BR-225 June 2004

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The (ESA) was formed on 31 May 1975. It currently has 15 Member States: Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.

The ESA Science Programme has launched a series of innovative and successful missions. Highlights of the programme include: Cassini-HuygensCassini- , which is a four- mission SMART-1, Europe’s first mission to the to investigate in unprecedented detail the Moon, which will test -electric propulsion interaction between the Sun and the ’s in flight, a key for future deep- UniqueUnique InsightsInsights intointo aa RingedRinged WorldWorld magnetosphere. space missions.

Double Star, following in the footsteps of SOHO, which is providing new views of the the Cluster mission, with its two spacecraft it Sun's and interior, revealing solar studies the effects of the Sun on the Earth’s tornadoes and the probable cause of the environment. supersonic solar wind. , which took the first close-up pictures of a comet nucleus (Halley) and , the first spacecraft to fly over the completed flybys of Comets Halley and Grigg- Sun’s poles. Skjellerup.

Hipparcos, which fixed the positions of XMM-Newton, with its powerful mirrors, the stars far more accurately than ever before is helping to solve many cosmic mysteries of and changed astronomers' ideas about the the violent X-ray Universe, from enigmatic ContentsContents scale of the Universe. black holes to the formation of galaxies.

Hubble Space , Solving the puzzles of and 4 a collaboration with NASA on the world's most important and successful orbital observatory. High ambitions for an outstanding planetary mission 6

Integral, which is the first space A long and rich journey 8 8 observatory that can simultaneously observe celestial objects in gamma rays, X-rays and What lieslies beneath?beneath? 12 12 visible . Mysterious Titan 16 ISO, which studied cool gas clouds and 2 -3 planetary . Everywhere it looked, it found water in surprising abundance. Vehicles of discovery 20

IUE, the first space observatory ever launched, marked the real beginning of ultraviolet astronomy.

Mars Express, Europe’s first mission to Mars, which consists of an orbiter and a looking for signs of water and life on the Red Planet. , Europe’s comet chaser, will be the first mission to fly alongside and land on a comet, probing the building blocks of the in unprecedented detail.

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More information can also be obtained via the ESA Science Web Site at: http://www.esa.int/science ©ESA 2004 Top: During this ring plane crossing observed by Hubble,Titan is casting a shadow on Saturn.The other moons appear white because of their bright, Solving the puzzles of Saturn icy surfaces. Bottom: The planet with its rings slightly tilted.Two other moons appear on the

Courtesy NASA/JPL-Caltech lower right and the upper left.

and Titan The typical orange-coloured .

When the first space missions flew past Saturn at beginning of the 20th century.Titan has its own coincides with the planet’s axis of rotation; this is unique the end of the 1970s, very little was known about this atmosphere, and in 1944 Dutch-born astronomer Gerard in the Solar System. Finally, discovered that mysterious and beautiful world.The most distant planet Kuiper discovered that it contained .This made Saturn radiates more heat than it receives from the Sun. that could be seen by ancient astronomers, Saturn had Titan unique in the Solar System. A relatively thin Originally it was thought, as with Jupiter, that this was appeared for centuries to be just another bright star in atmosphere has only ever been found on two other large due to the slow shrinking of the planet. However, from © E.Karkoscha (Arizona Univ. & Planetary Lab)/NASA the night sky. Only when the telescope was invented at moons, Jupiter’s Io and Neptune’s Triton. detailed calculations it emerged that Saturn cannot the beginning of the 17th century could human eyes see shrink any further and had already reached its current Saturn has also been a frequent target of the for the first time that this celestial body was actually a The space age is transforming our size 2000 million years ago. NASA/ESA , which has produced planet. knowledge of Saturn and Titan stunning views of long-lived hurricane-like storms in the When Voyager-1 encountered the planet in 1980, planet’s atmosphere.The world’s major , Saturn and Titan still had many secrets. Before the it saw cloud cover, storms and weather systems.To including Hubble, were recently trained on Saturn to late 1970s, ground observations had shown Saturn itself everyone’s surprise, the clouds above the equator observe an event known as a ‘ring plane crossing’ when to be a large ball of hydrogen and helium, flattened at appeared to travel with the highest velocity ever the rings are seen edge-on from Earth. During these the poles due to its rapid rotation. Occasionally, light and measured in the Solar System – up to 1800 kilometres an crossings, faint objects near the planet are easier to see. dark spots could be seen in the vague bands of cloud hour! Polar auroral were also observed for the first Many of Saturn’s known moons were discovered during running parallel to the planet’s equator.Virtually nothing time. Both Voyagers photographed the moons known at these crossings. was known about the moons, except for their orbits, that time, and discovered some new ones; most consisted orbital periods and approximate sizes. mainly of ice and rock, and were strewn with craters Titan was also observed by the two Voyagers, as sometimes so big that they would cover a large well as other telescopes. Both spacecraft could observe The US Pioneer 11 spacecraft provided sensational proportion of the moon’s surface.They observed a its mysterious orange atmosphere, rich in nitrogen,

Courtesy NASA/JPL-Caltech discoveries in 1979. Cassini’s ‘division’ was not empty, but breathtaking collection of ‘ringlets’,bands often no methane and other organic compounds. In 1998 ESA’s The so-called ‘Cassini division’ is visible in this picture of Saturn’s consisted of a myriad of thin dust rings. Outside the thicker than a few tens of metres in a subtle balance Infrared Space Observatory (ISO) discovered the presence ring taken by . outermost known ring, yet between the gravity of the planet and that of nearby of water vapour in Titan’s atmosphere. Basically Titan another ring was found. moons.The particles in the rings ranged in size from exhibits many similarities to conditions that may well To Galilei, the planet appeared as a ‘triplet’ (a Saturn proved to have a near-invisible dust grains to icebergs the size of a house. once have prevailed on Earth before life appreared. 4 -5 planet and two large moons), but in 1659 Dutch scientist strong magnetic field, much saw that the two ‘moons’ were in fact more powerful than that of a thin flat ring surrounding the planet. It later became Earth.This meant that, like clear that this ring was really a system of rings. Italian Jupiter, it had to have a core astronomer Jean-Dominique Cassini discovered in 1675 of liquid hydrogen and helium Courtesy NASA/JPL-Caltech that the ‘ring’ consisted of an outer ring and an inner ring, compressed so much that the Cassini-Huygens, named after the two celebrated scientists, is the joint NASA/ESA/ASI mission to Saturn and its giant separated by a darker band, now known as the ‘Cassini mixture has the properties of High-speed winds blow moon Titan. It is designed to shed light on many of the unsolved mysteries arising from those previous observations. Division’.Nearly two centuries later, the English physicist a metal. Strangely, the axis of parallel to Saturn’s James Clerk Maxwell proved that the rings had to consist this magnetic field almost equator. •What is the source of heat inside Saturn that produces 87% more energy than the planet absorbs from sunlight? of many small particles, all orbiting Saturn like individual •What is the origin of Saturn’s rings? moons. •Where do the subtle colours in the rings come from? •Are there any more moons? Of all the planets, Saturn still has the highest •Why has the moon such an abnormally smooth surface? (Has recent melting erased craters?) number of satellite bodies, currently counted at 31.The •What is the origin of the dark organic material covering one side of the moon ? largest moon,Titan, was discovered by Huygens in 1655. •Which chemical reactions are occurring in Titan’s atmosphere? Four more were first observed by Cassini and, thanks to •What is the source of the methane, a compound associated with biological activity on Earth, which is so abundant in better telescopes, nine more moons were known by the Titan’s atmosphere? •Are there any on Titan? •Do more complex organic compounds and ‘pre-biotic’ molecules exist on Titan? NASA/R. Beebe/D. Gilmore, L. Bergeron ,STScI/NASA Storms on Saturn as seen by the Hubble Space Telescope. High ambitions © ESA

for an outstanding Courtesy NASA/JPL planetary mission

In many respects, a journey to Saturn is a mounted.The mission calls for a journey back to the origins of the Solar sophisticated robotic spacecraft to orbit the System. Saturn’s rings and moons are a ringed planet over a four-year period and a Artist’s impression of the Cassini-Huygens mission at Saturn and Titan. model of the archetypal Solar System, when scientific probe called ‘Huygens’ to be fragments of rock and ice collided with released from the main spacecraft, each other and melted on a grand scale. parachuting through the atmosphere of Titan, Saturn’s most interesting moon, is Titan and eventually landing on its surface. similar to the primeval Earth in its frozen state. Perhaps the building blocks of life Cassini-Huygens is a masterpiece of have been preserved there? This collaboration that, from the initial vision in environment is certainly an intriguing place the early 1980s to the completion of the The Huygens probe descending onto where time appears to have stood still for nominal mission in July 2008, will span Titan billions of years. nearly 30 years. It is a joint endeavour by The Mission at a Glance the US National Aeronautics and Space Cassini-Huygens is the most ambitious Administration (NASA), providing the Cassini Orbiter Huygens Probe effort in planetary space exploration ever Cassini orbiter, the European Space Agency Mission name Named after the French-Italian astronomer Named after the Dutch (ESA), providing the Jean-Dominique Cassini scientist Christiaan Huygens probe, and the (1625-1712), who Huygens (1629-1695), Agenzia Spazio Italiana discovered the four who discovered (ASI) that, through a major moons – Iapetus, Saturn’s rings and bilateral agreement with , and Titan. NASA, is providing – and the ‘Cassini Division’ hardware systems for the Cassini orbiter and Destination Saturn and its moons Titan instruments. Objectives In-depth studies of Saturn, its atmosphere, First direct sampling of Titan’s 6 -7 The twelve scientific rings, magnetic environment and moons atmospheric chemistry and first instruments on the photographs of the surface Cassini orbiter will conduct in-depth studies Experiments 12 instruments, led by the USA, Germany 6 instruments, led by France, Italy, of the planet, its rings, and United Kingdom Germany, the United Kingdom and the atmosphere, magnetic USA environment and a large number of its moons.The Launch date 15 October 1997, from Cape Canaveral (Florida, USA) six instruments on the Huygens probe will Launcher Titan-IVB/Centaur provide our first direct Expected operational Four years, from 1 July 2004 (Saturn Orbit 22 days, including up to 2.5 hours of and detailed sampling of lifetime Insertion) to 1 July 2008 descent through Titan’s atmosphere and Titan’s atmospheric a few hours on its surface chemistry and the first photographs of its Ground operations NASA JPL, using stations of NASA’s Deep ESA’s European Space Operations Centre Space Network in California, Spain and (ESOC) in Darmstadt, Germany. hidden surface. Australia. Courtesy NASA/JPL-Caltech The complex Saturnian system resembles an archetypal Solar System. Along and rich journey

Cassini-Huygens has taken almost seven years to reach its final destination, the Saturnian planetary system, which lies more than 1200 million kilometres from Earth. Based on the alignment of the planets and the capabilities of the , the spacecraft was launched in October 1997.The launch boosted the spacecraft into a Venus-Venus-Earth-Jupiter gravity-assist trajectory to reach Saturn on 1 July 2004. During each planetary fly-by, the spacecraft increased its speed relative to the Sun and gained the cumulative boosts it needed to reach its target.The long journey also provided an excellent to make scientific observations of several targets on the way. Once at Saturn, Cassini-Huygens will make 75 orbits around the planet, fly-bys of several of its moons, including , Enceladus, Rhea, Dione, and Iapetus, with 45 encounters with Titan.

• 15 October 1997: Launch, at 08:43 UT, from Cape Canaveral by a Titan • 11 June 2004: Phoebe moon fly-by at about 2000 km from the moon.This fly-by marked the beginning of IVB/Centaur rocket. After being placed in a parking orbit around Earth, the the approach to Saturn. Phoebe’s darkness and irregular, retrograde orbit suggest that it is most likely a Centaur upper stage was fired to send the spacecraft on an interplanetary captured ‘Centaur’,i.e. an object coming from a region of the outer Solar System known as the ‘Kuiper Belt’.If trajectory to Saturn. this is the case, this is the first intensive opportunity scientists will have to study a Kuiper-Belt object.

• 27 April 1998: 1st Venus fly-by,at 287 km from the planet.The spacecraft • 1 July 2004: arrival at Saturn. Saturn Orbit Injection (SOI) will mark the end of the spacecraft’s searched for emissions from lightning in Venus’s atmosphere. journey through the Solar System, and the beginning of its tour of Saturn, its rings, moons and magnetosphere.Cassini-Huygens will approach Saturn from below the ring plane.On 1 July 2004, • 24 June 1999: 2nd Venus fly-by, at 600 km from the planet. The Cassini will fire its main engine for 96 minutes to brake and allow it to be captured as a satellite spacecraft studied the chemical composition of the Venusian of Saturn. Passing through the large gap between the F-ring and G-ring, Cassini will then environment and detected energetic neutral atoms escaping from Venus’s swing by close to the planet to begin the first of 75 orbits. Cassini-Huygens will then be atmosphere. No evidence of radio emission from lightning was observed. about 160 000 km from Saturn’s centre. One hour later the spacecraft will achieve its closest approach of the whole mission, passing less than 18 000 km above the cloud • 18 August 1999: Earth fly-by, at 1171 km from the planet. Cassini- tops. Cassini-Huygens will then coast above the rings for another hour and 44 Huygens provided a unique view of the Earth’s magnetosphere on a very minutes before descending back through the ring plane.The SOI burn is critical to short time scale and, in combination with other space and ground the success of the mission. In this phase, the spacecraft's proximity to Saturn and observatories, participated in an international campaign of observations its rings provides a unique opportunity for a close-range study. of Earth’s space environment. • 26 October 2004: 1st Titan fly-by at 1200 km distance. Cassini will make • December 1999-October 2000: crossing the asteroid belt,between the the first direct sampling of Titan’s atmosphere and the first radar probing of orbits of Mars and Jupiter. its surface.

• 30 December 2000: Jupiter fly-by at 9.7 million km distance from the • 13 December 2004: 2nd Titan fly-by at 2350 km distance. planet. During the six months in which Cassini was closest to Jupiter, it collaborated with NASA’s Galileo spacecraft, which had been orbiting • 15 December 2004: Dione fly-by at 84 000 km altitude, to study the Jupiter since 1995, for the rare opportunity to study the Jovian system moon’s icy surface and its irregular features,and investigate its formation from two different nearby perspectives at the same time. The two craft process. monitored how fluctuations in the solar wind approaching Jupiter affect 8 -9 the planet’s magnetosphere, a vast region of ionised gas under the • 25 December 2005: Huygens release window opens. After three influence of Jupiter’s magnetic field. They also examined Jupiter’s moons, revolutions around Saturn, the Huygens probe will be released to enter rings and storm clouds. During this phase, the Huygens probe helped the Titan’s atmosphere and will then coast for a maximum of 22 days before Cassini high-gain antenna to shield the orbiter and its instruments from arriving at Titan on 14 January 2005. No systems will be active during solar radiation. this coasting phase except for the timer that will wake up the probe and its instruments 4 hours before the initial encounter with the outer • 2001-2003: Gravitational-wave experiments. Cassini looked for ripples fringes of Titan's atmosphere. In the meantime, five days after Huygens’ in the fabric of space caused by the of massive bodies by release, Cassini will perform a deflection manoeuvre to position itself studying possible - almost imperceptible - fluctuations in the spacecraft’s with the proper geometry to collect the data during the Huygens speed relative to Earth. mission.

• February 2002: Cassini started its cruise-science experiments, to be • 1 January 2005: Iapetus fly-by at 64 000 km distance, to study this ‘two- concluded on arrival at Saturn: faced’ moon covered on one side with some of the darkest material in the February 2002: interplanetary dust detection, and detection of dust Solar System, and very bright on the other side. streams coming from Jupiter. Summer 2002: by measuring how much the Sun’s gravity bends an • 14 January 2005: Huygens descent. No later than 22 days after its release electromagnetic beam propagating between the spacecraft and Earth, from Cassini, Huygens will start its descent through Titan’s atmosphere to land Cassini tested the validity of Einstein’s Theory of General Relativity with a about 2.5 hours later.Meanwhile, Cassini will fly-by Titan for the third time, passing precision 50 times higher than previous measurements. within 60 000 km at its closest approach. January-June 2004: approach science. In a joint observation campaign with the Hubble Space Telescope, Cassini observed Saturn’s aurorae (polar lights),upstream of solar wind and radio emissions from the ringed planet. It also observed the planet’s atmosphere, rings and Titan. © NASA/JPL Courtesy NASA/JPL-Caltech

Enceladus as seen by .

Cassini’s Huygens descent profile • 15 February 2005: 4th Titan fly-by at continues about 950 km altitude. Instruments awoken, and radio link activated so that the • 17 February 2005: 1st Enceladus fly-by at

orbiter can listen to the probe for the next 4.5 hours After the Huygens portion of the mission, about 2900 km altitude.This smooth © NASA/JPL (after that Cassini will disappear behind Titan’s horizon). Cassini’s focus will shift to making moon is ten times as bright and reflective Cassini will also study Rhea measurements with the orbiter’s 12 as the Earth’s moon. Its orbit is embedded (top), Saturn's second largest satellite after Titan, and instruments and returning the information in the thickest part of the E-ring. Tethys (bottom). They are nd During the first three minutes inside Titan’s atmosphere, to Earth. Cassini will study Saturn’s polar • 9 March 2005: 2 Enceladus fly-by at both icy, densely cratered Huygens will decelerate from 18 000 to 1400 km/h.The regions in addition to the about 750 km altitude. satellites. temperature of the gas heated by friction with the planet’s equatorial zone. • 2005-2008: Cassini continues Saturn Huygens heatshield may reach 8000°C. Observations of seven observations and Titan fly-bys. selected icy moons will be • 1 July 2008: Nominal end of the Cassini- made, plus at least two Huygens mission. dozen more-distant Pilot parachute automatically deployed to pull out the fly-bys of other moons main parachute at a speed of about 1500 km/h. Speed reduced to less than 300 km/h within a minute. and there will be more than 44 encounters with Titan (also used for gravity- assist orbit changes that shape the Cassini orbital tour). The shell of the descent module falls away and exposes the scientific instruments to Titan's atmosphere at a height of about 160 km.The atmospheric temperature may then be about -120°C.

10 11 15 minutes later at about 120 km, the main parachute will be cut away and replaced by a smaller one, designed to allow a steady descent at about 20 km/h. History of a mission scenario At about 45 km altitude, the probe will go through the coldest layer of the atmosphere, at about –200°C. When a communications test on the Huygens probe was performed in February 2000, nobody foresaw the dramatic impact that its results would have on the Cassini-Huygens mission. An unexpected anomaly in the probe’s communications system meant that, during the descent to Titan, most of the scientific data would have been lost, A radar altimeter will measure the probe’s altitude during unless the two spacecraft were moving at relative velocities quite different from those initially planned. the last 10-20 km. During its descent, the probe's camera This was due to a design flaw in the Huygens receiver, which meant it was unable to compensate for the frequency shift will capture images of Titan’s cloud deck and surface. Data between the signal emitted by the probe and the one received by the moving orbiter, the so-called ‘Doppler’ shift. from Huygens will be relayed to Cassini passing overhead Given the impossibility of fixing an anomaly in the hardware on a spacecraft speeding towards Saturn, the Cassini- for later playback to Earth. Huygens engineers faced a huge challenge to save the Huygens mission. After six months of intensive work, the ESA/NASA Huygens Recovery Task Force came up with a solution: the first two Cassini orbits around Saturn would be shortened and an additional orbit, much higher than originally planned, would be inserted.The dates of the first two Titan fly-bys had to be delayed, with Huygens being released during a third fly-by (25 December 2004) from which the probe would get to Titan in a maximum of 22 days, seven weeks later than originally planned. In addition, the probe and instrument software had to be modified to cope with the earlier wake-up of Huygens. With this new scenario, Cassini will be able to receive Huygens’ data and to resume its original trajectory in February 2005.This solution has been achieved at the cost of reducing by only a quarter the amount of fuel available for Cassini to possibly extend its four-year mission. Pioneering space exploration is all about thrilling moments - problems can ©ESA © Jon Williams/NASA/JPL easily arise, and a measure of a team’s excellence is the manner in which it solves them! Saturn’s cloud structure. © NASA/JPL-Caltech Whatlies beneath?

Saturn, the second most massive planet in Winds and weather the Solar System after Jupiter, is a treasure - trove of opportunities for discovery.The Despite its serene appearance, Saturn’s Cassini orbiter has a host of investigations atmosphere is a violent place. Cassini will and experiments to perform once in orbit. It therefore monitor the ‘weather’ as this will will study the planet itself, the ring give clues to the chemistry of the structure, the moons, the magnetic field and atmosphere and the interior processes of Titan. Not only will it determine the present the planet.Wind speeds have been clocked state of these bodies and the processes at a staggering 1800 km/h near the equator. operating on or in them, but it is also Cassini will monitor these and also measure equipped to uncover the interactions that temperature variations across the planet. occur between them. Occasionally, white storms – bigger than © NASA/JPL-Caltech Earth – erupt through the cloud layers.The Atmosphere of Saturn. last of these occurred in 1994 and was observed by the Hubble Space Telescope. Facts about Saturn Gas planet Cassini scientists hope for another similar Type of planet: Gas giant such event to take place when their Saturn is a sister world to spacecraft is at Saturn and thereby get a Average distance from Sun 9.6 AU (1 Astronomical Unit is the average distance from the Earth to the Jupiter and both are essentially closer view. Sun, about 150 million kilometres) gigantic balls of gas. It is mostly a mixture of Between February hydrogen and helium, but many other Orbital period (Saturnian year) 29 years and 5.5 months and March 2004 chemical elements are present too. Saturn is Cassini has observed Rotation period around axis (Saturnian day) 10 hours and 39 minutes the least dense of all of the planets and, if two storms merging, 12 13 you could find an big enough, Saturn both with diameters Mass 95.2 Earth masses would float on water. close to 1000 km. Volume More than 750 times that of the Earth Saturn spins very quickly - once every 10 Diameter at equator 120 536 km (about 10 times the size of Earth). 1 Saturn radius is 60 268 km at hours and 39 minutes - so that the planet the equator bulges at its equator. Unlike Jupiter, the Diameter pole to pole 108 728 km colour of Saturn’s cloud layers is muted and subtle.This is because a deep layer of haze

Average density 0.7 g/cm3 (that of the Earth is 5.4 g/cm3) sits above the visible cloud layers, and has © NASA/JPL/Space Science Institute the effect of ‘washing-out’ the colours. Composition Mainly hydrogen (96 %) and helium (4 %) Computer simulations suggest that Saturn The deep interior Temperature at cloud surface -139°C has no surface to land on; instead the atmosphere gets denser with depth until it Deep inside Saturn there must be a source Temperature at core Expected to be around 10 000°C begins behaving like a highly compressed of energy. Mysteriously, the planet radiates Width of ring system From a few thousand to a few hundred thousand km above the cloud top liquid. Cassini will probe Saturn’s interior 87% more power than it absorbs. At ten indirectly by studying the effect of the times the distance of the Earth from the Average thickness of the ring system Between 30 and 500 metres planet’s gravity on the spacecraft’s Sun, Saturn only receives 1% of the sunlight trajectory and by measuring the planet’s that we do.The planet reflects one third of Number of known moons 31 (13 discovered by ground telescopes after the launch of Cassini-Huygens) magnetic field. it back into space, absorbing the rest.To Auroral displays Spokes in Saturn' rings. (polar lights) are Their sharp, narrow primarily shaped appearance suggests and powered by a short formation times. continual tug-of- war between Saturn’s magnetic © NASA/JPL-Caltech

© Jon Williams/NASA/JPL field and the flow of © J. T. Trauger (JPL)/NASA. charged particles from the Sun. This image was taken by Saturn’s ring particles in range in size from dust grains to rocks the size of a house. Hubble in 1997.

account for the extra energy radiated by On these moons, water freezes solid and so Saturn, scientists believe that helium may geological forces can sculpt it into Science experiments on Cassini be ‘raining’ through the atmosphere, mountains and impacts can leave craters. There are twelve instruments mounted on the Cassini orbiter.The scientific experiments are releasing energy as it sinks. Cassini can However, Enceladus is beautifully smooth, primarily mounted on two platforms: one takes care of the photography and spectroscopy, and measure the precise amount of energy as if its features have been rubbed away. the other is equipped for measuring magnetic and electric fields and for detecting neutral and radiated by Saturn, perhaps helping to solve Iapetus is bright on one side but dark on charged particles and dust. In addition, there are several instruments that are body-mounted. this mystery. the other. How did these contrasting surfaces come about? Instrument Purpose Puzzling rings Imaging Science Takes pictures in visible, near-ultraviolet and near-infrared light. Many of Saturn’s moons resemble Jupiter’s Subsystem – ISS Saturn’s rings are one of the wonders of the Europa, which has a global ocean under its (USA, F, D, UK) Variations in chemical Universe. Although they are over 250 000 icy crust, and many scientists expect to find composition are km in diameter, they are less than a similar oceans within some of Saturn’s Cassini radar – RADAR Maps surface of Titan using radar imager to pierce veil of haze. Also possibly responsible for kilometre thick.They are composed of dust moons. Cassini will investigate the (USA, F, I, UK) used to measure heights of surface features. the colour of Saturn’s grains and boulder-sized lumps of rock and characteristics of many of the moons, and rings enhanced in this Radio Science Searches for gravitational waves in the Universe; studies the image. ice. Pressed together, the rings would make will look for clues concerning their Subsystem – RSS atmosphere, rings and gravity fields of Saturn and its moons by a ‘moon’ only about 100 km in diameter. formation and evolution. (USA, I) measuring telltale changes in radio waves sent from the spacecraft.

From Earth, the The mystery of Saturn’s and Neutral Mass- Examines neutral and charged particles near Titan, Saturn and rings appear magnetosphere Spectrometer –INMS moons to learn more about their extended atmospheres and divided into three (USA, D) ionospheres. main bands, but in Saturn is surrounded by a giant magnetic Visible and Infrared Mapping Identifies the chemical compositions of the surfaces, atmospheres reality there are field, aligned with the rotation axis of the Spectrometer – VIMS and and its moons by measuring colours of visible thousands of planet.This cannot be explained by current (USA, F, D, I) light and infrared energy emitted or reflected. ringlets, all bunched theories. Cassini’s data may explain how the Composite Infrared Measures infrared energy from the surfaces, atmospheres and rings together. Cassini puzzling magnetic field is generated.This Spectrometer – CIRS of Saturn and its moons to study their temperature and therefore has plenty field may also cause strange features in the 14 15 (USA, F, D, I, UK) compositions. to do, mapping rings called ‘spokes’.These markings fall

© NASA/JPL-Caltech these ringlets and across the rings like spokes in a wheel, and Cosmic Dust Studies ice and dust grains in and near the Saturnian system. measuring their compositions. Scientists will may be caused by electrically charged Analyser – CDA be able to investigate why the rings are particles caught up in the magnetic field, (D, CZ, F, UK, USA, ESA) arranged like this and how they formed. but there are as yet no detailed theories Radio and Plasma-Wave Investigates plasma waves (generated by ionised gases flowing out They could be the debris of a shattered about them. Spectrometer – RPWS from the Sun or orbiting Saturn), natural emissions of radio energy moon, or the leftovers from a moon that (USA, A, F, S, UK, N) and dust. never formed. Cassini Plasma Explores plasma (highly ionised gas) within and near Saturn’s Discovering Saturn’s moons Spectrometer – CAPS magnetic field. (USA, F, FIN, HU, N, UK)

In addition to Titan, Saturn has 30 more Ultraviolet Imaging Measures ultraviolet energy from atmospheres and rings to study known moons, all of which are rocky and icy Spectrograph – UVIS their structure, chemistry and composition. worlds. Phoebe is the most distant and (USA, F, D) orbits the ‘wrong’ way around the planet, Magnetospheric Imaging Images Saturn’s magnetosphere and measures interactions between indicating that it was probably captured Instrument – MIMI the magnetosphere and the solar wind, a flow of ionised gases from the Kuiper Belt. (USA, F, D) streaming out from the Sun.

© David Seal Dual-technique Studies Saturn’s magnetic field and its interactions with the solar Large ice crevasse on the surface of Phoebe, the least known of all the Saturnian satellites. Magnetometer – MAG wind, the rings and the . (UK, D, USA, HU) Mysterious Titan

Titan is a truly fascinating world. It magnetic changes around the moon, is freezing cold, with temperatures Cassini may be able to determine whether reaching minus 180oC, and has a there are underground liquid-hydrocarbon very thick atmosphere whose reservoirs. Both Cassini and Huygens will origin is still unknown, consisting provide clues about the moon’s inner mainly of nitrogen, just like Earth, structure. Cassini will also see how Titan’s but also very rich in organic upper atmosphere interacts with the compounds which are constantly and if it has a reacting. In particular, a few significant magnetic field of its own.The

© NASA/JPL-Caltech percent of methane is continuously combined observations of Cassini and © Jon Williams/NASA/JPL What secrets has Titan still to reveal? © Jon Williams/NASA/JPL resupplied in Titan’s atmosphere by The Cassini spacecraft investigating Titan’s Huygens will help constrain the possible a mechanism that is still a mystery. atmosphere (left) and surface (right) from orbit. scientific scenarios for the formation and the evolution of Titan and its unique Orange-coloured clouds and mist due to atmosphere. the organic haze are so opaque that the surface can hardly be seen.They are created Titan also has a ‘greenhouse-warmed’ Both Cassini and Huygens will look at A drop into the unknown by sunlight and cosmic rays breaking down climate, as does the Earth, but sustained by Titan, and their combined data will the methane in the atmosphere, and different gases.Volcanoes and impacts greatly improve our understanding The whole of the Huygens scientific mission producing a range of complicated organic shape the surface and maybe provide of this mysterious moon.They will is to be carried out during just 2.5 hours of compounds that float down to the surface energy to make even more complex study its atmospheric chemistry exciting descent through Titan’s to accumulate over time. organic molecules.Water cannot exist in and investigate the energy source atmosphere and up to a few hours on its liquid form because the surface is far too that makes it so active.They will surface. Despite such a short mission cold, unless it exists for relatively short also look into Titan’s ‘weather’, duration, however, scientists will be able to periods because of the heat generated by measuring winds and gather a huge amount of scientific data that volcanism or asteroid impacts.Very little is temperatures, cloud physics and 16 17 known about the moon’s surface and circulation, lightning and scientists speculate that Huygens may find seasonal changes, as well as lakes or even oceans of a mixture of liquid possible climate changes. ethane, methane and nitrogen, and

© NASA/JPL/Space Science Institute possibly underground reservoirs. It seems The physics, topography In this image of Titan taken in May 2004, Cassini has already surpassed the best that the pressure and temperature on and composition of the views of the moon taken from Earth. Titan’s surface are sufficient to liquefy these surface will all be natural gases. investigated. By measuring Distance from centre of Saturn 1 221 870 km (= 20.3 Saturn radii) possible Orbital period (Titanic day) About 16 Earth days Diameter 5150 km (larger than Mercury - 4878 km, and slightly smaller than Jupiter’s Ganymede – 5262; Earth’s Moon is only 3476 km) Atmospheric thickness More than 1000 km Mass 1/45 of Earth’s mass (more massive than Pluto) Expected surface temperature About -180oC Surface pressure About 1.5 times higher than on Earth Average density About 1.9 times the density of water © ESA Huygens will be the first probe to descend to the surface of a world in the outer Solar System, and will make detailed studies of Titan. © Craig Attebery © Courtesy NASA/JPL-Caltech

Titan as seen by Voyager.

Weather and chemistry in the haze atmosphere (between 150 and 40 km will make their enormous efforts worthwhile. Even if the probe does altitude, and at around 20 km altitude). not survive the landing, the mission will be considered a tremendous Meteorologists will be fascinated by the success. parallels and contrasts with the weather on Hard landing or splashdown? Earth, in a world where clouds and raindrops Is there a primeval Earth around the corner? are made of methane and nitrogen.Winds of As the probe breaks through the cloud 500 km/h, which are expected to diminish deck, its camera will take up to 1100 Huygens may splash One of the main reasons during the descent, will propel Huygens pictures of the panorama and observe down on a sea of for sending Huygens to sideways when the main parachute opens. surface properties. Perhaps 50 km above it see volcanoes erupting with ammonia hydrocarbons. Titan is because in some The probe will be able to deduce the the surface, the haze may clear and give and water? If the probe survives the Simulations have shown that waves ways it is the closest prevailing wind speeds and provide detailed Huygens its first glimpse of the surface landing, the Surface Science Package will produced by Titan’s analogue to Earth weather information, such as temperature and between fluffy cloud-tops. A radar altimeter come into its own for the last phase of the winds would be up to before life began. Its pressure. It will also be able to measure the will help to determine Titan's surface mission. It will be able to tell whether it is in seven times higher than atmosphere and surface electrical properties of the atmosphere and characteristics by listening for echoes. a liquid, and the chemical composition of on Earth’s oceans but may contain many register radio pulses from lightning strokes, if A special lamp, turned on for the final that liquid. It may even detect waves and would move more slowly. chemicals of the kind they occur. A microphone will listen for stage of the descent, will allow accurate measure the depth. It will be able to deduce that existed on the thunderclaps. measurement of the colours of the surface the ratio of methane to ethane in the liquid, young Earth, and to help the probe’s spectrographs analyse which will give an indication of how long stocked the primeval Huygens will rotate as it drops, and its its composition. Titan has spent converting the one into the soup in which the first cameras will scan the scene over 360o, other. Scientists should then be able to living organisms imaging the cloud layers.The view will be Will Huygens land in an ocean of methane judge whether the ocean is as old as Titan, appeared. It is known very fuzzy, with the Sun plainly visible, but its and ethane, with coloured organic icebergs, or a later addition. On a dry surface, that complicated halo will allow or on solid land with geysers spouting Huygens will be able to measure its carbon molecules are the probe methane from underground reservoirs? Will hardness and whether the surface is level. present in cosmic to measure space, but ultraviolet the size and 18 19 light from the Sun, abundance of the Science experiments on Huygens cosmic rays and haze particles, Instrument Purpose lightning strokes could while the also manufacture spectrometers will Gas Chromatograph and Mass- Measures the chemical composition of gas in the atmosphere

© ESA carbon compounds on measure the heat Spectrometer – GCMS (USA, A, F,D) What chemical tricks can the Sun’s rays play in Titan’s planets like Earth. flows inwards atmosphere? Aerosol Collector and Pyrolyser – ACP Measures the chemical composition of aerosols (dust) from the Sun and (F,A,USA) Huygens will investigate this ‘home cooking’ on Titan. It will identify outwards from the complex molecules by their masses, and by their speeds of transit Titan into space. Descent Imager (panoramic camera) Takes images to study the distribution of aerosols (dust) and cloud droplets and through various filters. It will collect particles from the atmosphere These instruments and Spectral Radiometer – DISR to determine the nature of the surface. Makes spectral measurements to record and use an oven to vaporise them for identification. Even today, we will tell us about (USA, D, F, CH) the thermal properties of the atmosphere and measure its composition. still do not know how the self-sustaining assemblies of nucleic acids, the kind and Huygens Atmosphere Structure Measures the temperature, density and electrical properties of the atmosphere proteins and fats at the basis of life came into existence. By identifying number of Instrument – HASI during the entry, descent and after landing. Research into lightning on Titan the likely chemical precursors that filled the primeval soup, Huygens molecules in the (F,I,A,D,E,N,FIN, USA, UK, ESA, IS, P) will give a fresh impetus to the theories regarding the origin of life on atmosphere, and the Earth. will analyse Doppler Wind Experiment – DWE Measures the vertical wind profile and the horizontal winds via the propagation aerosol (dust) (D,I,USA) of radio signals through the atmosphere particles Surface Science Package – SSP Studies the state (physical properties and composition) of the surface at the distributed in two (UK, F, PL, USA, ESA) impact site layers of Titan’s Cassini orbiter’s main components. © NASA/JPL

Vehicles of discovery

Cassini, including the Huygens probe, is the an umbrella to shield the spacecraft’s getting far away largest, heaviest and most complex instruments from harmful rays. from Earth, interplanetary spacecraft ever built.With communications such a complicated space mission requiring Huygens is attached to Cassini by a were switched to the many instruments to operate separation mechanism, which will push it high-gain antenna. simultaneously, there is a major impact on off towards Titan at the right moment.The During operations at spacecraft resources, such as electrical mechanism will also start the probe Saturn, the scientific power.This demand – and the need for a rotating, to make sure that it is stabilised data collected by broadly based, diverse collection of and enters the atmosphere front-shield- Cassini’s instruments instruments – is the reason why the Cassini- first. Huygens is built like a shellfish, with a will be stored on the Huygens spacecraft is so large.When it was hard shell (carbon-fibre honeycomb solid-state recorders built, Cassini was also the first inter- covered by silica-fibre tiles) to protect its for about 15 hours planetary spacecraft to have on-board solid- delicate interior from extreme temperatures each day when the state recorders instead of tape recorders. (up to 8000oC) during the descent through high-gain antenna is Titan’s atmosphere.The probe consists of not pointed towards The antenna subsystem on the Cassini two parts: the Entry Assembly Module and Earth.Then, for nine orbiter consists of the 4-m high-gain the Descent Module.The Entry Assembly hours each day, the stored data will be antenna and two low-gain antennas.The Module carries the equipment to control Because of the very dim sunlight at Saturn’s relayed back to Earth. primary function of the high-gain antenna Huygens after its separation from Cassini, orbit, solar arrays were not an option on the is to support communication with Earth and has a front shield that will act both as a Cassini orbiter, and so the electrical power The Huygens probe remains dormant until and to receive Huygens’ signals during its brake and as thermal protection.The needed to operate the spacecraft and its 12 just before its entry into Titan’s atmosphere. mission. It is also used for scientific Descent Module contains the six scientific instruments is supplied by a set of radio- Contacts with Huygens during the cruise experiments. During most of the early part instruments.The probe will use three isotope thermoelectric generators, which phase were only possible via an umbilical of the journey, the high-gain antenna was different parachutes in sequence during the use heat produced by the harmless natural link with Cassini.This link has been used to directed towards the Sun, functioning like descent. decay of plutonium-238.These power subject the probe to periodic checkouts 20 21 generators were also used on the during the journey for health-monitoring Cassini orbiter Huygens probe ESA/NASA Ulysses mission.The energy to and instrument-calibration purposes. Dimensions More than 6.7 m high and more than 4 m wide. 2.7 m in diameter operate the Huygens probe is supplied by Deployed appendages extend up to 11 m from five onboard batteries, capable of The setting of the timers that will wake up the body of the spacecraft. generating 1800 Wh, which can power the the probe about four hours before it Launch mass Total Cassini-Huygens mass at launch was 5.82 tonnes, including 3.1 tonnes of propellant mission for up to eight hours. reaches Titan’s atmosphere will be the last Cassini (dry) mass: 2125 kg Huygens mass: 348 kg (=318 kg for the Huygens probe 1 Heat Shield “Cassini-Huygens to ground control…” and 30 kg for the Probe Support Equipment which 2 Front Shield remains attached to the orbiter after separation) 3 Back Cover During the early part of the Ground operations NASA’s JPL, using NASA’s Deep Space Network ESA’s European Space Operations Centre (ESOC) in interplanetary cruise, 4 Parachute Compartment stations in California, Spain and Australia. Darmstadt, Germany. communication between 5 Descent Module with 4 Prime contractor NASA’s JPL, Pasadena, California.The high-gain Alcatel, France, managed by ESA (ESTEC,The Netherlands) the spacecraft and Earth for 3 Scientific Intruments antenna is provided by ASI. checkouts and manoeuvres Cost 2245 MEuro* (2100 MEuro from NASA, plus 460 MEuro* (360 MEuro from ESA for the Huygens probe, took place via the two low- 145 MEuro from ASI for the high-gain antenna plus about 100 MEuro for the scientific instruments gain antennas onboard 5 1 2 and portions of three instruments) provided by European universities and research institutes Cassini. Six months after the funded by ESA Member States and NASA). Earth fly-by in August 1999, The Huygens probe in * Current economic conditions as the spacecraft was cross-section. © ESA

Laboratory (JPL) in Pasadena, California, An international endeavour using Deep Space Network stations in California, Spain and Australia. During a Hundreds of scientists and engineers from ‘low-activity’ day, Cassini can return 19 nations, including 17 European countries approximately one gigabyte of data, while and the USA, make up the team responsible in a ‘high-activity’ day the volume of data for designing, building, flying the Cassini- sent back to Earth can reach four gigabits. Huygens spacecraft and collecting their scientific data. Of these 19 countries, 17 The Huygens flight operations are being have been active participants since the conducted from ESA’s European Space mission started. Operations Centre (ESOC) in Darmstadt, A full-size model of Germany. All of the probe’s commands are The mission is managed by the Huygens probe was drop-tested and prepared at ESOC and sent via NASA's Deep NASA’s JPL, where the landed at Kiruna in Space Network to Cassini, which stores orbiter was designed and Sweden in 1995 them onboard for release to Huygens at a assembled. Development of during a successful pre-determined time. Huygens data are the Huygens probe was test of the parachutes. received back via the reverse path, and managed by ESA.The prime distributed to the scientific teams by ESOC. contractor for the probe is Alcatel in France. Equipment and instruments for the mission have been supplied from many European countries and the USA. © NASA/JPL The Cassini spacecraft 22 23

commands sent from the ground. Thereafter, Huygens will have to work autonomously.

The probe’s radio link with Cassini will be activated automatically in the early phase of the descent and the orbiter will listen to the © ESA probe for the next 4-5 hours, storing four identical copies (for redundancy) of the precious Huygens data in its memory units.

Within a couple of hours after the end of Prepared by: ESA Science Programme Communication Service this communication window, Cassini's high- Published by: ESA Publications Division ESTEC, PO Box 299 gain antenna will be turned away from Titan 2200 AG Noordwijk and pointed towards Earth to download the The Netherlands Huygens data. Editors: Bruce Battrick & Monica Talevi Design and Layout: Jules Perel Copyright: © 2004 European Space Agency Cassini flight operations are being ISSN No.: 0250-1589 ISBN No.: 92-9092-739-9 conducted from NASA’s Jet Propulsion Price: 7 Euros

© ESA Printed in the Netherlands The Huygens probe being assembled.