comets from Kuiper and Oort. • Comet Hyakutake, viewed in 1996 - nucleus three miles wide, tail stretching for millions of kilometers. • Cometary tails - how they form double tails of gas and dust upon approaching the Sun. What causes the tails to point away from the Sun. • The random, highly elongated orbits of comets around the Sun. Comets that graze the Sun, comets that plunge into it.

Jupiter's Pull • Fragile S-L9 is torn apart as it's drawn within 50,000 kilometers of Jupiter's cloud tops. • A graphic illustration of Jupiter's influence on comets - Shoemaker-Levy 9. • July 1994 and, one by one, SL9's fragments strike Jupiter - creating shockwaves as big as Earth and plumes over 1,000 kilometers high.

Halley's Comet • Comet Halley in history, as Giotto's star of Bethlehem and Hally's appearance in the Bayeux tapestry depicting the Norman conquest. • The Giotto spacecraft's journey to Halley in 1986. Flying through the inner coma, returning images of the nucleus to Earth. • Halley's capture by the Sun, and the resultant 76 year orbit.

Visitors from Afar • Hale Bopp's fly-by in 1997. • How comets only form tails as they approach the Sun. In the outer Solar System, they become tailless and inert.

Collecting Comet Dust • The Stardust craft's encounter with Comet Wild Two in 1999. The process of collecting samples and sending them to Earth. • Comets containing particles unchanged since the formation of the Solar System. Stardust provides an opportunity to analyze these building blocks of the Solar System.

Background The Mysterious Nature of Pluto Relatively little is known about the frozen ninth planet. New information will be hard to come by without a space mission. Nothing has been firmed up for the early 21st century. So much remains a puzzle, including the exact nature of the planet. Indeed, is Pluto a planet at all? It cannot have been formed in the inner Solar System as it contains too much ice. But the internal composition of this frozen world, with a density 2.02 times that of water, means it contains more rock than the satellites of the gas giants.

If Pluto is a planet, then should it really be called a double planet? Pluto's moon, Charon, is so closely matched in mass that the two bodies orbit a mutual balance point. It's the only known case in the Solar System where a moon is massive enough - compared to its planet - to swing its parent body around a point outside the planet. Pluto itself is tiny - one twenty-fifth the size of Mercury. Even when combined with its moon Charon, their joint mass is so small that they could not perturb the motions of a giant planet. So what exactly is Pluto? One theory is that Pluto may once have been a satellite of Neptune. Although containing more rock than the moons of the giant planets, Pluto is similar in mass, size and density. The same goes for Charon. Pluto's highly elliptical solar orbit suggests that it may have escaped from Neptune in some cataclysmic event. That would also explain the reversed orbit of Neptune's moon Triton and the fact that another moon, Nereid, has been flung into a highly elliptical orbit.

What could have happened? Possibly, a colossal planetesimal collided with Triton, Nereid and Pluto, radically perturbing their orbital paths. Tidal forces from the planetesimal then ripped Pluto in two, forming Charon, and catapulted the pair farther out in the outer Solar System. There exist, however, no other remnants of this planetesimal. The most widely held theory remains, therefore, that if Pluto is not a planet, then it originated in the Kuiper Belt.

The Nature of Comets A bright comet at dawn or dusk is a magnificent spectacle. It is hard to believe that that the entire cometary phenomenon originates in a tiny central nucleus, no more than a few kilometers in diameter. The nucleus is composed of "dirty ice" - a mixture of ices (mainly water ice) and dust. It is the interaction of the nucleus with the Sun's radiation - particularly the solar wind - that produces the familiar characteristics we see from Earth, especially the spectacular comet tails.

Cometary nuclei are irregular in shape. They range in size from just a few hundred meters to perhaps 100 kilometers across at most. Comets, therefore, are comparatively minor bodies on the cosmic scale. Most orbit the Sun in elongated, elliptical paths. When the nucleus is a long way from the Sun, it is completely frozen, inactive and invisible from Earth.

As the comet approaches the Sun, the temperature of the surface layers of the nucleus increases sufficiently for certain ices to begin to sublimate - in other words, to turn directly from solid ice into gas. Jets of gas erupt through cracks in the surface. They pick up dust particles and drag them away from the nucleus. This creates a temporary atmosphere around the nucleus - called a coma. The gases and dust of the coma completely envelop the nucleus and form the fuzzy head of the comet. The coma, which is more or less spherical, can extend from ten thousand kilometers to as far as one-million kilometers from the nucleus.

When relatively close to the Sun, a comet develops two or more tails, which generally point away from the Sun. It's these tails - appearing faintly blue and yellow – that make a comet such a awe- inspiring sight from Earth.

The blue tail is formed like this: Ultra-violet radiation in sunlight reacts with the coma. Neutral atoms and molecules of gas are split into negatively-charged electrons and positively-charged ions - a process called ionization. The ions are caught by the magnetic field of the solar wind - the stream of electrified particles constantly emitted by the Sun. Thus, ionized gas is carried at high speed from the comet - flourescing a blue trail, straight and narrow, millions of kilometers through space in the opposite direction to the Sun.

The yellow tail, comprising dust, is created by another process. Sunlight exerts a subtle influence - called radiation pressure - on solid grains of dust in the coma. This "push" is enough to carry dust from the coma in a shorter, broader tail. It's also slightly curved. But, like the blue gas tail, the yellow dust trail streams away from the Sun. Why yellow? It's the dust reflecting sunlight.

The blue gas tails stretch from ten-million to one-hundred-million kilometers. Yellow dust tails range from one-million to ten-million kilometers.

Every time a comet swings around the Sun, it trails huge quantities of fine dust along its orbit. At its most active in 1997, Comet Hale-Bopp was throwing off nearly 1,000 tons of dust every second! If the Earth passes through one of these trails, particles can be seen burning up high in the atmosphere. Such displays are called meteor showers - or shooting stars.

Cometary Storehouses - Kuiper Belt and Oort Cloud In the outer Solar System, beyond the planets, lies the Kuiper Belt - a vast, flattened and diffuse ring containing between ten-million and one-billion icy planetesimals or comets. The belt is an inner extension of the Oort Cloud - an immense spherical reservoir of comets at an even greater distance. Kuiper lies roughly in the same plane as the orbits of the planets. Kuiper Belt objects are primitive, icy remnants from the early phase of Solar System formation. The belt is probably the source of most short-period comets - that is, those with orbital periods of up to 200 years. The first Kuiper Belt object was identified in 1992. Since then many more have been discovered and the number is growing all the time. Computer simulations have shown that Kuiper Belt objects can jolted by planetary perturbations and sent hurling into orbits closer in to the Sun.

The Oort Cloud is a spherical shell of comets that envelops the Solar System at a gigantic distance. Conservative estimates place the inner radius of the Oort Cloud at 20,000 AU, with the outer radius lying near 100,000 AU. The outer radius, therefore, exists in interstellar space, and extends one-third of the way to the next nearest star. The Oort Cloud may contain up to between 100-billion and a trillion comets. Beyond a distance of 50,000 AU, the Sun's gravitational influence is so weak that the pull of nearby stars and giant dust clouds may periodically perturb the Ooor Cloud and send comets tumbling into the Solar System.

The Oort Cloud is generally accepted as the source of long-period comets - those with orbital periods of more than 200 years. The orbits of some comets can stretch to millions of years. Occasionally, during their passages through the outer Solar System, long-period comets may be "captured" into shorter orbits by giant planets like Saturn and Jupiter.

Theories about the formation of the Oort Cloud are highly speculative. One idea is that Oort Cloud comets formed in the outer regions of the primitive solar nebula – perhaps within the Kuiper Belt - and were subsequently ejected into the Oort Cloud by a combination of planetary and stellar perturbations. It is generally believed that all comets are in some way a by-product of formation of the Solar System.

Halley - The Most Famous Comet of All Halley's Comet is the brightest of the comets whose paths we can predict. It is also the only comet that has been seen regularly, through history, with the naked eye. Halley has been observed and recorded for more than 3,000 years - first noted by the Chinese in the winter of 1059/1058 BC.

Halley's Comet travels around the Sun in a highly elliptical orbit. When farthest away, it lies below the orbit of the planet Neptune, more than 35 times the distance of the Earth from the Sun. Halley passes our way once every 75 or 76 years - the return period varying because of planetary perturbations. It was last closest to the Sun on February 9, 1986. Halley rounds the Sun between the orbits of Mercury and Venus.

Halley is named after the second English Astronomer Royal, Edmond Halley - not because he discovered the comet, but because he was the first person to calculate its path around the Sun. Halley realized that comets seen in 1531 and 1607 were the same comet that he himself had seen in 1682. He predicted that the comet would return in 1758 - and it did! The comet was named in Halley's honor. Thanks to Halley's acuity, we now can now identify recorded observations of his comet at every return since that 240 BC. Halley's appearance in 1066, presaged the Norman conquest of England. The comet is depicted in the Bayeux Tapestry.

On some returns Halley passes too far from Earth to be seen well. This was so at the last return in 1985/86. The next, in 2061, will be no better. Nevertheless, the last pass was the first of the space age. A flotilla of spacecraft passed close to Halley. They sent back a feast of information on the comet's structure and composition. The only solid part of Halley is a tiny, elongated nucleus. It is shaped like a peanut, about 16 kilometers long by 9 kilometers wide. The nucleus is dirty ice, partly insulated from the heat of the Sun by a layer of intensely black dust. But, in places, the Sun gets through, burning deep fissures that expose the underlying ice. Jets of gas and dust shoot from the cracks, giving rise to all the activity of the comet, not least the heroic tails.

The Giotto Spacecraft Giotto was the name given to the European Space Agency's first interplanetary spacecraft. It flew within 600 kilometers of the sunward side of the icy nucleus of Halley's Comet on March 14, 1986. Giotto obtained the first ever close-up images of a cometary nucleus. The spacecraft was named after the Italian painter Giotto di Bondone who, in 1304, depicted Halley's Comet as the Star of Bethlehem on a fresco in the Scrovegni Chapel at Padua.

Launched by an Ariane 1 rocket from Kourou in French Guiana on July 2, 1985, Giotto was stabilized by spinning the craft at 15 revolutions per minute. Giotto carried 10 scientific experiments.

Although protected from dust particle impacts by a dual-sheet bumper shield during its breakneck fly-by of Halley's nucleus, Giotto was hit several times by relatively large dust particles - greater than one milligram. One impact, shortly before closest approach, caused Giotto to wobble, interrupting the communications link with Earth, so that scientific data were lost. About half of the experiments onboard suffered damage, including the Halley Multicolor Camera. On July 2, 1990, Giotto became the first spacecraft to use Earth in a "gravity-assist" maneuver to rendezvous with Comet Grigg-Skjellerup on July 10, 1992.

Links for Further Information Comprehensive Pluto page. Internal and surface features, its moon Charon, facts and myths, proposed missions to the planet, news and images. http://www.windows.umich.edu/cgi-bin/tour.cgi?link=/pluto/pluto.html

Comet Halley page. Images from ground-based telescopes and satellites. Spacecraft encounters with the comet, historical records and origins. http://ceps.nasm.edu:2020/ETP/COMETS/comet_halley.html

ESO's Comet Hale-Bopp page. Updated to June 1998, with images and data, plus links to other related websites. http://www.eso.org/outreach/info-events/hale-bopp/

Comet Shoemaker-Levy 9 page. Reports and images of the impacts, history of SL9, proceedings of the international conference on SL9 held in 1996. Observations from the Hubble Space Telescope and Galileo spacecraft. http://nssdc.gsfc.nasa.gov/planetary/comet.html

Stardust and Comet Wild Two page. History of the comet, details of its composition, overview of the process for imaging both the coma and nucleus. http://stardust.jpl.nasa/comets/wild2.html

Questions and Activities for the Curious 1. Describe the technique used by Clyde Tombaugh to discover Pluto at the Lowell Observatory, Arizona, in 1930.

2. Some people believe that Pluto should not be called a planet at all. What are the arguments for and against this viewpoint?

3. Imagine you are a comet perturbed from the Oort Cloud into the inner Solar System. Describe your million-year journey towards the Sun, and back again.

4. Describe the appearance of the two types of cometary tails, explaining the differences between them and how they are formed.

5. Give an account of the events which led up to the impacts of the 20 or so fragments of Comet Shoemaker-Levy 9 on Jupiter in July, 1994.

6. Why is Halley's Comet often called "the most famous comet of them all"?

7. When Halley's Comet returned in 1910, it was photographed for the first time. In 1986 it was visited by spacecraft for the first time. How do you think scientists might study the comet when next it returns in 2061?

8. What are the main discoveries made by the Giotto spacecraft in 1986 of Halley"s Comet?

THE COMPLETE COSMOS Chapter 5: Moon Born of collision, the story of the Moon. Its influence on Earth. Apollo landings and the recent discovery of water.

Outline The Moon - Earth's partner in space. The Moon's influence on our planet, from mystic links to ancient artifacts such as Stonehenge, in England, to more tangible influences like biological cycles - sea turtles laying eggs according to tidal cycles, and the Moon's effects on tides and timekeeping.

Lunar phases, from new to full and back to new again. As the Moon orbits Earth, we see varying amounts of the half of the Moon that is lit by the Sun. How lunar gravity influences the tides. How tidal drag causes Earth and Moon to move slowly apart. How tital drag slows Earth's spin, lengthening the day from six hours to 24 hours, and ultimately to 47 days in the far future.

The formation and evolution of the Moon. How, from an impacting planetesimal half the size of Earth, the Moon was formed billions of years ago. The dark lunar "seas" and rugged highlands. The barren, airless, lifeless lunar surface, a landscape scarred by craters, faults and rills - plus huge fluctuations in temperature between day and night. The Apollo 11 Mission - astronauts Armstrong, Aldrin and Collins head for the Moon. A montage of the six Apollo landings, the astronauts' experiences and experiments. The 1998 Lunar Prospector mission, and its confirmation of ice at both poles on the Moon.

Sub-chapters Our Nearest Neighbor • The Moon in the night sky above Stonehenge and how the Moon's surface looks from Earth. • Lunar influences over biological cycles - turtles laying eggs at high and low tide, when incubation periods are best. • How the Moon's gravitational pull has slowed Earth's spin, lengthening the day from six to 24 hours, and how the process continues. • Comparisons between Earth and Moon - Earth is nearly four times as wide as the Moon, and 80 times as massive. Traveling ten times around Earth's equator is the same distance as flying to the Moon.

Phases and Tides • Lunar phases: during the Moon's 27-day orbit of Earth, we see varying amounts of the half of the Moon that is illuminated by the Sun. • Tidal influences: tides rise and fall twice daily due to the Moon's gravity. Tidal drag slows Earth's spin, increasing the length of the day. The Moon also causes Earth's axis to wobble.

Giant Impacts and the Lunar Surface • A Mars-sized planetesimal hits early Earth. From the ring of debris, the Moon is formed. • Surface features: the dark lunar "seas", huge impact basins that filled with lava and solidified; bright, young ray craters, caused by the splash of impacts; faults (valleys) and clefts (rills) which cut through the lunar terrain. • The airless Moon maintains a pristine record of impacts.

Men on the Moon • 1969 Apollo 11 Mission - Armstrong and Aldrin's landing on the Moon. Includes Armstrong's "One small step..." dialogue. • A montage of the six Apollo landings - astronauts working and playing on the Moon. Lunar samples analyzed back on Earth.

Searching for Ice • 1998's Lunar Prospector sending signals to the Moon's surface and discovering ice at both poles. The promise of further expeditions to the Moon.

Background The Phases of the Moon The Sun continually illuminates exactly 50% of the Moon's surface. As the Moon orbits the Earth, however, we see varying amounts of the "lit" half, depending on the relative positions of the Sun and the Moon. As a result, the Moon appears to change shape from night to night: passing from crescent to first quarter, through full moon to last quarter, and back to a thin crescent just before new moon. New moon occurs when the Moon is between the Earth and the Sun, and the illuminated side is facing away from the Earth. Full moon occurs when the Moon is on the opposite side of Earth to the Sun, making the entire illuminated side visible. These apparent changes of shape are known as the "phases of the Moon".

The cycle between successive phases of the Moon, the time from one new moon to the next, is 29.53 days. This is 2.2 days longer than the 27.32 days it takes for the Moon to revolve around the Earth - a period measured relative to the stars. The reason for the difference is that the Earth has moved along its orbit around the Sun. It takes two extra days for the Moon to return to the same position relative to the Sun, producing the same phase.

Tides Gravitational forces act between all bodies. The gravitational pull of the Moon, and to a lesser extent of the Sun, causes the waters of Earth's oceans to rise and fall. This effect is called a tide. In simple terms, tides are caused by the difference in gravitational pull between parts of the Earth closest to and farthest away from the Moon. This difference causes the oceans to bulge towards the direction of the Moon - two bulges occurring on opposite sides of the Earth. As our planet spins on its axis, each point on the coast passes through two high tides and two low tides every day.

The Sun affects the oceans, but has only 40% of the Moon's influence. Every two weeks, when the Moon and Sun align with Earth, both at new and full moon, their join gravitational pull creates even higher high tides and even lower low tides. These are called "spring tides". In between, when Earth, Moon and Sun form a right angle, at first and last quarter moon, their gravitational pulls tend to cancel, and high and low tides are less pronounced. These are called "neap tides".

The Surface of the Moon Casually glancing at the Moon, the light and dark areas of its surface are still strikingly apparent. The dark regions are believed to be volcanic features. Early observers thought these regions were seas, applying the Latin word for sea when describing them - mare (pronounced mah-ray), a term still used today. The very largest impacts created huge basins, and cracked the Moon's crust allowing molten lava to flood the craters. This molten rock eventually cooled, solidifying to form the dark mare regions. A relative lack of craters suggests the mare regions are younger than the lighter, rugged lunar highlands.

The lunar surface is entirely covered with craters, although they are more abundant in the rugged highland areas. A small number of craters may be of volcanic origin, but most are believed to be the result of impacts, during an intense bombardment by cosmic projectiles early in the history of the Solar System. Craters vary in size, from craterlets of approximately one meter in diameter, to very large craters spanning over 100 km. Certain larger craters have flat floors and central peaks. Some impact craters are surrounded by light-colored rays of material, ejected by the impacts.

Links for Further Information Earth's Moon. Comprehensive Moon page featuring orbital, tidal and gravitational information. Facts on surface properties, interior structure, theories of Moon formation, plus images, film and sound links. http://cesp10.phys.utk.edu/astr161/lect/moon/moon.html

Good Moon page, with historical links, links to missions and other moons in the Solar System. http://www.phy.nau.edu/~danmac/Course_HomePages/A100/Misc/9planets/luna.html

Project Apollo. Detailed history of the Apollo program, including image archive, chronology of events, unmanned and manned missions and links to other pages. http://www.ksc.nasa.gov/history/apollo/apollo.html

Lunar Prospector Homepage. Comprehensive site, including mission data, results, images and links. http://lunar.arc.nasa.gov

NSSDC Image Catalog - the Moon. Impressive range of images, plus text, from US lunar missions, including Apollo, Clementine, Galileo and Lunar Orbiter. http://nssdc.gsfc.nasa.gov/imgcat/html/group_page/EM.html

Latest theories of Moon formation conducted by the University of Colorado. http://www.earthsky.com/specials/moonformation.html

Questions and Activities for the Curious 1. If the Earth were a football, how big would the Moon be to the same scale? How far from the Earth on this scale would the Moon be?

2. Sketch phases of the Moon, whenever the sky is clear, over a four-week period. Put the date and time underneath your sketch each time you draw it. Using your observations, estimate the dates of new and full moon, and of first and last quarter.

3. Why is the interval between one full moon and the next about 2.2 days longer than the time it takes the Moon to revolve around the Earth?

4. What has been the effect of the Moon on the Earth's rotation? How has this affected the length of our day, and what will happen in the far future?

5. How can you tell the difference between relatively young and relatively old craters on the surface of the Moon? Give three examples of each.

6. Produce a sketch-map of the Moon. Draw the major dark mare areas, and mountain chains, and mark in some of the largest lunar craters.

7. Which Apollo mission nearly ended in disaster? What happened?

8. Select one of the six successful Apollo landings on the Moon and find out what you can about the date of the mission, the activities the astronauts performed, and where on the Moon they landed.

THE COMPLETE COSMOS Chapter 21: Milky Way Our Galaxy explored, light years explained. The life and death of stars. Supernovae -and the clouds where stars are born.

Outline Time-lapse of the Milky Way traversing the night sky - the sideways view of a flattened disk of stars, our galaxy. From above, the Milky Way is a spiral, a family of 150 billion stars. Our Sun is but one of these stars - two-thirds of the way from the center and orbiting the galactic center once every 225 million years.

The heart of the Milky Way - choked with gas and dust - seethes with energy. Distances within our galaxy, and beyond, are measured in light years - the distance traveled by light in one year. The disk of the Milky Way is 100,000 light years across. Our Solar System lies 30,000 light years from the galactic center.

The distances to some of our nearest stars. A journey from the constellation of Orion, past the Pleiades, Aldebaran, Sirius and Barnard's Star to the Alpha Centauri system, a trio of stars four- and-a-quarter light years away from the Sun.

Emission nebulae, where stars are born. The violence of starbirth and the action of stellar winds. The more massive a star, the shorter its life. As they age, some stars swell to become red giants or supergiants. Most end by puffing off their outer layers to form beautiful planetary nebulae and collapsing into white dwarfs. In a binary star system, a white dwarf can provoke periodic nova eruptions.

The most devastating stellar explosions are supernovae - when a supermassive star blows itself apart. The Crab Nebula is an example. With a rapidly spinning pulsar at its core, intense radiation causes the gas of the nebula to glow. Eta Carinae is a star with 100 times the mass of the Sun. When it goes supernova, Eta Carinae may collapse beyond the pulsar stage to become - a black hole. Birth, life and death in the Milky Way.

Sub-chapters Milky Way • Time-lapse from Earth of the Milky Way delicately arching across the sky. • In spectacular animation, a sideways look at a flattened disk of stars, our galaxy. • From above, the Milky Way is a spiral containing 150-billion stars. • Our Sun is one of those stars. It lies two-thirds of the way from the galactic center, towards the edge of the disk - and orbits the center once every 225 million years. • The galactic center shines brightest of all. Veiled in gas and dust, it seethes with energy. • By contrast, our Sun's neighborhood is suburban - quiet stars leading ordinary lives. The Sun is no exception - yellow, average and middle-aged.

Nearest Stars • Distances in the Milky Way are too large for kilometers or miles. Measurement is based on the distance traveled by light in a given time. • Light travels from the Sun to Earth in eight-and-a-half minutes. A light year is the distance light travels in one year - equal to 9.46 million million kilometers. • The Sun's nearest neighbor, the Alpha Centauri system, is four-and-a-quarter light years away - or more than 40 million million kilometers. The Milky Way is 100,000 light years across. The Sun is 30,000 light years from the galactic center. • From Earth, stars that seem close to each other - like the familiar constellation of Orion - may, in reality, be hundreds of light years apart, the result of a line-of-sight effect. Relatively nearby stars lie tens or hundreds of light years away. • A journey past the stars of the Pleiades - 380 light years away - and the older, dimmer Hyades, 150 light years distant. Although it appears to be part of the Hyades, the star Aldebaran is at less than half as far. Sirius, the brightest star in the sky, is closer to home - less than nine light years away. In 10,000 years, the dim red dwarf Barnard's Star will be the Sun's closest companion. • But for now, the stars of the Alpha Centauri system are the Sun's nearest neighbors - at a distance of four-and-a-quarter-light years . Young Stars • The spiral galaxy M83 reveals regions of starbirth. They glow pink and are called emission nebulae. The Milky Way also has them - clouds of glowing hydrogen streaked with dark dust. • Where material clumps, stars fire into life. Stellar birth is violent. Young stars emit powerful winds. In the Rosette Nebula, a hole 12 light years across is the result of powerful stellar winds from young stars.

Red Giants, Planetary Nebulae • The Sun is an average star, consuming its hydrogen fuel in moderation. Massive stars have shorter lives - voraciously using their fuel and swelling to red supergiants hundreds of times their original size. • Antares is surrounded by a reflection nebula - a star so big that its atmosphere leaks into space, a cloud of its own material that catches the light of other stars. • As hydrogen runs out, stars switch to helium and other elements. Eventually the delicate equilibrium is lost. Radiation pushing outwards beats gravity pulling inwards. The star swells to a bloated red giant. • The outer layers are lost to form beautiful planetary nebulae. With the core exposed, the star collapses to a white dwarf. • Some white dwarfs are more spectacular. In a binary system, a white dwarf may draw material from its companion. On reaching critical mass, the overloaded outer shell of the white dwarf explodes as a nova. Both stars survive and the process repeats. • The Sun's lifespan is ten billion years. A supergiant star, with 30 times the Sun's mass, will survive just one million years. These massive stars explode as supernovae, the fate of any star more than eight times the mass of the Sun.

The Crab Pulsar • The supernova that created the Crab Nebula was observed from China in 1054. Within the nebula, a pulsar survives - a superdense relic of the original star spinning 30 times a second. • The pulsar emits shockwaves, particles spinning from the surrounding disk of material at almost the speed of light - seen in time-lapse from the Hubble Space Telescope.

Exploding Superstars • Eta Carinae, a star with a 100 hundred times the mass of the Sun, is venting great clouds of material - an indication that catastrophe is not far away. • When Eta Carinae explodes as a supernova, it will collapse beyond the pulsar stage to become a black hole.

Background The Light Year A ray of light travels at almost 300,000 kilometers per second. Consequently, it takes light only about 500 seconds to travel the 150 million kilometers between the Sun and the Earth. If a ray of light traveled non-stop for one year, it would cover a distance of about 9,460,000-million kilometers. This distance is called the light year. The concept confuses some people because a light year is a measure of distance and not of time.

Distances to the stars are so vast that measuring them in kilometers or miles (or even millions of kilometers or miles) is not very practical. Even the nearest star system - Alpha Centauri - is about 40 million million kilometers distant. Yet light would travel this distance in only four-and-a- quarter years. So astronomers say that the distance to the Alpha Centauri system is about four- and-a-quarter light years.

Other well known stars are much farther away. Betelgeuse, the red supergiant star in Orion, is about 590 light years from us. Deneb, the brightest star in Cygnus the Swan - or the Northern Cross - is even more remote, lying 1,800 light years away.

The distances to other galaxies is also measured in light years. One of the nearest galaxies is Andromeda - more than two-and-a quarter million light years away. Most galaxies are much farther. Some of the most distant objects known are called quasars. Many have distances of over ten thousand million (ten billion) light years.

The Milky Way Galaxy The Milky Way is our galaxy - sometimes known simply as the Galaxy. It contains at least 150 thousand million (150 billion) stars of which our Sun is one. They form a gigantic flattened disk. It is thin at the edges but has a bulge - or nucleus- at the center. Look more closely at the disk and a spiral structure is apparent. Our Sun, which is nowhere near the galactic center, lies close to the edge of one of the spiral arms, about two-thirds of the way out from the center, towards the edge.

As a result of the Sun's peripheral position, when we look up into the night sky from Earth, the stars of the Milky Way are not scattered evenly across the heavens. Instead, they appear as the edge of a flattened disk - the galactic plane. It is a luminous band stretching across the sky - fairly narrow and quite beautiful. Within it, stars appear to be crowded together. The galactic center lies in the direction of the constellation of Sagittarius. For observers in Europe and the United States, the band of the Milky Way passes through Aquila, Cygnus, Cassiopeia, Perseus, Auriga and parts of Gemini and Orion. The diameter of the Milky Way is about 100,000 light years. The Sun is just over 30,000 light years from the galactic center. The maximum thickness of the Milky Way – through the central bulge - is about 20,000 light years. A "halo" that surrounds the galaxy contains fewer stars than the disk of the galaxy itself. The extent of the halo is uncertain - but all the stars within it are old. They are believed to have formed early in the life of the Milky Way, maybe 12,000 million years ago. Most of the stars in the disk - and probably those in the nucleus - are stars of intermediate age, probably from 3,000 to 5,000 million years old. The youngest stars are confined to a layer – some 1,500 light years deep - along the disk's central plane.

Many of the brightest stars of the Milky Way are in its spiral arms. They wind outwards from the nucleus, close to the central plane of the disk. The Sun is only a few light-years north of the galactic plane, near the inner edge of one of the spiral arms. The youngest stars in the galaxy, together with very bright, short-lived hot blue-white stars, are mainly found in the spiral arms. Close to the Sun, astronomers have identified and traced out parts of three spiral arms. These are the Orion arm, in which the Sun is situated; the Sagittarius arm, which lies about 6,500 light years nearer to the galactic center; and the Perseus arm which lies about 6,500 light years farther out. About one-tenth of the mass of the Milky Way is probably gas and dust, occupying the space between the stars. Huge quantities have been detected within or close to the galactic plane. Vast clouds are also found in the spiral arms where stars are still being formed. But this gas and dust obscures many objects. We cannot directly see the center of the galaxy, because it lies beyond the star-clouds in Sagittarius. The star-count increases rapidly towards the center of the galaxy. A powerful source of radio waves, called Sagittarius A, appears to lie right at the galactic center.

The entire Milky Way rotates around its center. The galactic disk rotates more rapidly than the halo. In our part of the galaxy, the disk rotates at about 250 kilometers per second. The Sun takes some 225 million years to complete one revolution - a period known as the cosmic year. The halo rotates at only about 50 kilometers per second.

The Birth of Stars Stars are huge balls of gas held together, like everything else in the Universe, by the force of gravity. They differ from planets in more than just size. Stars generate energy in their cores by nuclear fusion - reactions that for all stars begin with the conversion of hydrogen into helium. This process releases energy which makes its way slowly to the surface. From there it pours into space. Planets have no light of their own. They shine only by reflected starlight. Stars are formed from the collapse of giant clouds of interstellar dust and gas. As more and more material falls in, the developing star grows hotter. Eventually, when the temperature in the core exceeds seven million degrees, nuclear fusion reactions begin. In fact, a star is just a pause in a long-term process of collapse. The halt is caused by a balancing act. On the one hand, the star is a naturally expansive body pushing outwards. On the other, huge forces of gravity are pushing inwards. This is how it works. Radiation is generated at the core by nuclear fusion - the combination of hydrogen atoms to make helium. The energy released has to force its way out. But everywhere within the star, gravitational pull exactly balances the outward thrust of the radiation pressure.

A star's mass will influence the course of its life. For a star to shine at all requires a mass of at least one-fifteenth that of our Sun - about 80 times the mass of the largest planet, Jupiter. Some stars, however, are much larger and more massive than the Sun, the biggest being around 100 solar masses. The mass of a star also determines its temperature and luminosity.

The Evolution and Death of Stars When nuclear fusion fires up, stars have temperatures and luminosities prescribed precisely by their masses - with a very slight variation caused by the chemical mix. It is these factors that define a star's Main Sequence - in other words, the major part of its lifespan.

Massive stars are very luminous and hot. They shine blue. Stars in the middle range are less hot and shine yellow - the Sun being a good example. The least massive stars are the coolest and glow a dull red. Again, mass is the vital factor in governing how long a star spends in the Main Sequence. For instance, our Sun's Main Sequence is ten billion years. Roughly half the sequence has already elapsed. For a star like Sirius, which is twice as massive as the Sun and 20 times as luminous, the Main Sequence is one billion years. For a star of 30 solar masses with a luminosity 100,000 times that of the Sun, the Main Sequence drops dramatically to a mere one million years. So, massive stars have a much shorter Main Sequence than the Sun, despite having more fuel at the outset. That is because they consume it so much faster in order to supply their greater luminosities.

Ructions ensue when a star grows old. As hydrogen fuel is consumed, hot but inert helium grows within the core. Meanwhile, on the outside of the core, hydrogen fusion proceeds. The core grows hotter and hotter - to a point where helium can also participate in nuclear fusion. This produces carbon and oxygen - and a sudden change called the helium flash. The inner core shrinks and becomes enormously dense, while the outer parts of the star expand. Luminosity increases - more for smaller stars than for larger ones – and the temperature falls rapidly. The star becomes a giant, perhaps 100 times its original diameter. Its outer layers cool so much that the star dims to red or orange-red. Slowly, the giant grows brighter and cooler. Eventually, it becomes a variable star, oscillating in size and fluctuating in brightness. Finally, the outer layers are shed, forming a bubble around the dying star called a "planetary nebula". Beneath, the hot core is revealed - a white dwarf of extreme density. The white dwarf cools - ending as a black dwarf, the cinder of a star.

This cycle applies only to stars like our Sun. If the star is much bigger, with a core whose mass exceeds 1.4 solar masses - the Chandrasekhar limit - the story is different. Such stars exhaust their hydrogen quickly and usually progress to the red giant stage in a more convulsive way. Indeed they will become much larger and more luminous than a conventional red giant. They become supergiants - so luminous that they can be easily seen right across the Milky Way galaxy, as well as in other galaxies.

In a big star, its great mass compresses and heats the core so much that carbon ignites. This keeps the star "burning" when its helium is gone. There follows a succession of nuclear reactions that create heavier and heavier elements which continue to fuel the star. Oxygen, neon, magnesium and silicon are formed. But, since higher and higher temperatures are needed at every stage, each new shell is confined to a smaller and hotter region around the core. Eventually, the star begins to burn silicon into iron. This signals the end. Nuclear fusion stops with iron. A star with an iron core is out of fuel - and out of time. The supergiant is blown to pieces in a spectacular and catastrophic supernova.

As we have seen, stable stars are a delicate balance between their internal radiation pressure and their own force of gravity. The failure of this balance is the underlying cause of a supernova. When fusion reactions cease, the internal radiation pressure disappears and the interior of the star begins to collapse. The effect of this "implosion" is to crush the superdense core, forcing protons and electrons together to produce a ball of neutrons. The star's plunging outer layers strike the neutron core, crushing it still more. The infalling gas is heated to billions of degrees. Finally, the pressure blasts away the outer layers in a titanic supernova. This cataclysm is not exactly at the center of the star, but on the exterior of the dense core. The explosion not only throws off several solar masses of gas at speeds of well over 10,000 kilometers per second, but it crushes the core with unimaginable force. At the very least, a superdense neutron star is formed, about ten kilometers in diameter. In some case, the compression may be so great that a black hole is formed.

Links for Further Information The Milky Way - images and text on our galaxy with links to specific features. http://csep10.phys.utk.edu/guidry/violence/ginfo1.html

Good image of the Milky Way's flattened disk acquired by the COBE spacecraft – with introductory text. http://map.gsfc.nasa.gov/html/milky_way.html

Origins of the Milky Way. Lecture by Barbara Ryden on the theories of formation of our galaxy. http://www-astronomy.mps.ohio-state.edu/~ryden/astro162_2/notes26.html

Constellation homepage with photographs of the Milky Way, the constellations, the brightest and nearest stars, plus supplementary information. http://www.astro.wisc.edu/~dolan/constellations/constellations.html

Informative page on starbirth with color photographs and detailed descriptions of features in the images. Includes photographs of the "Pillars of Creation" and the Orion Nebula. http://csep10.phys.utk.edu/guidry/violence.birth.html

The death of stars. Detailed descriptions of novae and supernovae with illustrations. Explains the difference between the two phenomena. Accompanying diagrams. Links to supernova remnants, including the Crab Nebula and Cygnus Loop, and a special link to Eta Carinae with a photograph of this unstable supermassive star. http://csep10.phys.utk.edu/guidry/violence/supernovae.html

Questions and Activities for the Curious 1. What is a light year? Give some examples of its use in defining astronomical distances.

2. Describe the size, shape and structure of our Milky Way galaxy.

3. Imagine you are flying a spaceship from the Pleiades star cluster to our Sun. Describe the star systems you pass.

4. What do we know of fierce stellar winds blowing from young stars?

5. Discuss the life and times of a star like our Sun - from birth to death.

6. Explain the difference between a nova and a supernova.

7. Outline the events leading to the destruction of a massive star in a supernova explosion.

8. What is the Crab Nebula and what would you find at the heart of this nebula?

THE COMPLETE COSMOS Chapter 2: Mercury The most comprehensive portrait ever of this scorched little planet. Double sunrises, craters, cracks and - incongruously - maybe polar ice. Outline From a flattened disk of gas and dust, the birth of the Sun and planets - outwards from the rocky inner planets, past the outer gas giants, to Pluto and the icy debris at the edge of the Solar System.

From Earth, Mercury is visible only in the evening twilight sky - because of the planet's tight orbit around the Sun. Mercury has a large, iron-rich core.

The first and, so far, the only space mission to Mercury - Mariner 10. Mariner's pictures of the planet's surface. Numerous craters, faults, lava plains, rugged highlands and a huge, multi-ringed basin.

Oddities: Mercury's day is twice as long as its year. From Mercury's surface, the Sun appears to grow and shrink in size. Why there are double sunrises. And, with such great extremes of temperature, could there be ice at Mercury's poles? Despite continued research, more than half of Mercury remains unseen by robotic probes.

Sub-chapters Building the Planets • The formation of the Solar System from a flattened, slowly rotating disc of gas and dust. • A tour of the planetary system, traveling out into the icy debris of the Kuiper Belt and Oort Cloud. • How the inner planets formed from the heaviest elements.

Closest to the Sun • Why Mercury is consistently difficult to see, being observable with the unaided eye only when low in the sky in the twilight of dusk or dawn. • The orbits of Mercury and Venus as viewed from Earth, plus transits of Mercury across the solar face. • Mercury as the densest planet, with a large iron-rich core.

Craters, Cracks and Basins • Mariner 10, the first space mission to Mercury, launched in November 1973. • Mariner 10's images reveal Mercury's surface detail for the first time. • Depiction of the giant impact which gouged a huge, multi-ringed basin on Mercury four billion years ago. • Mercury's surface features, including craters, faults, lava plains and mountainous highlands.

Weird Effects • Illustrating how Mercury's rotation causes the planet to spin one-and-a-half times during each orbit of the Sun, resulting in Mercury's day being twice as long as its year. • Presenting Mercury's elongated, oval shaped orbit. How this causes the Sun to grow and shrink in the sky. • Explanation of Mercury's double sunrise. When orbiting closest to the Sun, the planet's orbital speed increases, then slows again. The result is the Sun's apparent rising, setting, then rising again.

Temperature Extremes • Extremes of temperature on Mercury. • The surprising discovery that ice may be present at both poles on Mercury - in deep craters permanently shielded from the heat of the Sun. • Mercury is only partially studied - its proximity to the Sun ruling out the possibility of close scrutiny with large telescopes. • For this reason, more than half of Mercury remains to be photographed, awaiting a second space mission to the planet.

Background Formation of the Planets Most astronomers believe the Sun and the planets originated at the same time, from a cloud of interstellar material. The cloud collapsed, forming a slowly spinning disc of gas and dust, the hot center of which became the Sun. As the center contracted, the internal temperature rose to a point where nuclear fusion reactions occurred, and the young Sun began to shine.

Simultaneously, the outer limits of the cloud cooled, the material beginning to solidify. Small grains became pebbles and boulder-sized crags, which collided and coalesced – a process called accretion - forming larger objects known as planetesimals. These planetesimals eventually accreted to form the planets of the Solar System.

The outer gas giant planets, orbiting within the cooler regions of the system, retained vast envelopes of the lightest gases hydrogen and helium, surrounding a rocky core. The inner terrestrial planets, with higher temperatures and lower gravities, were unable to retain their gases, but became rich in minerals. For this reason, the four inner planets, Mercury, Venus, Earth and Mars, have far higher average densities than the four gas giants, Jupiter, Saturn, Uranus and Neptune.

The Density of Mercury Density is the amount of mass for every unit of volume of an object, i.e. - mass divided by the volume. Whereas we can weigh and measure objects on Earth, we have to estimate the amount of space a planet occupies (its volume) and the amount of matter it contains (its mass). Scientists do this by analyzing the gravitational pull of a planet on nearby objects and by using data obtained by space probes. The average density of a planet is obtained by dividing its mass by its volume. Mercury has considerable mass for it size. Although only slightly bigger than Earth's Moon, Mercury's mass is four times greater than the Moon's. Mercury's density (5.42 grams per cubic centimeter), therefore, is comparable with that of Earth (5.52 grams per cubic centimeter). This is due to Mercury's large iron core which accounts for 65 to 70% of the planet's mass, explaining both the high density and Mariner 10's detection of a significant magnetic field. Mercury is the most iron rich planet in the Solar System.

Mercury's Double Sunrise Due to Mercury's elongated orbit, its distance from the Sun fluctuates between 46 million and 70 million kilometers. When Mercury is closest to the Sun, the Sun appears one-and- a-half times as large as when Mercury is farthest away. The elongated orbit also means that Mercury travels one-and-a-half times faster when closest to the Sun, than when farthest away.

As Mercury orbits the Sun, its orbital motion causes the Sun to move from west to east across the sky. At the same time, its axial rotation causes the Sun to move from east to west across the sky. Normally, Mercury's east-to-west motion is greater than the west-to-east motion, so the Sun appears to move slowly from east to west - the time between sunrise and sunset being 88 days. When Mercury is closest to the Sun, however, and Mercury's orbital velocity is greatest, the increased west-to-east motion of the Sun in the sky exceeds the east-to-west movement caused by the axial rotation. The Sun, therefore, appears to slow down, stop, reverse direction for a while, stop again, then continue its east to west trek across the sky. Viewed from a particular location on Mercury, the result is a double sunrise - or double sunset.

Links for Further Information A detailed site covering the Mariner 10 mission to Mercury. http://leonardo.jpl.nasa.gov/msl/QuickLooks/mariner10QL.html

A good site for images of Mercury. http://ceps.nasm.edu:2020/RPIF/MERCURY/mercury.html

A comprehensive site about Mercury, offering information at three different levels - Beginner - Intermediate - Advanced. http://www.windows.umich.edu/mercury/mercury.html

An informative site featuring general topics of interest about the planet Mercury. http://pds.jpl.nasa.gov/planets/welcome/mercury.html

A selection of thumbnail images from Mariner 10's mission to Mercury and Venus, with higher resolution images available. http://www.jpl.nasa.gov/mip/mnr10.html

Questions and Activities for the Curious 1. The Sun constitutes 99.86 per cent of all the matter in the Solar System. Imagine you have been given a box of 100 apples. Devise a demonstration that could be used to illustrate this point.

2. If the diameter of Mercury is 4,878 kilometers, and that of the Earth is 12, 756 kilometers, find the volume of both Mercury and the Earth. How many times could a globe the size of Mercury fit inside the Earth?

3. When are the best times to observe Mercury with the unaided eye from Earth, and why is this so?

4. Describe the different kinds of surface features found on the planet Mercury.

5. Explain why Mercury's day, the interval between one sunrise and the next, is exactly twice as long as the length of its year - the time taken to complete an orbit of the Sun.

6. The distance of Mercury from the Sun varies between 46 million kilometers (at its closest) and 70 million kilometers (at its farthest). Make two sketches that show the relative sizes of the Sun as viewed from Mercury at these times.

7. The temperature on the surface of Mercury can rise as high as 430 degrees Celsius. If you had four metal pots, made of tin, lead, copper, and iron, research which ones would melt on the surface of Mercury. 8. Mercury is the closest planet to the Sun, yet some astronomers believe there is water ice at the poles of the planet. Discuss how this might be so.

THE COMPLETE COSMOS Chapter 7: Mars Cold, arid Mars, where we'll land next. Polar caps, volcanoes, the biggest canyon ever seen - Mars probably once had oceans.

Outline Mars is similar to Earth in that it has an atmosphere, a tilted axis, seasons and a day of twenty- four-and-a-half hours. But there are crucial differences. Mars has a sub-zero climate, global dust storms and an atmosphere 150 times weaker than Earth's. Space missions have revealed much of Mars - like the Viking Orbiter images of the planet's surface and the search for life in the planet's red soil by the Viking Landers, albeit unsuccessful.

A Martian guided tour takes in Mariner Valley, the 4000-kilometre long rift cutting across the planet's equatorial regions - plus the great shield volcanoes, such as Olympus Mons, the tallest volcanic peak in the Solar System.

Volcanic activity may once have been important - not only in triggering massive flash floods, but in giving Mars a respectable atmosphere and surface temperatures. But when the volcanoes shut down there were no eruptions to replenish the atmosphere, which leaked into space because of the weak Martian gravity.

Recent missions to the Red Planet include the Mars Pathfinder landing and Mars Global Surveyor's evidence for ancient Martian oceans and a weak magnetic field. Future expeditions are previewed, together with the continuing search for microbial life.

Sub-chapters The Red Planet * Mars as the "dusty Red Planet" and the relevance of Martian exploration in the light of the planet's similarities to Earth.

Seasons and Dust Storms • Bitterly cold temperatures because Mars is so much farther from the Sun. • Two tiny Martian moons. • How tilted axis and elliptical orbit of Mars cause and distort the Martian seasons. • The differences between the seasons in the northern and southern hemispheres. How the polar caps expand and contract. • First pictures of Mars from early Mariner spacecraft - leading to a steady growth in our knowledge of the planet's surface. • Dust storms that can envelop the entire planet.

The Vikings • The Viking Orbiters revolutionized our understanding of the Red Planet. Their pictures hinted at the presence of water in the form of sub-surface ice, and to past flash floods. • Frosts and early morning fog. • Touchdown of two Viking landers which dig for signs of life.

Giant Volcanoes • Mars is small. North America would fit across the planet. • AmMartian fly-over - Mariner Valley (the biggest rift), and the great volcanic peaks, including Olympus Mons, the tallest volcano in the Solar System.

Pathfinder & Global Surveyor • The theory of oceans forming on early Mars. The importance of great volcanic eruptions and climatic instabilities in explaining why Mars doesn't spark life. • Volcanic shut-down - main reason for today thin Martian atmosphere. • Touchdown of Mars Pathfinder and the work of its six-wheeled rover in analyzing the composition of the rocks - with results that may support the existence of ancient oceans. • Supporting evidence from Mars Global Surveyor in orbit, which finds a huge flat region. Surveyor also measures the weak Martian magnetic field.

Future Mars Missions • With the success of Pathfinder and Global Surveyor, further missions are planned, each with a lander and an orbiter. • The aim of the Mars Surveyor program is to understand the history of water on the Red Planet. Landers will continue the hunt for signs of life.

Background Seasons of Mars The rotation axis of Mars is tilted by 24 degrees to the vertical, very similar to the axial tilt of Earth. Mars, therefore, has seasons in much the same way as Earth. The orbit of Mars around the Sun, however, takes 687 days compared with Earth's 365. So Martian seasons are correspondingly longer than Earth's. The highly elliptical orbit of Mars has a noticeable effect on the seasons. When Mars is closest to the Sun (and traveling more quickly) the southern hemisphere is tilted towards the Sun. As a result, southern spring lasts only 143 Martian days, and southern summer 154 days. In addition, because the planet is closest to the Sun, the southern Martian summer is hotter and shorter than the northern summer. Northern spring lasts 194 days and summer, 178 days. Northern winter is as long as southern summer, and the other seasons are similarly reversed. Southern winter, when Mars is farthest from the Sun, is colder and longer than northern winter. Climate in the southern hemisphere of Mars, therefore, displays a wider range of temperatures than in the north.

The Mystery of the Missing Water of Mars When measurements by the US Viking landers confirmed low atmospheric pressure and low temperatures on Mars, it became obvious that the Red Planet was extremely dry – a "freeze-dried desert". Water can only exist as a liquid over a certain range of pressure and temperature. The conditions on Mars today would result in any liquid water boiling violently and rapidly evaporating into the thin Martian atmosphere. Liquid water, therefore, cannot be present on present-day Mars.

But when orbiting spacecraft obtained detailed photographs of the Martian surface, features which resembled dried-up riverbeds were apparent. There were also erosion features that looked like flash-flood plains. This supported the theory that liquid water once flowed across the surface. Scientists, therefore, were faced with the problem of the planet's missing water. Part of this water undoubtedly resides as ice in the polar caps of Mars. With arrival of summer, the upper layer of frozen carbon dioxide (dry ice) in the ice caps rapidly evaporates. This exposes a layer of water ice beneath that survives the chilly Martian summers. The thickness of this water ice, however, has not yet been determined.

The flash-floods on Mars seem to have gushed from regions of collapsed, jumbled terrain. This indicates that the water released came from a layer of melted permafrost beneath the Martian surface - identical to that found in the tundra in far northern parts of Earth. If heat from volcanic activity occasionally melted this sub-surface ice, the ground would collapse as vast quantities of water were forced to the surface, producing a brief flash-flood as the water boiled away. From photographs, scientists have estimated the depths and widths of the Martian flash-flood channels. Results suggest the Martian flash-floods must have been at least 100 times greater than anything known on Earth. Scientists disagree about the amount of water lying frozen in the polar ice and as sub-surface ice and permafrost, but some believe that if all this water melted, it would be sufficient to produce a shallow ocean, 200 meters deep, across the surface of Mars.

The Volcanoes of Mars The Martian volcanoes were first photographed by the US spacecraft Mariner 9 which entered orbit around the planet in 1971. The largest Martian volcano, Olympus Mons, rises 27 kilometers above the surrounding plains, making it the highest known volcano in the Solar System. Olympus Mons is three times higher than Earth's highest mountain, Mount Everest. The highest volcano on Earth is Mauna Loa in the Hawaiian Islands, but it is largely submerged in the waters of the Pacific Ocean. Its summit rises eight kilometers above the ocean floor. The base of Olympus Mons is ringed with cliffs reaching six kilometers in height and its diameter is nearly 600 kilometers. Only two of the volcanoes on Venus, Rhea Mons and Theia Mons, cover a larger area - and they are only six kilometers high. At the summit of Olympus Mons, there is a complex caldera, approximately 80 kilometers across, containing a number of overlapping volcanic craters. The colossal size of Olympus Mons strongly suggests that there is no plate tectonic activity on Mars - as there is on Earth. A "hot spot" in the mantle of Mars keeps pumping molten rock upwards through the same vent for millions of years. This produces one giant volcano, rather than a chain of volcanoes like the Hawaiian Islands on Earth. Olympus Mons is one of four large volcanoes clustered together on the Tharsis bulge, a region 2500 km across, lying just north of the Martian equator. The other three volcanoes of the Tharsis group lie in a rough line. They are called Ascraeus Mons, Pavonis Mons, and Arsia Mons. A separate, smaller group of Martian volcanoes is located almost on the opposite side of the planet. None of the Martian volcanoes appear to be active today, but it is not known whether they are truly extinct or merely dormant.

Links for Further Information General information about Mars. http:seds.lpl.arizona.edu/nineplanets/nineplanets/mars.html

An atlas of Mars, with answers to frequently asked questions. http://ic-www.arc.nasa.gov/ic/projects/bayers-group/Atlas/Mars

A compilation of facts about Mars, including arguments about the possibility of life on the planet, with comparative images of Mars and Earth. http://cass.jsc.nasa.gov/publications/slidesets/marslife.html

A chronology of space exploration to Mars. http://www.star.le.ac.uk/edu/solar/craft2.html#mars

'Earth invades Mars' - a full account of the Mars Pathfinder and Mars Global Surveyor missions. http://www.spacer.com/mars/mars-prev.html

Mars Pathfinder page. Comprehensive image gallery, with accompanying text, plus links to other images, animations and the Mars Sunset movie. http://www.brandx.net/dbajot/mpf/

Mars Pathfinder images from NASA. http://www.in-search-of.com/frames/events/mpf_archive.shtml

Mars Global Surveyor - Welcome to Mars! Comprehensive site with what's new, latest images, directory of all mgs links, and weekly status overview. http://mpfwww.arc.nasa.gov/mgs/

Questions and Activities for the Curious 1. Compare and contrast the seasons in the northern and southern hemispheres of Mars with those of Earth.

2. Describe the evidence from spacecraft which has led scientists to believe that liquid water once flowed on the surface of Mars.

3. Research and describe the similarities and differences between volcanoes on Earth and Mars. Why are volcanoes on Mars larger than those on Earth?

4. Explain why Earth's sky is blue, whereas on Mars it is pinkish in color?

5. Carbon dioxide is almost as abundant in the atmosphere of Mars as it is in the Venusian atmosphere. But why is there no runaway greenhouse effect on Mars?

6. Mars is often described as "the planet which is the least unlike Earth". Justify this statement.

7. Research and report on the current status of the debate concerning life on Mars. Do you think Mars could be as barren and sterile as the Moon?

8. Imagine you are an astronaut living on Mars and you have a motorized vehicle for exploring the planet. Where would you like to go, and why?

THE COMPLETE COSMOS Chapter 19: Light Fantastic Visible light reveals only part of the Universe. How other wavelengths fill out the picture - from gamma-rays to radio.

Outline Visible light is just part of a whole spectrum of radiation. Properly called the electromagnetic spectrum, it stretches from the longest wavelengths of radio, through infrared, visible light, ultraviolet and X-rays, to the shortest wavelengths of gamma-rays. All radiation is invisible - we only see light because it reflects off particles in the atmosphere.

To "see" radiation other than visible light, a new kind of astronomy is needed . Radio waves are caught in huge dish-like antennae - detecting beams emitted by pulsars and revealing the causes of galactic turmoil. Infrared observations unveil vast, warm clouds of cosmic gas and dust - the birthplaces of stars. Like ultraviolet and radio waves, relatively little infrared penetrates Earth's atmospheric shield.

The best "invisible" observations are from Earth orbit - like the Infrared Space Observatory and the Hubble Space Telescope which can sense not only visible light but infrared and ultraviolet radiation. Ultraviolet unmasks hot young stars and the churning surface of our Sun. At shorter wavelengths, X-rays pinpoint the hottest and most energetic areas of the Sun. They also reveal the violence of the cosmos - some of it associated with black holes.

Gamma-rays are the most energetic radiation, evident in the hottest and most violent parts of the Universe. "Gamma Ray Bursters" - blasts of intense energy from the farthest reaches of the cosmos - remain a mystery. A closing montage of our Galaxy, the Milky Way, at different wavelengths.

Sub-chapters Radiation Spectrum • The electromagnetic spectrum - radiation at many wavelengths - stretches from the shortest wavelength gamma-rays to the longest radio waves. • Visible light is only a tiny fraction of all possible wavelengths in the electromagnetic spectrum. In fact, all radiation is invisible. We only see visible light because it is reflected off particles in the atmosphere. • Sensing invisible radiation - infrared as heat, ultraviolet as suntan. • If we observe the Universe only in visible light, we miss the big picture. • An orchestra plays - but only a small range of notes are audible. It's like viewing the Universe in visible light alone. Hearing the full range of the orchestra is like observing all the radiation from the Universe.

Radio and Infrared • Radio waves are the longest in the radiation spectrum. Captured in huge radio dishes, the information is translated into images. • Radio waves reveal the turmoil of Centaurus A - the aftermath of a galactic collision. Also detected: the radiation beams of a pulsar, a tiny superdense star. • Infrared radiation is absorbed by water vapor in the air. Earthbound observation is limited to arid mountain tops and frozen wastes of Antarctica. • But like ultraviolet and the longest radio waves, little infrared penetrates Earth's atmospheric shield, so the Infrared Space Observatory (ISO) must work from orbit.

ISO's Universe * The coldest laboratory in the Universe, ISO picks out warm clouds of gas and dust - the birthplaces of stars. It reveals the spiral structure of the Andromeda Galaxy and penetrates the shells of dust that surround a cometary nucleus. • ISO also detects abundant water - vital for life - towards the center of the Milky Way.

Ultraviolet Views • The Hubble Space Telescope can view in visible light, infrared and ultraviolet. • Observations in ultraviolet reveal hot young stars, a cloud of hydrogen enveloping Halley's Comet, the churning surface of our Sun.

The X-Ray Sky • X-ray astronomy unveils the hottest and most energetic regions of the Sun, together with its corona and searing atmosphere. • The orbiting ROSAT observatory, which targets cosmic hot-spots, discovers 60,000 X- ray sources, a black hole at the center of the galaxy M87 and the pulsing and energetic relic of an exploded star.

Gamma-Ray Blasts • The Compton Observatory, launched from Space Shuttle, observes gamma-rays – the hottest, most energetic radiation of all. Compton detects jets of gas shooting from a supermassive black hole and mysterious gamma-ray bursters - blasts of energy from deep space whose origin is unknown. • In Our Galaxy: o Gamma-ray observations reveal cosmic rays colliding with hot gas. o Galactic hot-spots show up in X-rays. o Infrared spotlights clouds of dust. o The Galactic Center emits radio waves. o Finally, in visible light, the majesty of the Milky Way.

Background The Nature of Light After thousands of experiments and centuries of study, scientists still find it difficult to answer the question: What Is Light? The problem is that light is invisible. Before it can be seen, light must be reflected from something. When it enters Earth's atmosphere, light from the Sun is reflected and scattered by tiny particles of dust or water droplets. It is this interation that colors our clouds and makes the sky blue. This effect can be seen at the cinema. The beam of light that passes from the projector to the screen glints through millions of tiny dust particles floating in the air. Each particle is reflecting the light. They enable us to see the beam.

The ancient Greeks carried out the first experiments on the nature of light. Euclid, in the third century BC, knew about the reflection of light. A century later, Ptolemy investigated the refraction of light as it passed from one transparent substance to another. In the 17th century, Isaac Newton attempted to explain the properties of light. He suggested that a light beam consisted of a stream of tiny particles, which he called corpuscles. Newton theorized that light traveled in straight lines. He believed that the reflection of light by a mirror took place as corpuscles bounced off the surface of the mirror.

Newton thought corpuscles were attracted to certain transparent substances and moved faster in them. This was his explanation of refraction - when light passes from air into water it is bent. His notion was that the amount of bending depended upon how much faster corpuscles traveled in water or glass than through air. Newton thought there were different kinds of corpuscle for each color of light.

A little after Newton, the Dutch physicist Christian Huygens had a new idea. Huygens correctly reasoned that light traveled as waves. Drop a pebble into water and waves emanate from the point of impact. Huygens believed that light traveled that way – but that the waves were very small.

Today we understand that the distance between the "tops" of adjacent light waves is a few ten- thousandths of a millimeter. This tiny measurement is called the wavelength of the light. Such waves rise and fall several hundred million million times every second. This is called the frequency of light waves.

Although Huygens had explained reflection, refraction and several other properties of light, it took more than a century for his ideas to win support. The important difference between the ideas of Newton and Huygens was that Huygens' theory needed light to travel more slowly in glass or water than in air. Newton predicted the opposite. The matter was settled in 1850 when the French physicist Jean-Bernard-Leon Foucault showed that light traveled faster in air than in water. So Huygens was right and Newton was wrong. Finally, in the 1860s, the Scottish physicist James Clerk Maxwell showed that light waves were just one form of electromagnetic radiation.

But light still had mysteries. The wave theory did not explain all the properties of light. In 1900, the German physicist Max Planck demonstrated that light travels in separate "packets" of energy known as "quanta" - as do other forms of radiation. Then, in 1904, Albert Einstein suggested that electromagnetic radiation sometimes behaves as though it consists of particles. He called them photons. Thus, modern physicists were forced to recognize that, in some ways, electromagnetic radiation behaves as a wave but, in others, it behaves as though it consists of particles. No longer are the different explanations of Newton and Huygens regarded as rival theories.

Electromagnetic Waves and the Electromagnetic Spectrum From the early 19th century, it was known that visible light travels as a transverse wave where the vibrations move in a direction perpendicular to the advancing wave front. Physicists assumed that the wave needed something to travel through. So they dreamt up an invisible medium called "luminiferous ether".

Then, in the 1860s, James Clerk Maxwell laid out the theory of electromagnetic waves. Such waves, which have both electric and magnetic components, were produced, he postulated, by the oscillation or acceleration of an electric charge. Maxwell declared that light was an electromagnetic phenomenon - and did not need ether as a medium. Nevertheless, the ether notion took some time to disappear because it fitted with the Newtonian idea of an absolute space-time frame for the Universe.

Ether was seriously discredited in 1881 by the American scientists Albert Michelson and Edward Morley. Their work - vital in the development of the theory of relatively – led to the realization that the speed of electromagnetic waves in a vacuum is always the same, regardless of the velocity of the source or of the observer.

Indeed, all electromagnetic waves share two characteristics. Firstly, they need no material medium for transmission - they can move through a vacuum. Thus, light and radio waves are able to travel to Earth through the vacuum of space from the Sun and stars. Secondly, in a vacuum, regardless of their frequency and wavelength, electromagnetic waves all travel at a wave velocity of 299,792 kilometers per second. Any wave which has these two properties is an electromagnetic wave.

Electromagnetic waves form a spectrum - the electromagnetic spectrum - extending from waves of extremely high frequency and short wavelength to those of extremely low frequency and long wavelength. Visible light is a small part of the spectrum. In order of decreasing frequency, the spectrum consists of gamma rays, hard and soft X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves. All components of the electromagnetic spectrum show typical properties of wave motion, including diffraction and interference. The wavelengths range from billionths of a centimeter to many kilometers. The length and frequency of electromagnetic waves are important in determining their heating effect, visibility, penetration and other characteristics.

The Infrared Space Observatory Launched in 1995, the Infrared Space Observatory (ISO) proved both a cool customer and a great success. An international project of the European Space Agency, ISO carried four instruments: ISOCAM - an infrared camera, SWS - a short wavelength spectrometer, LWS – a long wavelength spectrometer, and ISOPHOT - a photometer and polarimeter. Infrared radiation was routed to these instruments by a reflecting telescope with a 60-centimetre aperture.

Most importantly, the telescope was cooled by 2,200 liters of liquid helium in continuous circulation. With a boiling point at below minus 270 degrees Celsius, the helium absorbed heat and vented it from the craft. This kept ISO super-cool - preventing its own infrared emissions from swamping those it was trying to detect from remote cosmic sources. ISO would last as long as its coolant.

ISO was 5.3 meters tall and, at launch, weighed 2.3 tons. Its observations were strictly programmed and executed. Data collected from space was immediately transmitted to ISO's ground station. The craft's elliptical orbit of Earth allowed long observing runs. While observing, ISO was stabilized by dual star-trackers that also provided "pointing" information. On April 8, 1998, ISO's liquid helium ran out - more than ten months after the official "expiry" date. This extra observing time allowed ISO to view more than 26,000 cosmic infrared sources. In part, ISO's longevity was due to the daily loss of helium coolant being 17 per cent less than expected. Among other triumphs, ISO's "pointing" accuracy was ten times better than detailed in the initial specifications and stability was five times better than the acceptable level. ISO viewed the cosmos for between 90 and 95 per cent of the observing time available.

Links for Further Information: Good illustration of the electromagnetic spectrum showing the location and wavelength range of each type of radiation. http://noradcorp.com/spectrum.htm

Deep Space Network radio astronomy page. Impressive information on radio astronomy, along with links to other useful sites and images. http://dsnra.jpl.nasa.gov/

ISO home page - comprehensive site featuring news, mission and spacecraft data, scientific discoveries, links, press releases and an extensive image gallery. http://isowww.estec.esa.nl/

Extreme Ultra-Violet Explorer page, containing information about EUVE, articles, history, daily operations, status information, image gallery and links. http://www.cea.berkeley.edu/~pubinfo/html/EUVE.html

ROSAT home page - news, history of the spacecraft, mission information, data analysis, image gallery and related sites. http://heasarc.gsfc.nasa.gov/docs/rosat/rosogof.html

Compton Gamma Ray Observatory page - general information plus data concerning CGRO's imaging instruments and links. http://erbscobe.gsfc.nasa.gov/CGROHomePage.html

Questions and Activities for the Curious 1. How is the color of light and its wavelength related?

2. Draw a diagram to show the principal regions of the electromagnetic spectrum – from the shortest wavelengths to the longest.

3. Why it is important for astronomers to observe the same astronomical objects at a variety of wavelengths?

4. Why are infrared telescopes sited on mountain tops and in Antarctica?

5. Which gases in the Earth's atmosphere absorb ultraviolet radiation

6. Why is it necessary to study the birthplaces of stars at infrared wavelengths?

7. Which regions of the Sun are best shown in X-ray images?

8. Is light really invisible? It is - and a simple experiment proves the point. Take a cardboard container about the size of shoebox. Make two cardboard tubes about 10 centimeters long and three centimeter in diameter. Cut circular holes precisely at the center of each end of the box, both holes three centimeters in diameter. Paint the inside of the box and the insides of the tubes with black watercolor. Push the tubes into the holes at each end of the box. Fix them in place with tape and make sure there are no holes elsewhere. Firmly glue down the lid of the box. Finally, cut a window in the side of the box and cover the window with a piece of transparent plastic. Take the box into a darkened room and put it on a table. Shine a torch into one of the tubes and hold a piece of white card a short distance away from the opposite tube. You will see that the torchlight reaches the piece of white card. But, if you look through the window of the box, you will see - nothing! The beam of light from the torch is invisible. Outer space is like the inside of the box: total darkness.

THE COMPLETE COSMOS Chapter 02: Lifequest Is there life elsewhere in the Solar System? Currently, on Jupiter's moon Europa? In the future, on Saturn's moon Titan?

Outline A giant star dies in a catastrophic explosion and debris splatters across space. That debris contains heavy elements only produced in gigantic stars. As it travels outwards, the debris encounters clouds of gas and dust. It enriches them with its heavy elements. One such is the cloud from which our Solar System condenses.

Once formed, Planet Earth finds itself in the "habitable zone" of the system - at just the right distance from the Sun for life to be possible. Earth has water and is rich in the chemicals - heavy elements included - essential for life. Earth is the perfect incubator.

Mars lies just beyond the habitable zone. Today, it is a freeze-dried desert, but it may once have been warmer and had oceans. Some scientists think we may find evidence of past life there. On Jupiter's moon, Europa, there could be a warm ocean beneath its icy crust. Hot vents could support primitive life. Saturn's moon, Titan, is like early Earth in deep-freeze. The ingredients for life are present. Heat is all they need to come alive.

Scientists are also searching for life beyond our Solar System. Giant dishes listen for extraterrestrials and robot probes head for the stars. Evidence is mounting that many other stars have planetary systems - formed from clouds of gas and dust that contain water and the building blocks of life. But, so far, none of the planets identified around other stars have the conditions necessary for life.

Sub-chapters Star Death, Planet Birth • Death of a giant star in a supernova explosion. • Shock wave passes through clouds of gas and dust, enriching them with heavy elements from the dead star, and triggering contraction. • From such clouds, new stars are born, and each is the potential center of a new star and planetary system. • Formation and evolution of our Solar System, with a shining star at the center. • Around the Sun are rings of gas and dust that eventually form into planets. • Earth forms in the 'habitable zone' endowed with the necessary conditions for the development of life.

Bountiful Earth, Barren Mars • Comets bring water to Earth. Life is triggered, perhaps, by lightning. The elements from the supernova are essential for living cells. Later, green planets breathe oxygen into Earth's atmosphere. • Mars, unlike Earth, is outside the habitable zone. Today, its temperature is too low to sustain any type of life, but this does not discount past life. There is evidence that Mars was once warmer and had oceans.

Europa's Oceans • Europa, a moon of Jupiter - with a possible warm water ocean beneath an icy crust. • The chance that hot volcanic vents may support primitive life. A possibility for organisms that feed on chemical rather than solar energy.

Titan - Laboratory for Life? • Farther out lies chilly Saturn with few chances for life. Saturn's largest moon, Titan, may be a laboratory for the origins of life. The basic building blocks of life are present, but for the time being Titan is like Earth in deep-freeze. • Five billion years from now, when the Sun swells to become a red giant star, Titan may briefly awaken.

Building Blocks of Life • The search for extraterrestrial intelligence (SETI). Giant radio dishes listen for signs of intelligent life. Robot envoys are traveling towards the stars. • Evidence mounts that other stars have planets. The clouds of gas and dust that pervade our galaxy contain water and organic molecules.

Search for New Planets • Within such clouds, the Hubble Space Telescope images protoplanetary disks that could eventually form planets. Such disks may be distorted by an orbiting planet. • Some stars wobble, a tell-tale indicator of a companion planet - and another star seems to have ejected a planet, but this is later discounted. • Brown dwarfs - too big for a planet, too small for a star. • Of the suspected extrasolar planets so far discovered, none has the conditions necessary for life - as we know it.

Background Signs of Life in a Martian Meteorite In August, 1996, scientists believed they had found evidence that primitive life may once have existed on Mars. The clue came from studies of a 1.9-kilogramme meteorite fragment, labeled ALH84001. It is one a dozen or so meteorites, known as SNC meteorites, found on Earth, but believed to have originated on Mars. Why Mars as the source of these SNC meteorites? The reasoning was that gas pockets trapped inside the meteorites had an almost identical chemical composition to measurements of the Martian atmosphere made by Viking.

ALH84001 is regarded as the most ancient of the SNC meteorites. It is thought to have originated in the ancient cratered highlands of the southern hemisphere of Mars. The fragment is an igneous rock that crystallized slowly from magma about 4.5 billion years ago and was fractured during intensive bombardment 3.8 to 4.0 billion years ago. Later, perhaps around 3.6 billion years ago, it spent some time immersed in water that contained significant amounts of carbon dioxide which penetrated the fractures. The Martian climate then changed leaving the planet barren with no liquid water. Then, about 15 to 16 million years ago, another major impact blasted out rocks (including ALH8400l) from Mars. These entered orbit around the Sun. Approximately 13,000 years ago, the rock which became meteorite ALH8400l fell to Earth in Antarctica - to be found by scientists in 1984.

ALH84001 is slightly friable and highly fractured It has a concentration of carbonates 50 times greater than other SNC meteorites. While under Martian water, small beads of calcium- magnesium-iron-rich carbonates formed in fissures and pores of the rock. There is some debate as to when the carbonates formed. An initial study fixed their age at about 3.6 billion years, but more recent dating indicated a much younger age of 1.3 to 1.4 billion years. The carbonates occur as 'globules', typically about 50 micrometers across, although some are up to 200 micrometers across. Concentrated within the carbonates are tiny grains of magnetite (iron oxide) and iron sulphide. Their occurrence indicates that the water was hot and chemically reducing at the time. This suggests volcanic vents as the formation source - and that is of great to those studying origins of life. Nevertheless, the existence of carbonates and grains of magnetite and iron supplied could be explained by non-biological processes.

Examination of fracture surfaces in ALH84001 show the presence of organic compounds called polycyclic aromatic hydrocarbons (PAHs). There are also tiny tubular or ovoid structures on the surface of the carbonates that have been dubbed 'microfossils'. Interestingly, the mixture of PAH's found on ALH84001 is very different to that found on dust grains and in other types of meteorite, suggesting the possibility of a non-biological origin. Furthermore, the microfossils are 100 times smaller than the smallest microfossils of ancient bacteria found on Earth. Although they are similar in size and shape to fossilized so-called 'nanobacteria', apparently present in some terrestrial rocks, there is considerable skepticism that these objects are fossils. While all the individual pieces of evidence for signs of past (fossil) life in ALH84001 can be explained by non- biological (geochemical) processes, some scientists think it significant that all these pieces of evidence were found together in one sample. They think that the idea of many separate non- biological processes occurring together is highly unlikely. It is this which, in their view, suggests a biological origin. All the evidence requires further rigorous scientific examination to test its authenticity, and the possibility of primitive life existing or having existed on Mars cannot be ruled out.

The Possibility of Life on Europa Many scientists believe that Europa, one of the four largest moons of Jupiter, may be one of the few places in our Solar System where we might find extraterrestrial life. The other are Mars and Saturn's moon, Titan. What makes Europa such an attractive place to look for some form of life is the possibility that it has both liquid water and volcanic activity. Liquid water is essential for life on Earth. Volcanic activity would provide some of the heat necessary to prevent the water on Europa freezing. It could also provide important dissolved chemicals needed by living organisms.

It was NASA's Galileo spacecraft that sent back high quality images of Europa's surface. That is how we know it is covered with ice. These images also hint at a layer of liquid water - a sub- surface ocean - beneath the ice. True, there is still no concrete evidence for such an ocean - but the Galileo has considerably strengthened the case. Firstly, the images provide clear evidence of near-surface melting together with movements of large blocks of icy crust in ways similar to icebergs or ice rafts on the Earth. Secondly, there are very few impact craters on the surface. This suggests that this activity took place recently, geologically speaking. The problem is that we have no precise way to measure the exact age of the surface. It is possible that we are looking at an ancient "frozen" ocean, not one that presently exists. Scientists feel the evidence favors an ocean - but that is by no means conclusive.

As for the possibility of life in Europa's oceans, this depends on when there was an ocean and how long it lasted - including up to the present. When the Viking landers went to Mars in 1976, most life scientists felt that to have a chance for extraterrestrial life you had to have light (for photosynthesis), liquid water, and oxygen. Since then we have discovered places on the Earth (for example, hydrothermal vents on the ocean floor and geothermal hotsprings) where life is currently sustained in the dark, without oxygen, using the heat and chemical energy from volcanic gases dissolved in superheated water. Some of these life forms such as thermophyllic (or heat- loving) bacteria are among the most ancient types of life on Earth.

Many scientists now speculate that Earthly life may actually have arisen under such conditions. So, of course, we are now more interested in places like Mars and Europa where there may have been liquid water and volcanic activity as a place to look for primitive life. If there is an ocean on Europa, there is no easy way to estimate the chances of life could existing there. But that is exactly the question scientists are trying to answer by continued exploration. In other words, we must go and look. It is no easy task. Europa's distance from the Earth and Sun and its thick layer of ice, will make exploration difficult.

For the time being, the Galileo spacecraft will continue to observe Europa. Beyond that, NASA is considering a possible Europa orbiter mission, perhaps with landers. Such a mission might answer the ocean question by combining a number of techniques. One is to use radar to penetrate the ice and measure its thickness. Another is to make very precise gravity and altimetry measurements to observe the tides raised by Jupiter on Europa. These will be much larger - as great as 40 meters or so - if there is a liquid layer.

Titan - An Example of Early Earth in Deep-Freeze Titan is the largest of Saturn's moons. With a diameter of 5,150 kilometers, it is the second biggest moon in the Solar System. Titan is the only planetary moon known to have a dense, opaque atmosphere. Many scientists believe that the chemistry in Titan's atmosphere may be quite similar to that existing on the Earth several billion years ago before life began releasing oxygen into the air. For this reason, Titan may provide clues to the primeval Earth.

Titan's fascination, meant that the Voyager 1 spacecraft was targeted to make a close fly-by in November 1980. Unfortunately, Voyager found that Titan's surface was completely hidden by a thick layer of photochemical haze at about 200 km altitude. Several distinct, detached haze layers were visible above the opaque haze layer. The haze layers merged with the main layer over the north pole of Titan, forming what scientists first thought was a dark hood. Under the better viewing conditions of Voyager 2, the hood was found to be a dark ring around the pole. The southern hemisphere was slightly brighter than the northern, possibly the result of seasonal effects. When the Voyagers flew by, the seasons on Titan were early spring in the northern hemisphere and early autumn in the south.

The Voyagers confirmed that most gas in Titan's atmosphere was nitrogen, as on Earth. The second most abundant was methane. There was also ethane and several other hydrocarbons. The thickness - the extent - of Titan's atmosphere was about ten times that of Earth. Titan's atmospheric pressure near the surface was 1.6 bars, some 60 percent greater than Earth's at sea level. The atmospheric temperature near the surface was about -178 degrees Celsius, only 4 degrees above the triple-point temperature of methane. This is the temperature at which methane can exist simultaneously as a solid, liquid and gas at the same temperature and pressure. Methane, however appeared to be below its saturation pressure near Titan's surface. This means that rivers and lakes of methane probably don't exist, in spite of the tantalizing analogy to water on Earth. All the same, scientists believe that lakes of ethane do exist and that methane is probably dissolved in the ethane.

Through a series of photo-chemical reactions, stimulated by ultraviolet light from the Sun, the methane in Titan's atmosphere is converted to ethane, acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. Such carbon-based compounds represent the first stages in the formation of the building blocks necessary for the formation of life. But Titan's very low temperature almost certainly inhibits more complex organic chemistry. Nevertheless, in some five billion years time, when the Sun swells into a blazing red giant, Titan may receive sufficient heat for more complex reactions to take place. This, however, will be a brief spell. The lifetime of the red giant Sun will probably be too short for the sustained evolution of life.

The Cassini spacecraft will provide more information about Titan. Launched on October 15, 1997, Cassini is due to arrive at Saturn in June, 2004. Later that year, Casini will release the European-built Huyghens probe for a parachute descent through Titan's atmosphere, followed by a touchdown (or splashdown!) on the surface. The Cassini orbiter will then have more than 30 encounters with Titan during its orbital tour of Saturn. The orbiter will map the moon's surface with a synthetic aperture radar similar to that used by the Magellan spacecraft when mapping Venus.|

Links for Further Information Lunar and Planetary Institute site, featuring a balanced view of the debate concerning the evidence for life in Martian meteorites, including explanations of recent scientific papers, with insightful and objective commentaries. http://cass.jsc.nasa.gov/lpi/meteorites/mars_meteorite.html

The possibility of life on Europa and investigations of its surface, with lists of related web sites, books and journal articles. http://www.msoe.edu/~tritt/sf/europa.life.html

The Galileo spacecraft page with many images of Europa and the other moons of Jupiter. http://www.jpl.nasa.gov/galileo

Images of the surface of Europa acquired by NASA spacecraft. http://www.msoe.edu/~tritt/sf/europa.images.html

A page with clues to the possibility of life on Europa, and supporting evidence which may gathered from the Antarctic. http://www-b.jpl.nasa.gov/galileo/news11.html

A page of facts about Titan, with evidence for the possibility of life. http://www.msoe.edu/~tritt/sf/titan.html

A page on the nature of possible life on other planets - including Europa - within our Solar System. http://www.msoe.edu/~tritt/sf/life.html

From Space Science News, a page about looking for life in extreme environments. http://science.msfc.nasa.gov/current/events/ast21aug98_1a.htm

Jean Schneider's comprehensive Extrasolar Planets Encyclopaedia, with latest news, overview of detection methods, searches, bibliography and reports, list of related web sites, and the Extrasolar Planets Catalog, which includes brown dwarfs and all extrasolar planet candidates to be confirmed. http://www.obspm.fr/planets

The Extrasolar Planets Catalog (extracted from the Jean Schneider's site). http://wwwusr.obspm.fr/departement/darc/planets/catalog.html

Questions and Activities for the Curious 1. When massive stars die in supernova explosions, why is this important for the formation of planets and the evolution of life?

2. Explain what is meant by the term "habitable zone". Give examples of one planet which lies in the habitable zone around our Sun and one that is outside.

3. Describe the conditions on the young Earth. Show how they were very different from the Earth we know today.

4. Explain why some scientists believe we might find primitive life on Jupiter's extraordinary moon Europa.

5. Research the scientific program known as SETI (Searches for Extraterrestrial Intelligence). Give arguments for and against this type of research activity.

6. What message would you send to an alien civilization and why?

7. If astronomers did detect conclusive signs of life on another planet, what effects do you think this would have on our society?

8. Describe the search for extrasolar planets - the hunt for planets around other stars. Give two examples of such planets. Say whether you think the conditions are right for life as we know it to have evolved there.

THE COMPLETE COSMOS Chapter 10: Jupiter Bigger than the other planets combined, a turbulent gas giant with 16 moons - a voyage through this mini Solar System.

Outline Larger than all the other planets combined, Jupiter lies 780 million kilometers from the Sun, has an almost 12-year orbit, and the shortest planetary day. Eleven Earths are needed to span its diameter and 1,300 to fill its volume. Jupiter captures Comet Shoemaker-Levy 9. Its fragments impact the planet.

The twin Voyager spacecraft pass the giant planet in 1979. Detailed images of Jupiter acquired by the Voyagers, including the Great Red Spot, a tempest three times larger than Earth. Ten years later, the Galileo probe parachutes into Jupiter's clouds and makes further discoveries before being crushed by atmospheric pressure. Jupiter's 30,000-degree Celsius core generates more heat than the planet receives from the Sun - and drives 500 kilometer-per-hour winds. Internal structure and its magnetic field. Galileo's orbital tour of Jupiter's 16 moons, focusing on the four majors - Io, the most volcanically-active body known; frozen Ganymede, largest moon in the Solar System with a surface temperature of minus 150 degrees; Callisto, Ganymede's crater- scarred twin; and Europa, icy world with a possible sub-surface ocean that may contain extra- terrestrial life. Jupiter and its moons are a mini solar system.

Sub-chapters Largest Planet • Jupiter, a giant ball of gas, bigger than all the other planets combined. • Spinning like a top, Jupiter's short day, orbit and distance from the Sun. • Jupiter/Earth comparisons: 11 Earths are needed to span Jupiter's diameter, and 1,300 to fill its volume. • Jupiter's role as a "cosmic vacuum cleaner", Jovian gravity capturing comets and drawing them into the planet. • Brief overview of the Comet Shoemaker-Levy 9 story.

Weather Bands and Storms • The two Voyager probes, launched in 1977, reach Jupiter two years later. • Whirling belts of weather, storms racing around the planet, and its most famous blemish, the Great Red Spot - an anti-cyclone much larger than Earth.

Descent into Hell • Galileo's circuitous route to Jupiter. The probe's descent through the Jovian clouds - buffeted by 500-kilometre-per-hour winds (driven by heat from Jupiter's core), and eventually crushed by atmospheric pressure. • Jupiter's composition: 90 per cent hydrogen, the rest mainly helium. The planet's atmosphere - layer by layer - and its internal structure, down to the rocky core. • Generation of Jupiter's magnetic field in a layer of liquid metallic hydrogen. • Jupiter's magnetosphere - after the Sun, the biggest entity in the Solar System. • Auroral halos around the Jovian poles, created by the solar wind.

Volcanic Io • Galileo's tour of Jupiter's four largest satellites. First stop, Io - flexed by the pull of Jupiter and neighboring moons. An extraordinary colored surface and erupting volcanoes.

Ganymede and Callisto • Ganymede, the Solar System's largest moon with a thick icy crust. The magnetic field detected by Galileo, suggests an internal structure of slush, rock and ice. • Callisto, Ganymede's twin, but heavily cratered with a multi-ringed impact basin.

Icy Europa * Europa, a possible water-world. Is a salty ocean hidden beneath its cracked, icy surface? * Tidal flexing may warm the waters, making Europa an incubator for life. With the surface ice so thick, any search for life must await a landing.

Background The Great Red Spot A large, reddish oval feature called the Great Red Spot is usually visible in Jupiter's southern hemisphere - lying within the planet's south tropical zone. Since the spot was first observed in the mid-17th century, observers have reported many variations in its size and color. At its largest, the spot was so huge that three Earth's could have fitted side-by-side along its length. In the mid- 1970s, however, the spot faded from view. At the time of the Voyager flybys in 1979, the Great Red Spot was only slightly larger than Earth.

The Great Red Spot is a very long-lived storm in the planet's ever changing atmosphere. Observations of cloud motions in and around the spot show that it spins anti-clockwise, completing a full revolution in about six days. Furthermore, winds to the north of the spot blow from the east, those south of the spot blow from the west. Consequently, the Great Red Spot is rather like a wheel spinning between two surfaces moving in opposite directions, producing a surprising stable wind pattern. The spot is a high pressure weather system that protrudes above the surrounding visible cloud tops.

Jupiter's Volcanic Moon In 1979, the Voyager 1 spacecraft flew past Io, the innermost of Jupiter's four large moons, uncovering an extraordinary world. The surface displayed a broad range of colors from white, yellow and orange, to black, which scientists jokingly compared to a cheese and tomato pizza, or a rotten orange. A major clue to this puzzling appearance was found several days after Voyager's Jupiter flyby when a scientist noticed a large umbrella-like plume protruding from Io in one photograph - an erupting volcano.

Careful examination of Io images obtained by both Voyagers 1 and 2 - and the more recent Galileo spacecraft - have shown that Io's surface is pocked with numerous volcanic vents from which eruptions occur. These vents appear as black spots typically ten to 50 kilometers in diameter, many with radiating lava flows. The plumes and fountains of material spewing from Io's volcanoes rise to heights of 70 to 300 kilometers above the surface. To reach these altitudes, material must be shot out of the vents at speeds of between 300 and 1000 meters per second.

The plumes from Io's volcanoes contain particles of sulphur and the acrid gas sulphur dioxide. When this gas is erupted into the vacuum of space it crystallizes into white crystals of sulphur dioxide frost or snow. It is the sulphur ejected from Io's volcanoes that causes the brilliant colors. Although normally bright yellow, when heated and suddenly cooled, sulphur can assume a range of colors from orange to red and black. Io's volcanic vents are not located at the tops of volcanic peaks, as on Earth, but are more akin to terrestrial geysers, similar to those at Yellowstone Park in the USA. Both sulphur and sulphur dioxide will be molten at depths of a few kilometers, but as they are forced up to Io's surface, they are explosively converted from a liquid to a high pressure gas. The heating of Io's interior is a consequence of the continual squeezing and flexing of the moon by tidal stresses caused by the gravitational tug-of-war between Jupiter, on one side, and Europa and Ganymede on the other.

Jupiter's Water-Worlds At least two of Jupiter's four largest moons are probably water-worlds, with hidden salty oceans beneath their icy surfaces. One of the moons, Europa, has long been speculated to have a sub- surface ocean of slushy or liquid water. Europa's water is kept warm by powerful gravitational tidal forces exerted by both Jupiter and the moons Io and Ganymede. Although nothing could survive on Europa's frozen crust, which looks like Arctic pack ice, scientists have postulated that water-based organisms, probably of a primitive microbial kind, could exist in Europa's hidden ocean. Measurements have shown that the liquid layer might be up to 100 kilometers deep, beneath an icy crust several kilometers thick.

More surprising was the discovery in October, 1998, that another Jovian moon, Callisto, may also have a hidden salty sea. Callisto was previously thought to be a solid sphere of rock and ice. A liquid ocean on Callisto, however, seems to be the only explanation for data sent back by the Galileo spacecraft while in orbit around Jupiter. Observations by Galileo have confirmed that neither Europa nor Callisto have an appreciable internal magnetic field. But evidence from the spacecraft shows that both moons disturb Jupiter's magnetic field as they pass through it. The only way this could happen is if electrical currents flowing within the moons induced a magnetic field. This would require sub-surface layers on both moons that are very good conductors of electricity. Deep salty liquid oceans on both moons seem to be the only adequate explanation.

Callisto is not subject to the same tidal forces as Europa. So flexing cannot be the reason that Callisto's sub-surface water - if it exists - doesn't freeze. Explanations could be warming from radiation, high pressures, eddy currents or the presence of some kind of "anti-freeze" in the form of salts or ammonia.

There may also be a sub-surface ocean on Ganymede. But Gannymede has an appreciable internal magnetic field which would obscure any induction effect. So, for the time being, there is no way of proving that Ganymede has a layer of water beneath the surface.

Links for Further Information Comprehensive Jupiter page, information on its atmosphere and internal structure, Jupiter's moons, facts, missions to the planet, recent news and images. http:// www.windows.umich.edu/cgi-bin/tour.cgi?link=/jupiter/jupiter.html

Good Jupiter page, images of the planet and moons with text links. http://pds.jpl.nasa.gov/planets/welcome/jupiter.htm

Voyager 1 mission page, including information on the mission, extensive images and FAQs. http://nssdc.gsfc.nasa.gov/ imgcat/html/mission_page/JP_Voyager_1_page1.html

Voyager 2 mission page, as above. http://nssdc.gsfc.nasa.gov/imgcat/html/mission_page/JP_Voyager_2_page1.html JPL's Galileo homepage, including up-to-date information on Galileo's orbital tour, details of the mission and the craft, background and images of Jupiter and its moons, plus frequently asked questions. http://www.jpl.nasa.gov/galileo/

Questions and Activities for the Curious 1. If the Earth were the size of a tennis ball, how big would Jupiter be on the same scale? And using this scale, how big would the Sun be and how far would be Jupiter from the Sun?

2. Describe the appearance of Jupiter's atmosphere. What kinds of features would you expect to see through a telescope?

3. The Galileo spacecraft, launched to Jupiter from the Space Shuttle in 1989, took more than six years to reach its destination. Describe its circuitous route from Earth to Jupiter, and research why it was necessary to use this route?

4. Discuss the various layers that make up Jupiter's atmosphere and its internal structure. Where is its magnetic field produced?

5. Imagine there had been a camera aboard Jupiter's Galileo probe as it parachuted through the outer layers of the planet's atmosphere. Describe what this camera might have seen during the probe's hour long descent, before it was destroyed.

6. Why do you think astronomers believe that Jupiter does not have a large, iron-rich core, even though the planet possesses a strong magnetic field?

7. Describe the colorful surface of Jupiter's moon Io, and explain how volcanic eruptions are responsible for its unusual appearance.

8. Describe the similarities and differences between the four largest moons of Jupiter.

Complete Cosmos Chapter 23: Infinity The structure of the Universe - galaxies, clusters, strands. How we measure to a nearby galaxy and to the farthest quasar. Outline In the Australian night sky, the unaided eye picks out two satellite galaxies of the Milky Way - the Large and Small Magellanic Clouds. Within the Large Cloud, in 1987, a supernova erupts - the first naked eye supernova for nearly 400 years.

We travel beyond - on a tour of the Milky Way's immediate neighbors. From the closest major galaxy, the Andromeda spiral, to more distant spirals like the beautiful Sombrero Galaxy, 40 million light years distant, and the elliptical radio galaxy Virgo A, a member of the Virgo Cluster - part of a "local supercluster" of about 1,000 galaxies. Galaxies exist in clusters, which gather as superclusters, which form strands across the Universe.

The cosmos is measured by "standard candles" - celestial objects of known intrinsic brightness or luminosity. They are used to estimate distance when their apparent brightness is compared to their known luminosity. Cepheid variables are an example.

Using the Hubble Space Telescope, such stars have been found in galaxies up to 80 million light years distant. Cepheids enable astronomers to estimate the distances of the galaxies.

Farther than 80 million light years, another standard candle is needed. It is found in a binary system - a white dwarf nearing the end of its life. The dwarf draws material from its stellar partner, reaches a critical mass, and explodes as a Type 1a supernova. The flash intensity is always the same. This Type 1a supernova is the standard candle that enables astronomers to measure distances up to a 100 times farther than with Cepheids. Edwin Hubble discovers the Universe is expanding - with galaxies speeding away in all directions. When galaxies race outwards, the lines in their spectra are shifted into the red. This "redshift" sizes up the cosmos and suggests that the distance to its farthest reaches is some 15 billion light years.

In the 1960s, the discovery of superluminous quasars - the most energetic objects in the Universe, and among the oldest and most distant. A review of radio galaxies, Seyfert galaxies and galaxies that swing through each other. Some merge to form supergalaxies. Such interactions can trigger intense bursts of star formation. Within ten billion years, our Milky Way galaxy will collide with the Andromeda galaxy.

Sub-chapters

Nearest Galaxies • In the Australian night sky - the Small and Large Magellanic Clouds, two mini satellite galaxies of the Milky Way. • A supernova in the Large Magellanic Cloud, seen in 1987. The explosion happened 160,000 years ago - but it's taken that long for its light to reach Earth. • Andromeda, the closest major galaxy - at over two million light years distant. • The Silver Coin Galaxy - a spiral 10 million light years away, possibly like our own galaxy. • Farther out, to the Sombrero Galaxy, 40 million light years from us. Incredible distances, but relatively close to home.

Clusters and Superclusters • The elliptical galaxy Virgo A, a member of the Virgo Cluster of galaxies, about 60 million light years distant. The cluster is part of a supercluster, containing about 1,000 galaxies. • Superclusters form strands across the Universe. To construct a model of these strands, astronomers use "standard candles", objects of known intrinsic brightness – or luminosity - to measure distance.

Standard Candles • Cepheid variables are standard candles. The brightness of these stars pulsates with consistent regularity as Cepheids expand and contract. Massive and bright Cepheids pulsate slowly, smaller and dimmer Cepheids more quickly. • By measuring the true distance and brightness of nearby Cepheids, the distance to remote Cepheids - which appear fainter in distant galaxies, but pulsate with the same regularity - can be calculated. • Ground-based telescopes only detect Cepheids up to 15 million light years away. The Hubble Space Telescope observes Cepheids to a distance of 80 million light years.

Type-One Supernovae • Beyond 80 million light years, another form of standard candle - Type 1a supernovae. These occur in binary systems where a white dwarf star draws material from its companion. On reaching a critical mass, the dwarf explodes as a supernova. Since the flash intensity is always the same, these supernovae can be used as standard candles. • Type 1a supernovae allow astronomers to measure distances up to 100 times farther into space than is possible with Cepheids. • In the 1920s, Edwin Hubble discovers the Universe is expanding, with galaxies racing out in all directions. As a galaxy speeds away, its spectrum is shifted towards red.

The farther away the galaxy, the faster it recedes and the greater its redshift. • Redshift is a vital tool for sizing and shaping the Universe. Current estimates suggest the distance to the farthest reaches of the Universe is 15 billion light years.

Quasars • Quasars are discovered in 1963, as astronomers locate sources of intense radio noise. At first, quasars appear as ordinary stars, but their redshifts put them billions of light years distant. • Radio galaxies and Seyfert galaxies. Like quasars, all three are fuelled by supermassive black holes. • Black holes at the heart of quasars are immense, consuming the equivalent of 600 Earths a minute. Quasars are the most energetic objects in the Universe - and among the oldest and most distant, with enormous redshifts.

Colliding Galaxies • Colliding and interacting galaxies come in all shapes and sizes. • The Antennae Galaxies are an example not of collision, but of two galaxies swinging through each other, like cosmic pendulums. Their gravitational interaction ejects two great tails like the antennae of an insect. The Antennae are a firestorm of starbirth. • The Cartwheel Galaxy is an example of an intruder galaxy ploughing into another larger - vast disruption. • Most spectacular of all is a collision from which neither galaxy emerges – both merging to form a supergalaxy. In ten billion years' time, this may happen to the Milky Way as it collides with Andromeda.

Background Redshift - A Measure of Distance The wavelength of visible light - or of any other electromagnetic radiation – is stretched to a longer wavelength when the source of that light is moving away from the observer. This phenomenon also affects the wavelengths of any dark absorption lines or bright emission lines in the spectrum of a galaxy - a spectrum that is, in fact, an average of the spectra of every star in the galaxy.

If a galaxy is moving away from the observer, the wavelengths of all the lines in its spectrum will be stretched to longer wavelengths. The lines, therefore, will appear shifted towards the long wave or red end of the spectrum. This is called the redshift. The greater the speed the galaxy is moving away from the observer, the greater will be the redshift.

Redshift is a most important tool in cosmology. It demonstrates that galaxies are receding from us and that the Universe is expanding. This was discovered by the American astronomer Edwin Hubble. Using the 100-inch Hooker reflector at the Mount Wilson Observatory, California, Hubble observed that all galaxies - apart from those in our immediate neighborhood - show redshifts in their spectra. The more distant the galaxy, the greater the redshift. In other words, the farther, the faster.

Later, Hubble found that galaxies were receding from us at speeds that were proportional to their distances from us. This was formulated in what came to be known as Hubble's Law. In reality, the galaxies are not receding from us at all. No matter from which galaxy you observe the expansion, you will see all the other galaxies receding with speeds proportional to their distances.

Initially, the cause of the redshifts in the spectra of galaxies was misinterpreted. It was thought that galaxies were racing through space as though blasted outwards by some huge explosion. Later it was realized that the Universe appears to be expanding not because the galaxies themselves are moving through space, but because the space between the galaxies is expanding.

As light emitted by the galaxies traverses this expanding space, its wavelength is increased, producing a redshift. The redshift is consistently proportional to distance and is a vital tool for estimating cosmological distances. Measurements for relatively close-by galaxies are compared with galaxies farther afield. Although there is some debate over the exact relationship between speed of recession and distance, there is no question about the usefulness of redshift in measurement.

A good analogy for understanding the concept of the expanding Universe is to think of a currant bun baking in an oven. Each currant is a galaxy and the dough is the space between them. As the bun cooks, it expands and the currants move away from each other. The currants farthest away from any single currant seem to move away faster. There is no real center to the expansion. Furthermore, the currants are not moving through the dough - the inflating dough is pushing them outwards. The cosmological redshift is a result of a similar process on a truly Universal scale.

The Rate of Expansion and the Age of the Universe In the 1920s, Edwin Hubble found that for all galaxies outside our local group, the lines in their optical spectra were shifted towards the red or longer wavelengths. Today, most astronomers support Hubble's interpretation - that each galaxy or cluster of galaxies is receding from every other one at speeds proportional to their distances. If galaxies are moving apart now, they must have been very close together at some time in the past. This idea is central to the theory of the Big Bang. The relationship between speed of recession and distance is known as Hubble's Law - and the constant of proportionality is called Hubble's Constant, which is generally quoted in units of kilometers per second per megaparsec (km/sec/Mpc).

Assuming that the Universe has been expanding at constant speed, it is possible to calculate the age of the Universe simply by dividing the distances of the galaxies by their speeds. This interval is called the Hubble Time. If the mutual gravitational attraction between galaxies is gradually slowing the rate of expansion, the galaxies must have been moving apart faster in the past than they are now. In this case, the actual age of the Universe will be less than the Hubble Time.

Although measurements of Hubble's Constant have been made for over 60 years, it is not known as precisely as astronomers would like. Its value probably lies somewhere between 50 and 100 km/sec/Mpc, which implies that the age of the Universe is between ten and 20 billion years. The large uncertainty in the value of the Hubble Constant and, consequently, in the value of the Hubble Time, arises principally from difficulties in determining the distances to galaxies which are far enough away for their motion to be dominated by the expansion of the Universe, rather than by "local" gravitational effects.

The Cosmological Distance Scale Close to home, the distances of galaxies can be determined from the apparent size of so-called HII regions - glowing clouds of hydrogen gas where stars are forming. Distances can also be measured from the properties of Cepheid variable stars. These vary rhythmically in brightness over several days and there is a direct link between a Cepheid's pulsation rate and its true brightness. Once a Cepheid's light variations are measured and its true brightness found, its distance can be calculated. At greater distances, astronomers can use the observed explosions of Type 1a supernovae. These can be seen at 100 times farther into space than Cepheid variables. In 1992, using the Hubble Space Telescope (HST), astronomers observed Cepheid variables in the faint spiral galaxy IC 4182, 16 million light years away. Back in 1937, a type 1a supernova also occurred in this galaxy. Both observations enable a tying together of the Cepheid and Type 1a supernova cosmological distance "ladders". They indicate a value for the Hubble Constant of about 45 km/sec/Mpc, implying the Universe is between 14 and 20 billion years old. This age is compatible with the ages of the oldest known stars, but contradicts other work, which favors a somewhat higher value for the Hubble Constant. In December 1993, during the first Servicing Mission, a new, much improved Wide-Field / Planetary Camera was installed on the Hubble Space Telescope. Subsequently, astronomers were able to detect Cepheid variables in the large Virgo Cluster of galaxies, estimated at about 60 million light years distant, as well as in galaxies out to 80 million light years. A problem soon arose when some measurements indicated a value for the Hubble. Constant consistent with an age for the Universe of only 11 billion years, younger than the age of the oldest stars known!

The Hipparcos satellite helped to resolve the issue. Its precise measurements of the distances to stars indicated that the Universe is between 13 and 15 billion years old. Hipparcos also reduced the age of the oldest stars to between 12 and 13 billion years, so the conundrum disappeared. Astronomers plan to improve their estimates of the Hubble Constant by using the Hubble Space Telescope to detect Cepheid variable stars in other galaxies which have had recent Type 1a supernovae. The ultimate goal is to refine the value of the Hubble Constant - and hence the scale and age of the Universe - to within ten per cent.

At greater distances, the methods of distance measurement become increasingly inaccurate. Eventually distances can only be estimated by measuring the redshift and relying on the accuracy of Hubble's Law. Unfortunately, for the most distant objects – particularly quasars - we cannot be absolutely certain that this method is valid.

Links for Further Information A page of introductory information about quasars. http://www.ph.unimelb.edu.au/~bholman/qso/qso1.html

Frequently asked questions about quasars - a good and detailed page. http://www/phys.vt.edu/~astrophy/faq/quasars.html

Mapping the distant Universe. An educational page detailing new methods for identifying high redshift galaxies - clear explanations, plus images and diagrams. http://astro.caltech.edu/~ccs.ugr.html

Good picture of a quasar-galaxy collision, with useful accompanying text and links. http://www/sai.msu.su/apod/ap951022.html

Close-up picture of the Antennae Galaxies, with accompanying text and links. http://phys.suvon.ac.kr/~kdh2/tdpic/ast971027.html

Introductory information about the Cartwheel Galaxy, including definitions of terms and a description of events. http://www.ccsn.nevada.edu/other/Planetarium/galaxy.html

Photograph of the Cartwheel Galaxy with accompanying text. http://antwrp.gsfc.nasa.gov/apod/ap950702.html

Image of the Andromeda Galaxy with explanatory text and links. http://www/phy.mtu.edu/apod/ap961009.html

Image of the Sombrero Galaxy with text and links. http://www.phy.mtu.edu/apod/ap980223.html

Questions and Activities for the Curious 1. Why are the Magellanic Clouds so named?

2. Give examples of spiral, elliptical and irregular galaxies and describe the appearance of each type.

3. What is the Local Group of galaxies? Name some galaxies from within this grouping, apart from the Milky Way.

4. In what way are Cepheid variables used to measure distance?

5. How far into space can Cepheids help astronomers measure distances from ground-based telescopes and from the Hubble Space Telescope?

6. What is a Type 1a supernova and how is it used as a "standard candle"?

7. What is redshift and how did Edwin Hubble use it to demonstrate that the Universe was expanding?

8. What happens when galaxies collide with each other.

THE COMPLETE COSMOS Chapter 18: Impact! The threat of comets and asteroids. What would happen if the spacerock which slew the dinosaurs hit New York today.

Outline The asteroid belt - and the ways that asteroids can be knocked into planet-crossing orbits. Two major impacts on planet Earth. In the most recent, some 65 million years ago, a mountain-sized rock is ejected from the asteroid belt and strikes Earth – slaying the dinosaurs. In the earlier, about four-and-a-half billion years ago, Earth is hit by a planetesimal the size of Mars. Debris splashes into space, some of which forms our Moon. The Moon's airless, cratered surface is testament to billions of years worth of impacts.

Comet Shoemaker-Levy 9's collision with Jupiter in 1994. Without Jupiter's powerful gravity, more comets would threaten Earth. In 1972, a spacerock skips off Earth's atmosphere and back into space. In 1992, a fireball breaks up over America's East Coast. In 1908, a cosmic missile explodes over Siberia with the destructive power of a huge nuclear bomb.

Space missions to assess the threat. The NEAR probe investigates Eros, an Earth-approaching asteroid 40 kilometers across. Rosetta, to be launched in 2003, will fly by two asteroids and a comet. Methods for protecting Earth from incoming dangers – a mirror that concentrates sunlight on to asteroids, solar sails which tug away intruders. As a last resort, the nuclear option - attaching a warhead to a rocket and firing it at an approaching asteroid.

The nightmare scenario - a comet from the Oort Cloud surprises us. A 100-million megaton blast vaporizes New York City opening up a crater 200 kilometers wide. As cosmic winter descends, the final curtain falls on humanity.

Sub-chapters Cosmic Collisions • Between Mars and Jupiter lies the asteroid belt. Two asteroids collide, an ejected fragment is drawn towards the Sun. A nudge from Mars' gravity knocks it Earthwards. This ten-kilometer projectile impacts Earth. Doomsday for the dinosaurs. • Four-and-a-half billion years ago, a Mars-sized planetesimal strikes Earth. The debris falls back to Earth or gathers to form the Moon. Over eons, the Moon slows Earth's spin and accelerates the evolution of life.

Impacts on Moon and Jupiter • The Moon's surface, pockmarked by 30,000 craters. With no wind or water, every impact is preserved. Earth's surface would look the same, if stripped of atmosphere, oceans, vegetation and geological movement. • Comet Shoemaker-Levy 9's impacts with Jupiter in 1994. Discovered in 1992, after the comet is drawn close to Jupiter and is torn apart by the planet's gravity. Without Jupiter's pulling power, more comets would threaten Earth.

Target Earth • In 1972, a spacerock, 80 meters wide, traveling at ten kilometers a second, crosses the sky. It could have destroyed a city, but skips off the upper atmosphere and returns to space. • A fireball fragments over America's East Coast in 1992. Fragments drop to the Earth as meteorites, the name for any cosmic debris reaching the ground. • Tunguska, Siberia, 1908. A fragment of a comet or asteroid, 60 meters across, explodes in the atmosphere like a nuclear bomb. More than 2000 square kilometers of uninhabited forest is flattened.

Asteroid Hazard • More than 100,000 asteroids are in Earth-crossing orbits - at least 2000 are large enough to pose a threat. • The NEAR craft that investigates the 40-kilometre asteroid Eros in 1999. An asteroid less than a quarter its size would threaten life on Earth. • In July 1999, the Deep Space 1 probe intercepts asteroid 1992 KD, sending pictures back to Earth. Currently in a Mars-crossing orbit, gravitational perturbations could send it Earthwards. • Rosetta, to be launched in 2003, will pass two asteroids and fly with comet Wirtanen for two years. A lander will touchdown on the surface of its nucleus.

Fighting Back • For every asteroid and comet known to travel in near-Earth space, 20 more are yet to be discovered. Various ideas for fighting incoming threats have been proposed. • A parabolic mirror could be towed into space. Concentrating sunlight onto a threatening asteroid, it could scorch it off course. • Huge mylar sails could be attached to the asteroid, to catch the solar wind, and tug the rock into a new orbit. • Another solution - a mass-driver is attached to an asteroid to shunt it away from Earth. • The nuclear option is our only quick response we have to an imminent impact - attaching a warhead to a rocket and firing it at the asteroid. But launching rockets with a nuclear cargo carries inherent dangers.

Checking for Intruders • Currently, 90% of our skies go unchecked. Suppose a comet, dislodged from the Oort Cloud by a gravitational disturbance, took us by surprise. • At ten kilometers wide, the comet would increase speed as it approached the Sun. Upon striking Earth, it would be traveling at 20 kilometers per second. • Accompanying the 100 million megaton blast and 200 kilometer wide crater, massive earthquakes and tsunami would sweep the globe. • Cosmic winter would probably finish those of us who survived the initial destruction.

Background The Tunguska Event Siberia, June 30,1908, 7:17 am local time. In the Podkamennaya Tunguska region, a fireball screeched across the sky. Eyewitnesses said the ball of fire touched the horizon and a "tongue of fire" rose into the air. An explosion was followed by tremors and massive bangs, heard over 800 kilometers away. Closer in, hot winds blew with hurricane ferocity.

The cause of this catastrophe was an asteroid or cometary fragment exploding in the atmosphere. The subsequent air-blast wave was recorded by meteorological barographs thousands of kilometers away in England. Seismograph stations detected a magnitude-five earthquake. In the days that followed, abnormally bright nights were recorded, outshining all but the most luminous stars, across Europe.

The turmoil of World War One and the Russian Revolution meant that an expedition to the remote, uninhabited site did not occur until 1927. Led by Leonid Alekseevich Kulik, a meteorite expert at the Mineralogical Museum in St. Petersburg, the team discovered a vast area of destruction, approximately 2,200 kilometers square. Within the central 15 kilometers, many forest trees stood stripped of branches. Farther out, trees were snapped, or were felled in a radial pattern up to distances of 40 kilometers from the end point of the fireball's trajectory. Thermal radiation caused the spontaneous combustion of large numbers of trees in this region. No fragments of an impacting body or impact crater were ever found. The theory is that the incoming fragment of a comet or asteroid was 50-100 meters in diameter, exploding in a terminal burst about eight to nine kilometers above the ground, with an energy equivalent to 15-20 megatons of TNT. Near-Earth Asteroids.

Near-Earth asteroids (NEAs) are sub-divided into three categories, Apollo, Amor and Aten asteroids, depending on the precise characteristics of their orbits. Near-Earth asteroids all have perihelion distances less than 1.3 AU. Members of the Amor group cross the orbit of Mars - often called 'Mars crossers' - but do not quite reach Earth's orbit, and are occasionally referred to as "Earth-approaching asteroids".

Apollo and Aten asteroids do cross Earth's orbit. They are known as "Earth-crossing asteroids". The three groups are not entirely separate from one another. Planetary perturbations can shift an asteroid's orbit from one group to another. The NEAs are mainly small objects, ten kilometers or less in size. Compositionally, the NEAs span all common asteroid types. They originate from main-belt asteroids, through a mixture of collisional fragmentation and chaotic orbital dynamics.

The number of known NEAs is constantly increasing. The total number of NEAs with diameters over 100 meters is estimated at more than 100,000. Only a few hundred currently known. Most Earth-crossing asteroids will eventually leave the Solar System, due to planetary perturbations. Almost all the rest will collide with one of the terrestrial planets. One-third are calculated to collide with the Earth.

Asteroids and Comets in Close-Up The Near-Earth Asteroid Rendezvous (NEAR) mission is the first spacecraft to orbit an asteroid and make comprehensive measurements of its composition and structure. Crucial questions about the nature and origin of near-Earth asteroids will be answered.

Launched on February 16, 1996, NEAR encountered the main belt asteroid 253 Mathilde on June 27 1997, and met the Earth-approaching asteroid Eros in 1999. Eros was spotted in 1898, the first Mars-crossing asteroid to be discovered It is currently a member of the Amor group. In relation to other NEAs, Eros is a colossus. With dimensions of 36 x 15 x 13 kilometers, it is almost four times larger than the asteroid thought to have hastened the extinction of the dinosaurs 65 million years ago.

In 1931, Eros came within 0.15 AU (23 million km) of Earth, an astronomical close shave. The most recent close encounter was in 1975. The perihelion distance of Eros is currently 1.13 AU, its aphelion distance 1.78 AU. Eros has a rotation period of 5.27 hours. Instruments aboard NEAR are recording the size, shape, volume, mass gravity field and spin of Eros, plus its elemental and compositional make-up, geology, topography, mass distribution and magnetic field.

The Rosetta mission, to be launched in 2003 or 2004, will observe the short-period comet Wirtanen over a long period of time. Observations of its behavior in relation to its distance from the Sun will be carried out. The plotted route for Rosetta will be once past Mars and twice past Earth, negotiating two asteroids en route. Rosetta will rendezvous with comet Wirtanen in 2011, when it is at a distance of 5 AU from the Sun, remaining with the comet from then on. Two probes will be deployed on to the comet's nucleus in 2012, with the comet passing perihelion in August 2013. Instruments on the spacecraft will monitor surface processes on the nucleus and investigate the composition of the cometary dust, gas and ices, along with the plasma environment around the comet. Instruments on the probes will analyze the surface material of the icy nucleus.

Studying this primitive cometary matter may aid understanding of the most basic composition of the Sun and planetary system. The significance of the mission has been likened to that of inscriptions on the Rosetta Stone, from which ancient hieroglyphic writing was deciphered - hence the choice of name for the mission.

Links for Further Information The Spacewatch page. Good site, with information on Near-Earth-asteroids, including a complete list of discoveries. Very good image section, including images of comets and asteroids with accompanying text. http://xlr8.lpl.arizona.edu/spacewatch/

Catastrophism. An incredibly detailed page concerning the threat of impacts and how comets and asteroids have helped to shape the Solar System and planets. Includes extensive links and images. http://pibweb.it.nwu.edu/~pib/catastro.htm

Comet Shoemaker-Levy 9 home page. Comprehensive site featuring the discovery of SL9, its impacts with Jupiter, plus a good gallery of images. http://manbow.ori.u-tokyo.ac.jp/tamaki-html/sl9/sl9.html

Rosetta Home Page, including mission objectives, key facts, spacecraft information, and image gallery. http://sli.esa.int/rosetta

The Near-Earth Object home page. http://cfa-www.harvard.edu/cfa/psNEO/TheNEOpage.html

The Probability of Collision with Earth. No images, but informative text on previous Earth impacts, and the possibility of collisions in the future. http://bang.lanl.gov/solarsys/comet/appendc.htm

Questions and Activities for the Curious: 1. Describe the location of the asteroid belt. Explain how some asteroids end up in Earth-crossing orbits.

2. Research the theory that the extinction of the dinosaurs was caused by a major cosmic impact 65 million years ago.

3. Show how the Moon may have been formed by a major impact of a Mars-sized object with Earth.

4. Is there evidence that the Earth has been hit by asteroids or comets in the past?

5. What is thought to have caused the flattening of pine trees over a huge area of Siberia in June 1908?

6. Assuming an asteroid was discovered on a collision course with Earth, what could be done to tackle the problem.

7. Explain why it is possible for a comet approaching us from the edge of the Solar System to be on a collision course with Earth and yet remain undetected until it is too late.

8. Predict what would happen if a ten-kilometer diameter asteroid or comet was to hit Earth.

THE COMPLETE COSMOS Chapter 22: Hubble's Eye After astronauts fix its faulty optics, the Hubble Space Telescope peers back through time to the depths of the cosmos.

Outline Images of the cosmos from the Hubble Space Telescope. In orbit high above Earth's blurring atmosphere, Hubble's pictures are spectacular. Seeing into deep space is do with gathering sufficient light - how it's collected and focused in a telescope, a comparison with the function of the human eye.

The largest telescopes on Earth - like the Keck in Hawaii and the Very Large Telescope (VLT) in Chile - overcome atmospheric distortion. They use clever optical systems to produce pin-sharp images. But for the best observing, there is nowhere like the clarity of space.

Orbital observing began in earnest in 1990 with the launch of the Hubble Space Telescope (HST). It promised pristine and detailed images that would dramatically improve our view and understanding of the Universe. But there was a problem with HST's main mirror. The images were fuzzy. Computer techniques were used to sharpen the images - but it was a make-shift solution.

Then, in December 1993, came the first Hubble Servicing Mission. In a series of spacewalks, Shuttle astronauts undertook the complex task of installing optics to correct the distortion of the main mirror. The mission was a resounding success. Hubble's sight was dramatically restored. Today Hubble fulfils its potential - unveiling the cosmos as never before, from the nearby planets to the farthest reaches of the observable Universe.

Sub-chapters Collecting Light • Masterpieces from the Hubble Space Telescope - "Pillars of Creation" in the Eagle Nebula where new stars are born, a dying star shrouded in the beautiful Cat's Eye Nebula, another in the Hourglass Nebula. • How the human eye collects light and brings it to a focus. • Seeing deep into space is to do with gathering plenty of light.

Improving the View • How a large reflecting telescope collects light and brings it to a focus. • The difficulties of observing space from the Earth - clouds and dust impair the view, atmospheric turbulence causes images to shimmer. • Today, complex computer-controlled optical systems compensate for much atmospheric distortion at the world's largest Earth-bound telescopes - the Keck Telescopes on Hawaii and the Very Large Telescope (VLT) in Chile.

Hubble Trouble • For the very best observing there is nowhere like the clarity of space. • 1990: Launch of the Hubble Space Telescope - positioned in Earth orbit by the robot arm of Space Shuttle. A two-billion dollar eye-in-the-sky, 600 kilometers above the planet. • Hubble is supposed to see ten-times finer detail than telescopes on Earth and to be 30- times more sensitive to light. • But there is mirror problem. Hubble's images are blurred. Computer correction helps sharpen them until a repair can be effected in orbit. • Despite the flaw, Hubble makes a series of discoveries - a supermassive black hole with two jets, the double core of the Andromeda Galaxy suggesting it once consumed another, a suspected black hole in the Whirlpool Galaxy, a supernova remnant.

Repairing Hubble • Actuality of the first Hubble Servicing Mission in December 1993. Astronauts attempt to correct Hubble's sight. • Superb photography of spacewalking "repair-men", working half the time under spotlights, performing the most complex fixes ever attempted in orbit. • They fit new solar arrays, install the corrective optics module, replace the Wide-Field / Planetary Camera, and make numerous minor repairs. Hubble was designed to be serviced in orbit, but this is major reconstruction. • The mission is a resounding success - Shuttle's finest hour. The distortion of the main mirror is corrected. Hubble's sight is completely restored.

Corrective Optics • Hubble realizes its potential. The corrective optics bring light to a perfect focus.

The real show can begin. • Mars in exquisite detail, even remote Neptune has features. • But Hubble's most outstanding views are of deep space - like a vista of starbirth in the Orion Nebula, clusters of hot young stars, the debris of a supernova called the • Lagoon nebula, and galaxies in unprecedented detail • Another mission by Shuttle astronauts sees Hubble fitted with two new instruments, NICMOS and STIS. They reveal the Egg Nebula in visible light and infra-red, and gas jetting from the core of an active galaxy.

New Vistas * The breathtaking penetration of HST is demonstrated by the Hubble Deep Field. It sees objects four-billion time fainter than can be discerned by the human eye. Hubble reveals galaxies never before seen.

Background

The Hubble Space Telescope The Hubble Space Telescope is a joint venture of America's National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). The objective is to operate a long-term orbiting observatory for the benefit of the international astronomical community.

HST was dreamt of in the 1940s, designed and built in the 70s and 80s and operated from the 90s onwards. From the start, Hubble was planned a project that would require regular servicing - vital to protect it from instrument and equipment failures.

HST has a 2.4-metre reflecting telescope. It was deployed in low orbit - 600 kilometers above Earth - by the crew of the Space Shuttle Discovery (STS-31) on April 25, 1990. Two months later, however, came the horrifying discovery that the curvature of the 2.4-metre primary mirror was slightly incorrect.

The edge the mirror was too flat by an amount equal to 1/50th the width of a human hair.

This defect - called "spherical aberration" - was caused by the incorrect adjustment of a testing device used in the mirror's manufacture. It meant that light focused by the main mirror was spread over a larger area than intended. This caused a fuzzy halo which slightly blurred the images of planets, stars and galaxies. Initially, NASA coped with the problem by using advanced computer processing techniques to sharpen Hubble's images. As a result, HST made some dramatic astronomical discoveries. But something had to be done to free-up Hubble's full potential. A mission was planned for December 1993. The objectives were to properly establish the craft's scientific capabilities, to restore the reliability of its systems, and to validate the on-orbit servicing concept. This first Servicing Mission accomplished all that – and more.

Operating from the Space Shuttle "Endeavour", two pairs of astronauts, working on alternate days, performed a total of five seven-hour spacewalks. They replaced faulty components, fitted two new solar arrays, installed a corrective optics module called COSTAR in the place of the High-Speed Photometer, and replaced the Wide-Field / Planetary Camera with a completely new version that contained the corrective optics.

The mission was a great success. With its new lenses, HST embarked on some of its most exciting scientific research - programs that required images of the very faintest objects and the finest detail. Hubble has not disappointed. It has delivered an array of breathtaking images that have led to important scientific discoveries.

Responsibility for Hubble's science operations rests with the Space Telescope Science Institute (STScI) at the Johns Hopkins University, Maryland, USA. STScI is run for NASA by the Association of Universities for Research in Astronomy, Inc. (AURA).

Hubble's science instruments include three cameras, two spectrographs, and fine guidance sensors mainly used for astrometric observations. With HST well clear of Earth's atmosphere, these instruments are vastly more effective than most of their terrestrial counterparts. Ground-based telescopes can seldom provide resolution better than 1.0 arc-seconds, except momentarily under the very best observing conditions. HST's resolution is about ten times better or 0.1 arc-seconds.

HST's Science Instruments Wide Field / Planetary Camera 2: The original Wide Field / Planetary Camera (WF/PC1) was replaced by WF/PC2 on the STS-61 shuttle mission in December 1993. WF/PC2 was a spare instrument developed in 1985 by the Jet Propulsion Laboratory in California. Near Infrared Camera and Multi-Object Spectrometer: NICMOS is used for infrared imaging and spectroscopic observations of astronomical targets. It detects light whose wavelengths are between 0.8 and 2.5 micrometers.

Space Telescope Imaging Spectrograph: Light gathered by Hubble is spread out by the spectrograph. It analyses the nature of celestial objects - chemical quantity and composition, temperature, radial velocity, rotational velocity and magnetic fields. STIS studies these objects across a spectral range from ultraviolet (115 nanometers) through the visible red and the near- infrared (1000 nanometers).

NICMOS and STIS replaced the two earlier spectrographs installed on the HST when launched in 1990 - the Goddard High Resolution Spectrograph and the Faint Object Spectrograph. NICMOS and STIS were installed during the second Servicing Mission in February 1997.

Corrective Optics Space Telescope Axial Replacement (COSTAR): COSTAR is not a science instrument. It's a corrective optics package that displaced the High Speed Photometer (installed on the HST at launch) during the first Servicing Mission in December 1993. COSTAR is designed to optically correct the effects of the primary mirror's aberration on the Faint Object Camera (FOC). All the other instruments, installed since HST's initial deployment, were designed with their own corrective optics. When the FOC is replaced by another instrument, COSTAR will no longer be needed.

Faint Object Camera: Built by the European Space Agency, the Faint Object Camera (FOC) has two complete detector systems. Each uses an intensifier tube to produce an image on a phosphor screen 100,000 times brighter than the light received. This phosphor image is then scanned by an electron-bombarded silicon (EBS) television camera. It is so sensitive that objects brighter than 21st magnitude must be dimmed by the camera's filters to avoid saturating the detectors. Even with a broad-band filter, the brightest object that can be accurately measured is 20th magnitude.

HST Mission Operations and Observations Although it operates around the clock, HST is not observing all the time. Each orbit lasts about 95 minutes with time allocated for "housekeeping" and for observations. Housekeeping includes turning the telescope on to a new target, avoiding the glare of the Sun or the Moon, switching communications antennae and data transmission modes, receiving commands from Earth and downlinking data to Earth - plus calibrating and similar activities.

When the Space Telescope Science Institute (STScI) completes a master observing plan, the schedule is forwarded to Goddard's Space Telescope Operations Control Center (STOCC). There, the science and housekeeping plans are merged into a detailed operations schedule. Each task is translated into a series of commands to be sent to Hubble's onboard computers. Computer loads are uplinked several times a day to keep the telescope operating efficiently.

If possible, two of Hubble's scientific instruments are used simultaneously to observe adjacent targets in the cosmos. For example, while a spectrograph is focused on a chosen star or nebula, the WF/PC (pronounced "wiff-pik") can image a region offset slightly from the main viewing target. During observations the Fine Guidance Sensors (FGS) track their respective guide stars to keep the telescope pointed at the right target. If an astronomer wants to monitor a Hubble observation, he or she can watch consoles at STScI or at STOCC. These display the images or other data coming in to HST from deep space. If an observation program has been set up for it, a little "real-time" commanding is possible. An astronomer can even control target acquisition or filter changing - but spontaneous guidance from Earth is not yet possible.

Engineering and scientific data from HST, as well as uplinked operational commands, are transmitted through the Tracking Data Relay Satellite (TDRS) and its companion ground station at White Sands, New Mexico, USA. Up to 24 hours of commands can be stored in Hubble's onboard computers. Data can be broadcast immediately from HST to the ground stations or stored on tape for later downlinking.

For a "quick-look" analysis, an observer on the ground can examine Hubble's "raw" images and other data within a few minutes of their transmission from Earth-orbit. Within 24 hours, STOCC formats the data for delivery to STScI which is responsible for data processing - i.e., the calibration, editing, distribution and maintenance of the data for the scientific community.

Competition is keen for Hubble Space Telescope observing time. Only one in ten proposals is accepted.

Links for Further Information The official Hubble Space Telescope web site at the Space Telescope Science Institute - news, images and links. http://www.stsci.edu/

A page that includes a graphic image of the HST showing all of its principal components. http://www.ball.com/aerospace/hst_img08.gif

Images and info from the first Servicing Mission to HST where COSTAR was installed. http://www.ball.com/aerospace/hstphoto.html

An overview of HST. http://oposite.stsci.edu/pubinfo/HSToverview.html

Text and images on the various repairs and upgrades to HST. http://www.skypub.com/news/hstsm2.html

A page that includes HST's image of starbirth in the Eagle Nebula. http://eis.jpl.nasa.gov/origins/poster/starbirth.html

Questions and Activities for the Curious 1. Why are astronomers constantly campaigning for larger optical telescopes?

2. How does a reflecting telescope collect and focus light?

3. What advantages does the Hubble Space Telescope (HST) have over ground-based telescopes? And what disadvantages?

4. Describe the initial problem with HST's main mirror and how it was corrected.

5. How did computers help in processing the images from the HST before the first Servicing Mission in December 1993?

6. What problems might be encountered by astronauts in servicing the HST in the weightlessness of space?

7. Name some of the discoveries made by the HST. Was the telescope a good investment of resources?

8. If you were given observing time on the HST, which astronomical object(s) would you observe and why?

THE COMPLETE COSMOS Chapter 13: High Life

Living and working in space. Triumphs, tragedies and everyday practicalities on the Russian space station Mir and America's Space Shuttle. Outline The effects of gravity on Earth, the joy of weightlessness in space. How hurtling around Earth in "free-fall" creates zero-gravity in a spacecraft and how its velocity keeps the craft in orbit. The velocities required to achieve orbit and to escape entirely the pull of Earth's gravity.

The story of the Space Shuttle. Why it replaced the expendable rockets of the Moon program. The main characteristics of Shuttle - and its functions. The Challenger Shuttle disaster. The Mir space- station and its link-ups with the Shuttle. Aspects of life in space, working aboard Shuttle and Mir. The practicalities of eating, drinking, sleeping and washing in zero gravity. The importance of a strict exercise regime while living in space. Leisure activities - like partying and Earth-watching.

Mishaps in orbit including Mir's collision, in 1997, with a cargo module. A tour of Mir's labyrinthian interior. Shuttle's role in space, including the deployment and retrieval of satellites, with assistance from spacewalking astronauts.

Sub-chapters Pull of Gravity • Concept of Earth's gravity and its effects on us. • Depictions of a weightlessness aboard American and Russian spacecraft. • Launch velocities required for space travel. Insufficient velocity results in a sub-orbital hop. A 27,000 kilometer-per-hour thrust is required to reach Earth orbit. • Speeds in excess of 40,000 kilometers per hour will achieve escape velocity - and break free from the pull of Earth's gravity.

Shuttle and Mir • All about Space Shuttle. It was developed to be more cost-effective than the expendable rockets of the Moon program. Shuttle launches as a rocket, orbits as a space station and lands on a runway like a glider. Shuttle's limit is Earth orbit. • January 28, 1986: the Challenger Shuttle disaster. A ruptured seal triggers an explosion, killing all seven crew members. • The Mir space-station, a permanent laboratory, 400 kilometers above Earth. • Dockings between Mir and Shuttle.

Eating and Sleeping • Living aboard Shuttle and Mir. Monitoring the effects of weightlessness on the human body. • Space food is glutinous to prevent crumbs and spicy to stimulate appetite. • Sleeping quarters - either a cubicle or strapped to the superstructure. • Mir's 90-minute orbit of Earth -16 sunrises and sunsets each day.

Body and Soul • Haircutting with a vacuum cleaner to collect floating hairs. • Shuttle's $15 million toilet and Mir's more primitive facilities. • Hair-washing with waterless shampoo. • Russians know most about selecting compatible crews. • Relaxing in space - a sing-song.

Earth-Watching, Working Out • The joy of observing features on Earth. • The importance of exercising in space. Weightlessness leads to muscle wastage and other problems.

Mir Problems • Mir's many troubles - cosmonauts making repairs outside the spacecraft. Internal fires, computer breakdowns and collision with a cargo module in 1997. • A tour of Mir's cramped, tunnel like interior. Uncomfortable, but Mir generates a wealth of invaluable scientific data. • Shuttle's role as a "space truck", launching satellites and retrieving them for repair. Observing the Earth for scientific, agricultural and military purposes.

Background Defying Gravity If a ball is thrown into the air, it reaches a certain height then falls back to the ground. It returns because of the force of gravity, the same force that keeps our feet firmly on the ground. The Earth, the Moon, the Sun and all the stars have their own gravity.

If the ball is thrown a little harder, it will rise to a greater height before falling back. If, however, you could throw the ball into the air at a speed of 40,000 kilometers per hour, it would never return to the ground. Earth's gravity would not be strong enough to pull the ball back, and it would escape into space. The speed of 40,000 kilometers per hour is called the Earth's "escape velocity". For a spacecraft to completely escape Earth's gravity and journey outward into space, the craft must travel at 40,000 kilometers or more.

The 19th century had some fanciful ideas on how escape velocity might be achieved. Jules Verne proposed firing space vehicles from a huge gun. He hadn't reckoned that the sudden jolt of the firing gun would kill any people on board. Another means had to be found to overcome the pull of Earth's gravity. Balloons and conventional aircraft are no good because they only work when surrounded by air. It supports them and provides the required lift - unlike the airless vacuum of space.

In 1895, a shy, deaf Russian teacher named Konstantin Tsiolkovskii suggested the use of rockets. He calculated the speed and the amount of fuel a rocket would need to carry it into space. He proposed using liquid instead of solid fuels, and using multi-stage rockets to get into orbit. Tsiolkovskii did not build a rocket himself. At the time nobody took much interest in his idea.

It wasn't until the 1920s that American rocket pioneer Robert Goddard launched the first successful liquid-fuelled rockets. Rocket research was also carried out in Germany before the Second World War by men such Werner von Braun. He developed the first really effective high- altitude rocket, the V2. After the war, the V2 and later designs were developed for space travel.

The US Space Shuttle does not need to travel as fast as 40,000 kilometers per hour, because it remains in Earth orbit. Shuttle doesn't have to reach escape velocity. But to achieve orbital velocity, Shuttle has to generate sufficient thrust to attain a speed of 27,000 kilometers per hour.

Space Shuttle has three main rocket engines that use fuel from a huge separate External Tank (ET). At launch, Shuttle rides on the back of the giant ET. This holds the liquid hydrogen and oxygen fuels that are pumped to Shuttle's main engines. Shuttle blasts off with the help of two giant solid rocket boosters. After about two minutes, when Shuttle is 45 kilometers above the ground, the boosters drop away and the Shuttle continues into space using only the thrust of its three main engines. From lift-off to Earth orbit takes less than ten minutes.

Living in Space Astronauts can live relatively comfortably in space. The cabin of a spacecraft is filled with normal air - pressurized, so that it pushes on the human body just as air does on Earth. Within the craft, astronauts wear day-to-day clothes. Life-support systems maintain the right air pressure. They also keep it clean so that it can be used over and over again. The air is kept smelling fresh by filters which remove the water vapor and poisonous carbon dioxide that astronauts breathe out.

On the US Space Shuttle, fuel cells generate electricity by means of a chemical reaction between hydrogen and oxygen, producing water as a by-product. Until recently, the Russians used solar panels to make electricity from sunlight. Fresh water was brought up from Earth. The latest Russian spacecraft will also use fuel cells.

The main difference between living on Earth and in space is the feeling of weightlessness. Everything, including the astronauts, drifts around inside the spacecraft if not fixed down. To envision weightlessness, look up at the ceiling and imagine a book lying there. To retrieve it, you push gently from your chair. You slowly rise to the ceiling. Then, a soft push against the ceiling returns you to your seat.

Sleeping in space is like "sleeping on a cloud". The body feels no pressure from bed or couch. You can sleep just floating around - or inside a lightweight sleeping bag taped to the side of the spacecraft. And you can sleep in any position, even upside down. When you are weightless, it all feels the same.

Liquids don't flow in weightlessness, so drinks won't stay in cups. If they escaped, drops of orange juice, for instance, would just float around inside the spacecraft. Drinks have to be sucked through a plastic straw from a sealed container.

In space, you can eat normal food, but it must be sticky to stay on your spoon or fork. Food is eaten from special packages on a tray that clips to a seat or table. When not in use, magnets stop knives, forks and spoons from floating away. Trays and cutlery are wiped down and re-used. Some food is dried (de-hydrated), requiring hot or cold water to be added before consumption.

Coping with Weightlessness Once in space, astronauts are busy people. During their first few days in orbit, however, space sickness is quite common and causes temporary discomfort. To keep fit and well, astronauts must take plenty of exercise, particularly on long-duration missions. In the weightlessness of space, muscles are deprived of their constant battle against gravity on Earth. Muscles lose tissue and function less well.

In addition, body fluids are no longer pulled downwards by gravity. Instead, they migrate towards the astronaut's head. This causes fat faces and blocked nasal passages. Belts are worn at the top of each leg to help control fluid flow until the body adjusts to space. The upward movement of fluids also causes the kidneys to excrete more urine, upsetting the body's salt concentration.

To prevent serious physical illness and to make the return to Earth-gravity less uncomfortable, astronauts must have about two hours of exercise a day. They use exercise bicycles and treadmills. Legs and arms and the heart - the most important muscle of all - are put through their paces in space. While exercising or working, astronauts stay put by using special hand holds, footholds and seat-belts.

From 1986 to 1999, Russia's Mir space station was home to astronauts from many countries. Two or three crew members were usually on board, but Mir could take up to six. Mir was visited by crews of the US Shuttle, which docked with the station. Astronauts have remained on board Mir for many months at a time. They have collected valuable information about how the human body copes, both mentally and physically, with long periods of weightlessness. The record for the longest unbroken stay by one person - 438 days by Dr Valeri Polyakov - was achieved aboard Mir.

The fact that cosmonauts have returned to Earth after long stays aboard Mir with no lasting ill effects bodes well. It has proved that well disciplined people can survive long periods of weightlessness in the cramped confines of a space station. This is good news for future ventures like the International Space Station, which will be permanently-manned, and for the colonies of humans that one day must inhabit the Moon and Mars.

Working in Space Astronauts work both inside and outside a spacecraft. Inside, there is routine maintenance and the monitoring of onboard systems, as well as scientific testing and experimentation. This can include investigations on the effects of weightlessness on the human body, the testing of new space products, and research on behalf of commercial organizations who pay to send experiments to be performed in weightlessness.

Work outside the craft may involve the deploying or repairing satellites, setting up experiments, testing new equipment, or evaluating techniques for the assembly of future space stations in orbit.

Together with the Shuttle commander and pilot, a crew includes "payload specialists" and "mission specialists". These are scientists with special skills who carry out experiments or handle payloads. Shuttle's cargo bay can accommodate several small loads and two or three satellites.

Satellites are pushed gently out of the bay by a special robot manipulator arm. Then, they are tested in orbit. Lifting requires no effort in space. Nevertheless, satellites must be moved with great care. The manipulator arm is worked by remote control from inside the Shuttle, the operator looking through a window on to the cargo-bay. The arm is over 15 meters long and is jointed like a human arm. Shuttle can be used for repairing or retrieving faulty satellites in low Earth orbits. Fixing a satellite is a tricky business. Shuttle is first brought up very close and the manipulator arm is extended towards faulty object. Astronauts then have to take a spacewalk to carry out repairs. This is called an Extra-Vehicular Activity (EVA). The astronauts may have to fit new parts to the satellite or replace damaged ones. Often the EVA workers need to attach themselves to the manipulator arm to avoid drifting off into space.

Clad in spacesuits, the crew leaves and enters the spacecraft through an airlock. Each suit has over 15 layers of plastic and metal to protect the astronaut against the extremes of hot and cold in space. Suit and helmet are perfectly sealed and correctly pressurized. A backpack contains a life support system, supplying the wearer with air, food and water. Sometimes an astronaut uses a manned maneuvering unit (MMU) for moving around outside the spacecraft. The MMU is shaped like a chair and worn like a huge backpack. It has 24 small thrusters that squirt jets of gas propel astronaut hither and thither.

And leisure time? Much as on Earth. Astronauts keep in touch with family and friends via laptop computers. Then, there's reading, photography, listening to music or playing cards. Whatever the preferences, these orbiting astronauts spend a considerable time just gazing at the ever-changing Planet Earth.

Links for Further Information Comprehensive page detailing all aspects of Shuttle missions, including launch dates, objectives and highlights - plus images. http://www.ksc.nasa.gov/shuttle/missions/missions.html

Mir space station home page, including news, history, research projects, archive, images and links. http://www.maximov.com/MIR/index.html

Living in space - an informative page with links to health, fitness, loneliness, living conditions and crop cultivation in orbit. http://www.tv.cbc.ca/national/pgminfo/space/livingspace.html

Excellent spacewalking site, detailing the history of spacewalking and spacesuits, the conditions in space, and the Manned Maneuvering Unit now used by astronauts working outside Shuttle. Extensive images. http://quest.arc.nasa.gov/space/teachers/suited/1intro.html

Interesting page, describing various effects on the body of a weightlessness. http://shuttle.nasa.gov/index.html/aging.html

Exhaustive Shuttle images library. http://www.ksc.nasa.gov/mirrors/images/html/shuttle.htm

Questions and Activities for the Curious 1. Describe the main components of the US Space Shuttle at lift-off.

2. Research the circumstances that led to the explosion of the Challenger Shuttle in January 1986.

3. What characteristics are important when choosing people to live and work together within the cramped confines of a space station?

4. Describe a typical meal-time in orbit. How it would differ from meal-time on Earth.

5. Discuss the problems that long periods of weightlessness pose for the fitness and health of astronauts.

6. Summarize achievements aboard the Soviet/Russian space station Mir since the launch of its first module in February 1986.

7. Imagine you are to spend six months orbiting the Earth aboard Mir. What would you most look forward to and what would you least relish?

8. What particular hazards are posed by a fire aboard a space station?

THE COMPLETE COSMOS Chapter 11: Earth Patrol Launched into Earth orbit, the satellites that monitor the health of our planet - ozone, melting ice-caps, weather, deforestation, navigation. Outline From Earth-orbit, satellites keep watch for us. Meteosat scans global weather patterns. It tracks severe storms - saving lives and minimizing disruption. And to reduce flooding, satellite images assist engineers select the best locations for dams and barriers.

The Topex-Poseidon satellite measures changes in sea level and ocean temperature. Our weather is driven by exchanges between the oceans and the atmosphere. In the Pacific, satellites reveal the phenomenon of El Nino.

Thanks to satellites, ships avoid fog banks, icebergs and rough seas. Satellites enable mariners to make accurate navigational fixes. Earth's ever-changing environment is monitored by satellites - volcanic eruptions, industrial emissions, the ozone hole, global warming and the damage inflicted by oil spills, fires and the destruction of the rainforest.

The SPOT satellite provides a fresh perspective on urban and rural developments – by combining images to produce 3-D pictures. Satellite radar gets beneath clouds of volcanic dust and the sands of the Sahara. Agriculture benefits as false-color satellite images check the health of crops and direct spray planes to where they are needed.

Sub-chapters Keeping Watch • Satellites chart the Earth. They help us understand the workings of planet. They speed information around the globe. • The launch of Meteosat - and its vantage point 36,000 kilometers above the equator. • Global weather. How satellites keep track of hurricanes and tornado-generating storms.

Storms and Floods • How meteorologists use satellite data to produce weather forecasts that help save lives and minimize disruption. • To reduce the risk of severe flooding, engineers use satellite images to pinpoint the best locations for barriers and dams.

Oceans and Atmosphere • From orbit, Topex-Poseidon scans the oceans, measuring changes in sea level and ocean temperatures. • The exchange of heat between the oceans and the atmosphere helps drive our weather. By monitoring ocean currents and temperatures, we can predict the weather. • In the Pacific, satellites reveal the curse of El Nino. It brings heavy rain to South America and drought to Indonesia and Australasia.

Eyes in the Sky • Satellites enable vessels to skirt fog banks and heavy seas and to fix navigational positions to within ten meters. Satellites spot the best way through pack-ice and keep track of icebergs. • Monitoring volcanic eruptions - and a global temperature drop as volcanic dust girdles the Earth. • Tracking industrial emissions and levels of atmospheric gases. • The ozone layer, our shield against ultra-violet radiation from the Sun. • Above Antarctica, a great hole is revealed in the ozone layer. Chemical emissions are blamed.

Environmental Monitoring • Earth may be warming up. Our eyes-in-the-sky watch the mercury. • They monitor the damage to Earth's environment. • Images from orbit assist with the clean-up after major oil spills. Satellites reveal the environmental cost of war. • They watch the destruction of tropical rainforests - the "lungs" of our planet.

Getting the Picture • The SPOT satellite makes for better use of land - with a fresh perspective on urban and rural developments. • By combining images from different positions, SPOT produces 3-D pictures. Then, by adding computer visualizations of prospective developments, their suitability can be assessed. • With satellite radar, we can peer through clouds of volcanic dust and beneath the sands of the Sahara. • False-color imagery provides farm reports from space. • Season by season, satellites monitor the health of crops, alerting growers to pests and blight. • Satellite information shows where pesticides should be sprayed.

Background El Nino El Nino is an oceanic and atmospheric phenomenon. It happens in the Pacific Ocean when unusually warm ocean conditions appear along the coasts of Ecuador and Peru. The result can be climatic disturbances of varying severity. El Nino occurs every two to seven years. It can affect weather around the world for months, even years, before patterns suddenly switch back to normal.

El Nino, which is Spanish for "the child", refers to the infant Jesus Christ. The ocean current associated with the phenomenon usually begins around Christmas, the season of Christ's birth. And because El Nino is accompanied by a fluctuation of air pressure and wind patterns in the southern Pacific, the phenomenon is correctly known as El-Nino-Southern Oscillation (ENSO).

El Nino happens when ocean current are sufficiently warm and persistent to cause a reversal in the usual weather conditions of the eastern and western Pacific. Normally, in the tropical areas of the western Pacific, the waters are warm and the air pressure above them is quite low. Moist air rises, causing the clouds and heavy rain characteristic of south-east Asia and northern Australia.

Meanwhile, in the eastern Pacific, the waters are cold and the air pressure high. This creates the arid conditions typical of the western coast of South America. The trade winds blow from east to west, pushing Sun-warmed surface waters westwards and exposing cold water to the surface in the east. During El Nino, however, the easterly trade winds weaken and even reverse. This causes a change in the surface temperature of the ocean. With the wind-change comes an increase in atmospheric pressure. As a result, the warm waters of the western Pacific start to flow back eastwards and the surface temperatures of the ocean increase significantly off South America.

The result is that the normal wet weather of the western Pacific moves to the eastern Pacific. And the usually arid climate of the eastern Pacific moves to the western Pacific. This brings heavy rains to South America and droughts to south-east Asia, India and southern Africa. It can also bring severe weather to large parts of North America.

The Ozone Hole The ozone layer lies between 19 and 48 kilometers above Earth's surface. Created by the action of sunlight on atmospheric oxygen, ozone is a form of oxygen that packs three oxygen atoms into each molecule, instead of the normal two. The great benefit of ozone is that it absorbs up to 90 per cent of the harmful ultraviolet (UV) radiation that reaches us from the Sun. Full solar UV radiation can damage crops, marine life and human health. It is, therefore, vitally important that ozone levels are maintained in the atmosphere .

Since the 1970s, chlorofluorocarbons (CFCs) have been accused of damaging the ozone layer. CFCs have long been used as refrigerants, in foam plastics and in aerosol cans. When released into the atmosphere, CFCs are broken down by sunlight and the chlorine constituent reacts with and destroys ozone molecules. CFCs have now been banned in many countries but the ozone layer continues to be attacked by bromine halocarbons, nitrous oxide and other chemicals.

In 1982, a periodic loss of ozone was detected above the southern polar region. This "ozone hole" develops at the start of the Antarctic spring in early October. After a few months, the hole closes again - but it never disappears completely. By 1995, it covered an area more then twice the size of the USA. More recently, the hole has stretched as far north as the southern tip of South America - to inhabited areas. The authorities have warned people to stay indoors during late morning and early afternoon.

Research has now shown that the ozone layer is periodically thinning over Arctic regions, although not to the extent as above the Antarctic. Whilst there is a general decline in the amount of ozone throughout the atmosphere, the Antarctic is particularly vulnerable due to the enhancing effect of ice particles in the air and the effects of the wind, especially the south polar vortex.

The international response has been to support further research - and to curb the use of ozone- depleting chemicals. The 1985 Montreal Protocol was signed by 49 countries. Its aim was to phase out the use of CFCs by 2000.

Scanning the Earth Since the 1960s, our planet has been continuously scanned by a series of Earth resources satellites fitted with high quality cameras and sensors. They have enabled us to see the Earth from a scale and perspective never previously possible. Two kinds of satellite are currently in operation.

The first passes over the north and south poles, scanning both the atmosphere and whatever is directly below. These satellites orbit at a height of about 1500 kilometers. During successive orbits, each lasting about 100 minutes, the Earth has rotated by 25 degrees in longitude. This means that at every orbit the satellite conveniently scans a different pole-to-pole strip.

The other type of satellite orbits above the equator at an altitude of about 35,900 kilometers. These satellites are geostationary - orbiting at precisely the speed of Earth's rotation and remaining over exactly the same spot. Satellites use remote-sensing to view Earth in both visible light and infrared. Images are interpreted by receiving stations on Earth. By digitizing the images, computers analyze and manipulate the information beamed from orbit.

Colors can be added to highlight differences picked up by the images. For example, "false color" is used to monitor plant growth - to differentiate between healthy plants and those suffering from disease and drought. Remote-sensing detects these differences because plants absorb large quantities of red wavelengths when plants are photosynthesizing normally.

As well as monitoring the health of crops and forests, remote sensing helps with map-making and pin-pointing floods, droughts and large-scale fires. Remote-sensing images reveal pollution in the atmosphere and oceans. They plot the movement of icebergs and are even used to locate precious minerals.

Weather satellites have greatly improved the accuracy of weather forecasting. In the case of geostationary satellites, they can look at a complete hemisphere of the Earth - useful in viewing cloud formations and global weather systems. Satellites can measure wind speed at sea level and higher in the atmosphere. They can observe the height and the distance between ocean waves. And satellites are invaluable in tracking devastating tropical storms.

The idea geostationary satellites was first suggested by Arthur C. Clark in 1945. They have revolutionized communications - relaying everything from phone call to television pictures. Signals can be bounced around the world at the speed of light. By 1997, there were 150 communication satellites in Earth-orbit - and that doesn't include meteorological or scientific satellites. Nor does it include spy satellites. Such satellites eavesdrop on radio messages from thousands of sources across the world. They watch troop movements, track ships and detect submarines. Spy satellites photograph military bases and missile launching sites.

Links for Further Information Data and links to Meteosat imagery. http://ftp.csr.Utexas.edu/sst/

A site about El Nino including most frequently asked questions. http://www.usis.it/wireless/wf971021/97102118.htm

A theme page about El Nino - with forecasts. http://www.pmel.noaa.gov/toga-tao/el-nino/home.html

A general page about the ozone layer. http://www.calidad-del-aire.gob.mx/sima/sima/o3e.html

Information about the Upper Atmosphere Research Satellite - images, information and data about its launch and mission. http://uarsfot08.gsfc.nasa.gov/UARS_HP.html

Rainforests and link to other sites - news, campaigns, information. http://www.ran.org/ran/

A site about SPOT with images, interpretation of information, programs, production, projects and cartography. http//www.orstom.fr/hapex/spot/spotimg.htm

Questions and Activities for the Curious 1. How do weather satellites help to reduce loss of life from severe tropical storms, such as hurricanes, cyclones and typhoons?

2. Explain how ocean currents play a major role in determining the Earth's weather patterns.

3. What are the probable effects of El Nino on the fishing industries of Peru and Ecuador?

4. If satellites had existed when the Titanic sailed in April 1912, would the liner have sunk?

5. What are the effects of major volcanic eruptions - like Mount Pinatubo in 1991 – on climate, both locally and worldwide?

6. Why is the Antarctic ozone hole largest at the start of Antarctic spring (early October)?

7. What is the role of satellites in assessing the rate at which tropical rainforests are being destroyed? What are the consequences of such destruction?

8. Describe the importance of satellites to farming and agriculture.

THE COMPLETE COSMOS Chapter 4: Earth Evolution of the Earth - and of life. Internal structure, continental drift, day length, seasons, oceans, climate, weather and El Nino. Outline Earth as a beautiful blue world, with an abundance of water, water vapor, ice – and life. Seismic waves reveal its internal structure, and the cause of its magnetic field. Magma upwelling from below produces mid-ocean ridges, continental drift, and volcanoes. Life may have begun in deep- sea ocean vents. Green plants transform Earth's early poisonous atmosphere into breathable air.

Earth's rotation is responsible for the apparent motion of the Sun - and stars – across the sky, and for day and night. Earth's tilted axis and orbit around the Sun cause the length of the day to vary and gives rise to seasonal changes.

The Sun drives the circulation of air between the equator and poles, and powers the winds. Earth's spin causes winds to slew east and west instead of blowing north and south. The interaction between the oceans and atmosphere. The great ocean currents drive Earth's weather systems. The global consequences of El Nino, a climatic upheaval in the Pacific Ocean.

Sub-chapters Blue Planet • Earth and its uniqueness among the planets. The importance of water to the existence of life.

Inside Earth • From its early violent beginnings, Earth differentiates into core, mantle and crust. • Seismic waves reveal Earth's internal structure. • The solid inner core moving inside the molten outer core cause the magnetic field. • The magnetic axis and wandering of the magnetic poles.

Continents and Atmosphere • Movement of plates in the Earth's crust creating continental drift. The locations of mid- ocean ridges, volcanoes - and deep ocean vents, where life is thought to have begun. • Role of green plants in converting Earth's early toxic atmosphere into breathable air - containing oxygen.

Day, Night and Seasons • The effect of Earth spinning on its axis - day and night, and the daily east-to-west motion of the Sun across the sky. • The 23.5-degree tilt of Earth's axis of rotation. How this causes the seasons and varying length of day and night during the year. • Graphics showing sunsets at different times of the year; solstices and equinoxes.

Winds and Ocean Currents • The influence of hot and cold ocean currents on Earth's climate. • Three great engines of air circulate between equator and poles - driven by the Sun. • Earth's pattern of winds - how our planet's spin slews wind direction to the east or to the west. • The Gulf Stream as a bringer of mild climatic conditions to northern Europe.

Climatic Tantrums • Generation of tropical storms - the power of a hurricane. • El Nino - a climatic tantrum in the Pacific Ocean, caused by a change in the pattern of trade winds. How El Nino can influence weather patterns around the world.

Background Orbit of the Earth The Earth's orbital path around the Sun is not quite circular, but follows a path called an ellipse. Earth takes exactly one year to travel once round its elliptical orbit of the Sun. On average, Earth moves along its orbit at 30 kilometers per second, this being almost 2.6 million kilometers every day. As it orbits, Earth's distance from the Sun varies slightly, between 147 million and 152 million kilometers. Earth is closest to the Sun - a position known as perihelion - in early January. Earth is farthest from the Sun - a position known as aphelion - in early July.

The plane of the Earth's orbit around the Sun is called the ecliptic. If Earth's axis of rotation were perpendicular to this plane, all points on Earth would experience equal day and night throughout the year. Earth's rotational axis, however, is tilted by 23.5 degrees to the ecliptic. This gives rise to the seasons, and variation in the length of day and night.

Sundials and Solar Time During a day on Earth, the Sun rises in the east and sets in the west. This movement is caused by Earth's spin on its axis. The same effect occurs at night, when the stars appear to rise in the east and set in the west. The apparent motion of the Sun across the sky in the course of a day can be studied with a shadow stick or sundial. Time as indicated by the Sun using a shadow stick or sundial is called apparent solar time. The Sun, however, does not always move at the same speed across the sky, but sometimes travels slower or faster than "average". This is because of Earth's elliptical orbit around the Sun, and the tilt of its axis of rotation to the plane of its orbit.

Time, as shown by a clock, is based on the movement of a fictitious "mean Sun", which travels at a constant rate across the sky whatever the time of year. This is called mean solar time. The difference between mean solar time (or clock time) and apparent solar time (or sundial time) is called the equation of time. Usually this difference amounts to several minutes. In mid-February sundials lag behind the mean Sun by roughly 14 minutes, but in early November sundials are ahead of the mean Sun by approximately 16 minutes. Sundials and clocks agree only four times a year - on April 16, June 15, September 1 and December 25.

Earth's Seasons The tilt of Earth's axis results in a given point of its surface being heated unequally by the Sun over the course of a year. During summer months in the northern hemisphere, Earth's north pole is tilted towards the Sun. Sunlight falls on the northern hemisphere at its highest angle, creating the greatest heating effect, earlier sunrises and later sunsets, and hence longer hours of daylight. Six months later, during winter in the northern hemisphere, Earth's south pole is tilted towards the Sun. Sunlight now falls on the northern hemisphere at its lowest angle, reducing the heating effect - the Sun is lower in the sky, and provides fewer hours of daylight than in the summer. Seasons in the southern hemisphere are always the opposite of those in the northern hemisphere.

Solstices and Equinoxes The positions along Earth's orbit, when its axis is tilted exactly towards the Sun's direction, are called solstices. Currently, they occur on or around December 21 and June 21. On December 21, the Sun is directly overhead at local noon for places on the Tropic of Capricorn (latitude 23.5 degrees south), and on June 21 at places on the Tropic of Cancer (latitude 23.5 degrees north). In between these times, there are two occasions when the Sun is overhead at local noon for places on the Earth's equator. These are known as the equinoxes, currently occurring on or around March 21 and September 22. On these two dates, day and night are equal the world over.

Links for Further Information A page with various links to topics on the environment. http://www.cnie.org/

A site with shots of Earth and climatic changes. http://edcwww.cr.usgs.gov/earthshots/slow/tableofcontents

A list of links for references. http://www.usgs.gov/education/othered.html

Provides other links to different aspects of planet Earth. http://www.arts.ouc.bc.ca/geog/links/environment.html

Questions and Activities for the Curious 1. Describe the various ways in which Earth is unique among the planets of our Solar System.

2. Research the reasons why the sky on Earth is blue, and why the Sun appears red when low in the sky, near sunrise or sunset.

3. Describe the internal structure of the Earth.

4. Identify the major tectonic plates on Earth and indicate regions of volcanic activity associated with them. Name the highest mountain range on Earth and identify the two tectonic plates that caused its formation.

5. As a result of continental drift, Africa and South America are currently moving apart at a rate of approximately three centimeters per year. Assuming this rate has been constant, calculate when these two continents must have been in contact.

6. Use a piece of wood, such as a long pencil or piece of dowel, placed vertically in a container (e.g. a plant pot) of soil or sand, to make a shadow stick. On a sunny day, mark the point where the end of the stick's shadow falls on a piece of white card placed under the container. Do this every ten minutes from 10am to 2pm. Work out the time when the length of the shadow was shortest. This is the time of local noon.

7. Find out the exact dates of the equinoxes and solstices for the year. What are the sunrise and sunset times on these days? Calculate for how many hours the Sun is up.

8. In winter, the fjords in Norway remain largely free from ice, even though they are farther north than the St. Lawrence river in Canada which freezes over. Why is this so?

THE COMPLETE COSMOS Chapter 16: Space Pioneers From the ancient sky-watchers of Babylon to space-age cosmology, the story of astronomy - Copernicus, Kepler, Galileo, Newton, Hubble. Outline Astronomy is born of the need to measure time - motions of the heavens indicate the date, the season, when to sow, when to prepare for winter. Some 6,000 years ago, the Babylonians chart celestial movements, group stars into patterns and observe "wandering stars" - in reality, the five nearest planets. The Chinese map the cosmos and record comets.

The Egyptians divide the year into 365 days, while the Maya make a calendar of 584 days. The Greeks place the Sun, the Moon, their five known planets, and the background stars in crystal spheres that orbit Earth. Eratosthenes shows that the Earth is round.

Hipparchus maps the sky and works out the relative distance to the Sun and Moon. Ptolemy believes the planets orbit Earth and develops a theory to fit his observations. He's wrong, but his ideas persist for nearly 1,500 years. Then Copernicus puts the Sun at the center of our planetary system and Johannes Kepler devises his three laws of planetary motion.

The start of modern astronomy as Galileo turns his telescope on the sky. Isaac Newton works out gravity, splits light, and improves the telescope. William Herschel discovers Uranus, builds the world's biggest reflecting telescope, catalogues the stars and identifies the shape of our galaxy, the Milky Way. The Earl of Rosse builds a larger telescope and sees the spiral structure of galaxies.

The 19th century yields two key tools - spectroscopy and photography. In the 1920s, Edwin Hubble employs both to record vast numbers of galaxies and the fact they are all racing outwards. The advent of radio astronomy and ever more powerful telescopes peer back to the beginnings of space and time.

Sub-chapters: First Astronomers • The need to measure time - to know the date, the season, when to sow, when to prepare for winter. • In ancient times, people were familiar with the motions of the heavens. The establishment of calendars. • The Babylonians, about 6,000 years ago, chart celestial movements, group the stars into constellations and create the signs of the Zodiac. The "wandering stars" observed by the ancients were, in fact, the five brightest planets. • Meanwhile, Chinese astronomers map the cosmos and carefully record phenomena such as comets.

Calendars and Spheres • The Egyptians use the dawn rising of Sirius to foretell the annual flooding of the Nile. Although believing their gods "control" heavenly cycles, the Egyptians devise a 365-day calendar. • Much later, in Central America, the Maya invent a 584-day calendar based on the cyclical movements of Venus. • Early ideas that "explain" the mechanics of the heavens - an intriguing Hindu view. • The Greeks place the Sun, the Moon, the five known planets and the background stars in a series of concentric crystal spheres that surround the Earth.

Measuring and Observing • In 200 BC, Eratosthenes shows that our world is round by using geometry to work out the circumference of the Earth. • Hipparchus compiles the first stellar map, records the brightnesses of stars and, by observing eclipses, calculates the relative distance to the Moon and Sun. • Ptolemy believes that the planets orbit Earth. He devises complex explanations to fit his observations - even the strange little "reverses" made by some planets. Although he's wrong, Ptolemy's ideas persist for almost 1,500 years.

Planetary Orbits • Nicolaus Copernicus, the 16th century Polish cleric, reasons that Earth and other planets orbit the Sun and that the Moon orbits Earth. But the orbits, he believes, are perfect circles. • In the early 17th century, Johannes Kepler devises three laws of planetary motion. • Kepler uses the precise observations of Tycho Brahe to show that the planets orbit the Sun in ellipses, not circles. Kepler demonstrates that a planet moves quickest when closest to the Sun and slowest when farthest away. He also reveals that the revolution periods of planets nearer to the Sun are shorter than those farther away. • Importantly, Kepler correctly explains why some planets appear to make a backwards loop in the sky as they are overtaken by Earth.

Through the Telescope • Early in the 17th century, Galileo is first to turn a telescope on the sky. Among many discoveries, he sees craters on the Moon, moons around Jupiter's and spots on the Sun. • Christiaan Huygens discovers Saturn's largest moon and that the Saturnian rings are detached from the planet. Giovanni Cassini finds a gap in the rings, now called the Cassini Division. • Isaac Newton revolutionizes astronomy. He works out gravity, splits light and shortens the telescope with the use of mirrors. • In the 18th century, William Herschel discovers Uranus, builds better reflecting telescopes, catalogues the stars and recognizes that our galaxy, the Milky Way, is a flattened disk.

Spectroscopy and Photography • In Ireland, in the mid-19th century, the Third Earl of Rosse builds an even large telescope. He glimpses galaxies beyond our own and sketches their spiral structure. • The 19th century yields two new astronomical tools - photography and spectroscopy. • The latter is the analysis of light to work out the nature of objects in space. • In the 1920s, Edwin Hubble uses both tools to record galaxy after galaxy. He discovers they are all racing outward. • Where optical telescopes cannot see, radio telescopes detect ever more distant objects. • Today, increasingly powerful telescopes penetrate back towards the beginnings of time and space.

Background The First Telescopes It is not clear who invented the telescope. Indeed, it may have been invented and re-invented many times. By the beginning of the 17th century, spectacle lenses had been in use in Europe for about 300 years. During that time, on several occasions, two spectacle lenses and a tube must surely have been arranged, purely by chance, to form a telescope.

By 1608 a Dutch spectacle maker, Hans Lippershey, had built an eyeglass that could make distant objects appear much closer. He called it a "device for seeing at a distance" - which is what "teleskopos" means in Greek. Hence, our modern word telescope. The Italian astronomer, Galileo Galilei, heard about the invention. The following year, in1609, he constructed a telescope of his own. He used an organ pipe and two lenses - one convex and one concave. Later, he built a number of improved telescopes. They were the finest in the world. Resting in a cradle on a stand, Galileo's best telescope could bring the heavens 30 times closer.

Galileo's Discoveries Galileo first turned his telescope on the sky in 1610. He saw craters and mountains on the Moon, previously believed to be smooth. By measuring shadows cast by the lunar mountains, he calculated that some of them must be as much as six kilometers high. Suddenly, thousands of stars, never before seen by human eyes, were discerned through Galileo's telescope. Although magnified 30 times, they still appeared as mere points of light. Galileo reasoned that they must be a very great distance. On January 7, 1610, when observing Jupiter, Galileo noted three bright star- like objects close to the planet. The following night, their positions had shifted. On subsequent nights, he saw them move again and again. As he sketched the planet and the attendant objects, Galileo concluded that they must be moons orbiting Jupiter, just as our Moon orbits Earth. He was right.

Galileo also observed that Venus appeared quite different to Jupiter. Venus had no moons and it appeared to change size and shape as it moved across the sky night by night – much like our Moon. At times Venus would be a large, thin crescent. A few weeks later it would be a smaller "half Venus". And just before disappearing into the glare of the Sun, Venus would appear as a tiny, fully illuminated disk. Galileo realized that this could be explained only if Venus moved around the Sun - and not around Earth as was previously believed.

In his solar observations, Galileo discovered dark sunspots and recognized that the Sun rotated.

Isaac Newton and the Spectrum The English genius Isaac Newton (1642-1727) first investigated how colored light could be made from white light. In 1665, Newton was doing some experiments with lenses. He noticed that the images formed by the lenses - which he had made himself - were not clear. They seemed blurred and surrounded by a narrow fringe of colored light. Newton made more lenses, taking great care when polishing them. But he always met the same problem. Finally, he concluded that the fault was not with the lenses. His hunch was that it was something to do with the refraction of light itself.

Newton projected a narrow beam of sunlight - about 8 mm across - from a hole in a window-blind across a darkened room. About five meters from the window, the beam produced an image of the Sun on a white screen. When he placed a triangular glass prism in the beam, the rays bent upwards. Newton observed that the image on the screen was stretched out into a broad band. It was colored at either end. Subsequent experiments, using a narrow slit, revealed that the image was actually made up of a number of overlapping colored patches.

Newton had discovered what we now call a spectrum. The colors were red, orange, yellow, green, blue, indigo and violet. By separating each of the seven main colors from the rest, Newton showed that the colors themselves could not be changed by refraction through a further prism.

Newton then allowed the whole spectrum to fall on another prism. This was placed the opposite way up to the first prism. A white image was obtained. If just one color was removed from the spectrum before passing it into this second prism, white light was not produced. Newton realized that sunlight, or white light, was a mixture of seven different colors.

So why is white light separated into its main colors by a prism? Each color of light travels as a wave and each has a different wavelength. The wavelength of red light, for instance, is seven ten- thousandths of a millimeter. The wavelength of violet light is four ten-thousandths of a millimeter. When passed into the glass prism, the movement of the waves is hindered. They travel more slowly in glass than in air. As a result, each color is bent or refracted. The color with the longest wavelength - red - bends the least. That with the shortest wavelength - violet - bends the most. This is because violet light waves travel more slowly through glass than do red light waves. The more slowly the colored wave travels through the prism, the more it is bent or refracted.

Spectroscopy The key to determining the composition and conditions of a moon or a planet or a star or a galaxy - indeed of any astronomical object - is its spectrum. The technique used to capture and analyze such a spectrum is called spectroscopy.

In spectroscopy, the light emitted or reflected by an object - or, more correctly, its electromagnetic radiation - is collected by a telescope. The light is then spread into its component colors to form a spectrum. By studying the spectrum of a particular object, scientists can work out what it's made of - among other things.

That is because light is emitted from atoms when the electrons within the atoms shift between orbits. An atom of hydrogen, for instance, will emit a different light to an atom of helium. This enables astronomers to search for the "signature" of different elements in an object by measuring how much light is present at each wavelength of the object's spectrum. Spectroscopy is a vastly important tool for astronomers.

Newton's Law of Gravity Isaac Newton is towering figure. As well as splitting sunlight into its component colors, he shortened and improved the telescope by using mirrors. He formulated three laws of motion. And even today, he influences scientific investigation. Newton began the practice of comparatively testing theories by experiment and then refining ideas as necessary.

Perhaps his greatest contribution to science was his work with gravity. Newton discovered that gravity operates the same way throughout the Universe. Gravity abides by a universal law. On the basis of his studies, Newton concluded that every mass exerts a force of attraction on every other mass. He showed, moreover, that the strength of the force is directly proportional to the product of the two masses, divided by the square of the distance between them. If m and M are the masses of any two bodies, and the distance between their two centers is r, then the strength of the attractive force, F, between them is given by: F = GMm/r2 The factor G in this equation is a constant of proportionality whose value is found by measuring the force between two bodies of known mass and separation very precisely. If M and m are measured in kilograms and r is measured in meters, and F in SI units, then G = 6.67 x 10-11 m3 kg-1 s-2.

Kepler's Laws of Planetary Motion The German mathematician Johannes Kepler (1571-1630) formulated three laws of planetary motion. These laws were based on empirical evidence taken from Tycho Brahe's detailed and very precise visual observations of the motions of the planets, particularly Mars.

Kepler's Laws are: 1. The orbit of each planet is an ellipse - with the Sun at one focus of the ellipse.

2. As each planet orbits the Sun, an imaginary line connecting the Sun and the planet - known as the radius vector - sweeps out equal areas in equal intervals of time. This means that the speed of a planet in an elliptical orbit will vary as the planet orbits the Sun. The planet will be moving fastest when it is closest to the Sun - a position known as perihelion - and slowest when farthest away - a position known as aphelion.

3. The squares of the sidereal periods of the planets are proportional to the cubes of their mean distances from the Sun.

Although Kepler's Laws vastly increased knowledge of planetary motion and behavior, the physical basis of the laws was not understood until Isaac Newton formulated his Law of Gravity.

Links for Further Information:

Detailed page explaining Newton's Universal Law of Gravitation - recounting the ideas Newton formulated and how he tested them. Includes images and the equations used by Newton - plus the application of theories on weight and gravity to planets and stars. http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

Johannes Kepler's Laws of Planetary Motion. Explains elliptical orbits and Kepler's three laws - with images. http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

The Hubble Constant explained - the rate at which the Universe is expanding from the time of the Big Bang. http://csep10.phys.utk.edu/guidry/violence/hubble_constant.html

Galileo's observations through the telescope and his theories on the law of motion. Includes images illustrating how Galileo's observations altered theories on the structure of the Solar System. http://csep10.phys.utk.edu/astr161/lect/history/galileo.html

The Universe of Aristotle and Ptolemy - explaining their theories on the structure of an Earth- centered Universe, complete with illustration. http://csep10.phys.utk.edu/astr161/lect/retrograde.aristotle.html

Calendars through history - from the Roman Lunar Calendar to the Gregorian Calendar. http://csep10.phys.utk.edu/astr161/lect/time/calendars.html

Interesting page explaining how the motion of the sky can be used to record time. http://csep10.phys.utk.edu/astr161/lect/time/timekeeping.html

Questions and Activities for the Curious 1. Describe the method used by Eratosthenes to measure the diameter of the Earth.

2. Outline Kepler's three laws of planetary motion.

3. What are the main astronomical discoveries made by Galileo?

4. Explain, with a diagram, why, each year, planets that are farther from the Sun than the Earth make a loop in the night sky.

5. What are the major difference between the planetary systems of Ptolomy and Copernicus?

6. Describe three of William Herschel's major contributions to astronomy.

7. Some people still believe that the Earth is flat. What proof is there that our planet is round?

8. Tycho Brahe argued that the Sun orbited the Earth but that other planets orbited the Sun. Could Tycho's model explain the phases of Venus as observed by Galileo?

Complete Cosmos Chapter 25: Black Holes, Dark Matter Although invisible, black holes betray their presence. It's the same with dark matter - perhaps the missing 90% of the Universe. Outline Black holes are invisible. So too is dark matter - hidden mass that constitutes perhaps 90 per cent of the Universe. Invisible they may be, but the effects of black holes and dark matter can be observed. The outer parts of galaxies spin faster than their visible mass justifies, indicating the presence of hidden mass. Gravitational interaction within star clusters suggests they contain up to 100 times more matter than can be seen. Yet the nature of dark matter is a mystery.

How do black holes betray their existence? Look for a star continuously losing mass and orbiting apparently empty space. A black hole often forms in the aftermath of a supernova - where the exploding star has a core with a mass at least three times that of our Sun. Other evidence of an invisible presence is the distortion of space and light at the rim of a black hole. Anything and everything caught by a black hole is forever trapped. Even light cannot escape. Approach too close and the result is "spaghettification".

The bending of light may also disclose the presence of dark matter. Light traveling to Earth can be distorted by the gravity of a massive intervening object. So-called "gravitational lensing" and the degree of light-bending can reveal the amount of dark matter in the Universe.

At the center of galaxies, supermassive black holes are believed to lurk. They may have swallowed the equivalent mass of millions of Suns. Thirty-thousand could lie within a supermassive black hole at the center of our own Milky Way. So what makes a supermassive black hole? They may be triggered by galactic mergers or collisions. The most powerful black holes are associated with fiercely bright quasars. Quasars are fuelled by the hungriest black holes - consuming the equivalent of 600 Earths per hour. Albert Einstein accurately predicted the effects of supermassive objects in space. A final thought - could a black hole be a wormhole to another part of the cosmos or even to another Universe?

Sub-chapters The Invisible Universe • Black holes are invisible, their gravity so powerful that not even light can escape. • Nine-tenths of the Universe is thought to be invisible. This "dark matter" is undetectable at any wavelength from radio waves to gamma-rays. • Dark matter may be largely responsible for the development and structure of the Universe. • The gravity of unseen objects betrays their presence. • Outer parts of galaxies spin faster than their visible mass justifies. Dark matter, up to ten times the visible mass, accelerates the spin of these galaxies.

What Is Dark Matter? • Dark matter is not the visible black dust that peppers galaxies. Often, clouds of such dust are where stars are born. Invisible dark matter pulls on these stars. • Interactions between galaxies in clusters suggest up to 100 times more matter than can be seen. Yet the nature of dark matter remains a mystery. Could it be dead stars, failed stars, black holes or exotic sub-atomic particles? • When superstars die in supernovae, their cores collapse to form black holes - but only if the cores have at least three times the mass of the Sun. A telltale sign of a black hole is a star continuously losing mass and orbiting "empty space". • A black hole can also be detected by the distortion of light and space around its rim. • Material caught in the intense gravitational pull of a stellar mass black holes is spaghettified.

Bending Light • To become a black hole, Earth would have to be compressed into a ball just eight millimeters across. • Light can be bent by gravity - and it is one way of detecting dark matter. If a massive object, such as a galaxy, is placed in the path of a beam of light, the light is distorted as though passing through a lens. The galaxy acts as a "crazy lens", splitting the bent light into arcs and bright knots. • The degree of light bending enables scientists to "weigh" the intervening galaxy, revealing the weight of both the visible matter and the dark matter.

Galactic Black Holes • A swirling ring of gas at the heart of the Milky Way may be evidence of a massive black hole - with a mass 30,000 times that of our Sun. • Galactic mergers and collisions - where the galaxy cores unite - produce supermassive black holes. The strange galaxy Centaurus A is an example. At its center is a black hole with a thousand times the mass of the black hole at the center of the Milky Way.

Supermassive Black Holes • Supermassive black holes are surrounded by an accretion disk, matter swirling inwards due to intense gravitational pull. Some matter jets away at right angles to the disk. • Such a supermassive hole consumes the equivalent of four Earths an hour. • The supermassive black hole at the core of the galaxy Virgo A has the mass of two-and- a-half billion Suns. It fires a jet of material across space.

The Ultimate Distorter • Fiercely bright quasars play host to the most powerful black holes. Quasars are fuelled by supermassive black holes that consume the equivalent of 600 Earths an hour. • Einstein's General Theory of Relativity - explaining how massive objects can bend light and distort space. • What's inside a black hole? Could it be a wormhole to another part of the cosmos – or even to another Universe?

Background Black Holes Black holes are regions of space where the gravitational field is so strong that photons- particles of light traveling at 300,000 kilometers per second - cannot escape. Stellar mass black holes are formed when massive stars of greater than eight solar masses explode in a supernova. Supermassive black holes are much larger. They are formed during galactic mergers.

The first conclusive evidence for a supermassive black hole came in 1994. It was discovered at the center of the giant elliptical galaxy M87 by scientists using the Hubble Space Telescope (HST). Their evidence came from the effect of the black hole on its surroundings.

The speed of objects surrounding a black hole indicate the amount of mass within a certain radius. HST measured the speed of rotation of the accretion disk at the heart of M87. It indicated a mass of 2,500 million solar masses concentrated within an area so small that, according to basic physics, light could not escape - the criterion for a black hole.

From the moment of this discovery, supermassive black holes were no longer theoretical concepts. As well as lying at the hearts of many giant elliptical galaxies, radio galaxies, Seyfert galaxies and quasars, supermassive black holes are now believed to exist at the center of many spiral galaxies - including our own Milky Way. Scientists are trying to find the gravitational wave signatures of black holes as predicted by Einstein's General Theory of Relativity.

Back in 1783, the English cleric John Michell suggested that a "dark star" could be the product of a gravitational force so strong that light could not escape from it. A hundred years earlier, Ole Romer had accurately measured the speed of light. This enabled Michell to work out that our Sun - without changing its mass - would have to shrink to a body with a radius of just three kilometers if it were to prevent the escape of light.

Michell postulated that if such objects existed, light could not escape from them and they would be dark. Little more than a decade later, the French mathematician Pierre Simon Laplace reached the same conclusion. He published his ideas in1796 with the remark that, although these bodies might be invisible, we could infer their presence by the "revolvement of other luminiferous bodies around them". So black holes would most easily be found if they occurred in binary star systems.

Not until 1916 did Albert Einstein's General Theory of Relativity show that gravity is related to the curvature of space and that a black hole is a region of space where curvature becomes so extreme that a hole forms. Karl Schwarzschild analyzed the implications of Einstein's theory. He worked out a formula for calculating the radius of a black hole of any particular mass - in other words, the radius of the so called "event horizon", the point of no return beyond which light cannot escape. This radius is now called the Schwarzschild Radius.

Is the Universe Open or Closed? The amount of dark matter in the cosmos is crucial to answering the question of whether the Universe is "open" or "closed". If it is open, then the total volume of space is infinite and the Universe will expand forever. If it is closed, the Universe contains a finite amount of space and will eventually collapse back on itself - ending in a Big Crunch as the matter comes together.

It has been suggested that the Big Crunch could be followed by another Big Bang, giving rise to the birth of a new Universe. If so, the Universe might oscillate between Big Bang and Big Crunch and could go on forever. If the Universe is neither open nor closed it is said to be "flat". Watch this space.

Dark Matter It is possible that 99 percent of matter is hidden. Cosmologists have pondered the form this dark matter might take. Suggestions have included old dead stars, very low mass stars called brown dwarfs, rocky bodies the size of planets or asteroids, black holes or "exotic" particles. Recently, in 2000, the very existence of dark matter has been seriously questioned. Again, watch this space.

Gravitational Lenses: Nature's Giant Telescopes Nature has lent astronomers a hand with the discovery of "gravitational lenses". These naturally occurring "telescopes" can be used to study quasars and the faint light of remote galaxies that would otherwise be beyond the reach of even the most powerful telescope. As more gravitational lenses are identified, they promise to be a powerful tool in probing the early Universe.

If a faint, distant galaxy or quasar lies behind a nearer cluster of galaxies – and provided the cluster is sufficiently massive - then light rays from the distant object will be bent by the strong gravitational pull of the cluster. The overall effect is that the cluster of galaxies acts like a gigantic glass lens - magnifying and distorting the image of the faint galaxy or quasar behind it and producing arcs or multiple images. Gravitational lenses are already used to investigate the first era of star formation in high redshift galaxies, the mass of galaxy clusters, the existence of dark matter, the size and structure of distant quasars, as well as the size scale of the Universe.

Links for Further Information

Black holes and beyond. An interesting page with a brief history of the subject, reasons for studying black holes, a thorough account of black hole formation - and images. http://www.ncsa.uiuc.edu/Cyberia/NemRel/BlackHoles.html

Good page on black holes - introductory information, images with accompanying text, clear diagrams to show the effects and behavior of black holes. http://www.damtp.ca.ac.uk/user/gr/public/bh_critical.html

Dark matter in the Universe. An educational introduction to dark matter - its effects on galaxy clusters, gravitational lensing and images. http://www.zebu.uoregon.edu/1996/ph123/19.html

An interesting picture on the appearance of the Universe with dark matter, plus accompanying text. http://www.ucolick.org/~deep/overview/darkmatt.html

An excellent page on the theory of relativity. Includes links relating to Einstein, gravitational lensing, pulsars and other relativity links on the web. http://csep10.phys.utk.edu/astr162/lect/cosmology/gravity.html

Radio image of the region around the black hole at the center of the Milky Way. http://dnausers.d-n-a.net/dnetGojg/Black/mw.htmat

Questions and Activities for the Curious 1. Summarize the evidence supporting the idea that galaxies and galaxy clusters contain a great deal of dark matter.

2. What is dark matter? Give examples of what it might be.

3. Describe the events leading up to the formation of a stellar mass black hole in a supernova explosion.

4. What would happen to someone falling into a stellar mass black hole - and why?

5. What does the term "escape velocity" mean? Use it to explain why black holes are said to be "black".

6. Explain the principle of gravitational lensing and give some examples.

7. Why do astronomers believe that a supermassive black hole inhabits the center of our Milky Way galaxy?

8. What is meant by an "open" or "closed" Universe? Explain why dark matter is important in this regard.