Lecture 24 Comets

Home , Comet dust, Comet Hyakutake, Comet Kohoutek, Comet tail, Comet West

Lecture 24

Comets Asteroids and Trojans

Asteroids are planetesimals left over from the formation of the solar system that orbit mostly between the orbits of Mars and Jupiter 4. Planet formation Mercury, Venus, Earth Mars and Venus, Mercury, Does the solar system end at Neptune? at end system solar the Does Jupiter, Saturn, Uranus and Neptune and Uranus Saturn, Jupiter, + Asteroids + (a dwarf planet) dwarf (a + Pluto Pluto +

So, where did all these planetary systems come from? !"#$%&'

Comet Halley

Comet Hyakutake

Comet Hale-Bopp Comet West

Two reservoirs of comets exist beyond the orbit of Neptune and are the sources of the comets that we see in the inner solar system !"#$%&'(#$)&

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Comet Kohoutek Comet Giacobini-Zinner

Comets are discovered in the inner solar system but do not originate there 14.2 Comets

Comets that come close enough to the Sun to be detectable from Earth have very eccentric orbits

Observation of orbits tells us that comets originate from the distant outer solar system beyond the orbit of Neptune

(In the above diagram, the comet should not have a tail - it is too far from the Sun) The outermost reservoir of comets is the Oort Cloud

Extends to 100, 000 AU The Oort comet cloud

The Oort comet cloud

Extends to 100,000 AU

More than 200 billion comets hibernate in the remote Oort comet cloud, shown here in cross section. It is located in the outer fringes of the solar system, at distances of about 100,000 AU from the Sun. By comparison, the distance to the nearest star, Proxima Centauri, is 0.27 million AU, while Neptune orbits the Sun at a mere 30 AU. The planetary realm therefore appears as an insignificant dot when compared to the comet cloud, and has to be magnified by a factor of 1,000 in order to be seen. This comet reservoir is named after the Dutch astronomer Jan H. Oort (1900-1992) who, in 1950, first postulated its existence.

The Oort cloud has never been seen - it’s existence has been deduced from an analysis of the orbits of new comets

Nearest star is 270,000 AU

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-;-,+*& •! !'",&).'/0& –!-12+"+&'(&,"#..#'%-&'(&3'*+,-& –!-'*+&(<..&,'?<"0-&@&'":#,&–!456555&7898& –!)'*+,-&0#-,/":+0&:;& -/%&#%&2#$2.;&+..#1A3<.&'":#,-&1<--#%$&*<--#=+&':>+3,& •! .<"$+&3.'/0&'"&-,<"& –!-'*+&+>+3,+0&("'*&-'.<"& -;-,+*& –!-'*+&(<..&,'?<"0-&@&'":#,& -/%&#%&2#$2.;&+..#1A3<.&'":#,-& The second reservoir is the Kuiper Belt 1992 QB1 outside the!"#$#%&'(&)'*+,-& orbit of Neptune

•! ./#0+"&1+2,& –! -'/"3+&'(&-4'",& 0+"#'5&3'*+,-& •! '"6#,72&0+"#'5-&&&&&& 2+--&,47%&899&&&&&&& :+7"-&& –! ;/-,&6+:'%5&'"6#,& '(&<+0,/%+& –! 5#-3'=+"+5& '6-+"=7>'%722:& #%&?@@8& The Kuiper belt 100 million to 10 billion comets

A repository of frozen, comet-sized worlds resides in the outer precincts of the planetary system, just beyond the orbit of Neptune and near the orbital plane of the planets. Known as the Kuiper belt, it is thought to contain 100 million to 10 billion, or 108 to 1010, comets. Many short-period comets are tossed into the inner solar system from the Kuiper belt.

We have now seen about 1000 Kuiper Belt objects !"#$%&'()%*&

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A “Dirty Snowball” is a mixture of silicate dust and ice !"#$"%&'"() •! *+,-.+%)/)0&123)%("456--) –!%"-&0)

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•! )*#+( –! !"#$%"&' –! ,-,.(/&(0"(+0#&*$*1( 1 to 10 km –! ()*+' –! 2(,..3...(/&(0"( Text 100,000 km +0#&*$*1( •! ,+-$&'( –! 2(,..(&0440%"(/&(4%"5( 100 million km –! #46#'7(8%0"$*+(#6#'( 91%&($:*(;<"( What you see when looking at a comet depends on how you look at it. The nucleus of a comet is usually invisible, unless a spacecraft is sent in to take a glimpse. A comet ﬁrst becomes visible when it develops a coma of gas and dust. When the comet passes closer to the Sun, long ion and dust tails become visible, streaming out of the coma in the direction opposite to the Sun. The comet’s tail always points away from the Sun, due to the solar wind. The ion tail is straighter than the dust tail. Exit Approaching Sun

The comet’s tail develops as it approaches the Sun and disappears as it moves away from the Sun. The ion tail always points away from the Sun; the dust tail curves a bit as the comet gets ahead of it in its orbit. In 1984 IRAS - the infrared satellite - detected cometary TRAILS as well as TAILS Comet Trail and Tail

This is an artist's concept of a comet dust trail and dust tail.

The trail can only be seen in the light of radiated heat. The dust trail is made of particles that are the size of sand grains and pebbles. They are large enough that they are not affected much by the Sun's light and solar wind.

The dust tail, on the other hand, is made of grains the size of cigarette-smoke particles. These grains are blown out of the dust coma near the comet nucleus by the Sun's light. When a comet nears the Sun, its ices can sublimate into gas and carry off dust, creating a coma and long tails. Formation of dust TRAIL

Dust particles are “massive” compared to individual atoms. They move slowly (recall: F = ma) away from nucleus and Sun. They lag behind the comet because they drift into orbits that are bigger than the comet’s orbit—therefore, they must slow down because of conservation of angular momentum Formation of dust TRAIL

Dust particles are “massive” compared to individual atoms. They move slowly (recall: F = ma) away from nucleus and Sun. They lag behind the comet because they are drifting into orbits that are bigger than the comet’s orbit— therefore, they must slow down because of conservation of angular momentum Formation of dust TRAIL Dust particles are “massive” compared to individual atoms. They move slowly (recall: F = ma) away from nucleus and Sun. They lag behind the comet because they are drifting into orbits that are bigger than the comet’s orbit— therefore, they must slow down because of conservation of angular momentum

Note: the gas atoms are very low mass, and are pushed out so fast that their paths are straight lines! (not curved like the dust paths) Typical cometary mass: 1012 to 1016 kg

Each trip close to the Sun removes some material; Halley’s Comet, for example, is expected to last about another 40,000 years

Sometimes a comet’s nucleus can disintegrate violently

Comets seen in the inner solar system must be replenished from unseen reservoirs in the outer solar system

Lifetime of Halley about 40,000 years !"#$%&'%(&)$#*%

•! (+"*,%-.#&%/0.% •! (&)$%#&&%12&*$%#&%/0.%3%4$%5$*#+&6$5% •! /0+7-7$%-.-8"2%9"**":$%.$"+%/0.%3%&+4-#% –! $"1,%"99+&"1,%#&%/0.;%-#%2&*$*%)"#$+-"2% •! <$%-.=0$.1$5%46%:+"7-#6%&'%92".$#*% •! -)9"1#%#,$%92".$#% •! 4$%*9$$5$5%09%3%$>$1#$5%'+&)%*&2"+%*6*#$)% •! 4$%9$+#0+4$5%-.#&%".%&+4-#%?-#,%"%*,&+#$+%9$+-&5% 14.2 Comets Kuiper Belt is the dominant source Most comets that enter the inner solar system reside in the Kuiper belt outside the orbit of Neptune. Occasionally a comet from the far larger Oort cloud wanders into the inner solar system as well. Halley’s Comet - the most famous comet 14.2 Comets Halley’s Comet is one of the most famous; it has a period of 76 years and has been observed since antiquity. Its most recent visit, in 1986, was not spectacular. Left: The comet in 1910, as seen with the naked eye Right: The comet in 1986, as seen through a telescope Halley's Comet or Comet Halley is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years.

Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and thus the only naked-eye comet that might appear twice in a human lifetime.

Other naked-eye comets may be brighter and more spectacular, but will appear only once in thousands of years. 14.2 Comets Halley’s Comet has a shorter period than most comets, but its orbit is not in the plane of the solar system, probably due to an encounter with a larger object

Next appearance is 2061 - 50 years from now 164 BCE The Adoration of the Magi (circa 1305) by Giotto, purportedly depicting Halley.

Observation of Halley's Comet, recorded in cuneiform on a clay tablet between 22–28 September 164 BCE, Babylon, Iraq. In 1066, the comet was seen in England and thought to be an omen: later that year 1066 Harold II of England died at the Battle of Hastings;

it was a bad omen for Harold, but a good omen for the man who defeated him, William the Conqueror.

The comet is represented on the Bayeux Tapestry as a ﬁery star, and the surviving accounts describe it as appearing to be four times the size of Venus and shining with a light equal to a quarter of that of the Moon. Halley came within 0.10!AU of the Earth at that time. Birth of Christ?

The Adoration of the Magi (circa 1305) by Giotto, purportedly depicting Halley. The Adoration of the Magi (circa 1305) by Giotto, purportedly depicting Halley.

Observation of Halley's Comet, recorded in cuneiform on a clay tablet between 22–28 September 164 BCE, Babylon, Iraq.

A photograph of Halley's Comet taken during its 1910 approach The Adoration of the Magi (circa 1305) by Giotto, purportedly depicting Halley.

Observation of Halley's Comet, recorded in cuneiform on a clay tablet between 22–28 September 164 BCE, Babylon, Iraq.

A photograph of Halley's Comet taken during its 1910 approach

Halley's comet in 1986 Nucleus of Comet Halley Giotto Image 1986

The Earth’s orbit intersects a stream of meteoric material left along the orbit of Comet Halley, producing two meteor showers, the Eta Aquarids in May and the Orionids in October. Other comets intersect the Earth’s orbit just once during their trip around the Sun. Annual meteor showers are created when the Earth enters the intersection point, such as the August Perseids produced by debris from Comet Swift Tuttle.

A composite image of the nucleus of Comet Halley (right) obtained using images taken in March 1986 with the camera on board the Giotto spacecraft, from a distance of 6.5 million meters before comet dust destroyed the camera. It is compared with a schematic drawing (left) that highlights the major features recognizable in the photograph. The nucleus is about 16 kilometers long and 8 kilometers wide. Dust and gas geyser out of narrow jets from the sunlit side of the nucleus, but about 90 percent of the surface is inactive. The gas is mainly water vapor sublimed from ice in the nucleus, while a signiﬁcant fraction of the dust may be dark carbon-rich matter. A dark surface crust, which insulates most of the underlying ice, is blacker than coal, reﬂecting about 4 percent of the incident sunlight. “Mountains” rise about 500 meters above the surrounding terrain, while a broad “crater” is depressed about 100 meters. Comet Hartley 2 seen by NASA EPOXI mission

Date: 4 Nov 2010 This close-up view of comet Hartley 2 was taken by NASA's EPOXI mission during its ﬂyby of the comet. It was captured by the spacecraft's Medium-Resolution instrument. Nucleus of Comet Borrelly NASA Deep Space 1 on the way to Comet Tempel 1 What you see when looking at a comet depends on how you look at it. The nucleus of a comet is usually invisible, unless a spacecraft is sent in to take a glimpse. A comet ﬁrst becomes visible when it develops a comapassing of gas and dust. When the comet passes closer to the Sun, long ion and dust tails become visible, streaming out of the coma in the direction opposite to the CometSun. When Borrellylooking at a comet in ultraviolet light, the hydrogen atoms in its huge hydrogenin 2001cloud are detected.

A camera on board the Deep Space 1 spacecraft peered into the icy heart of Comet Borrelly on 22 September 2001, taking this image from a distance of 3.4 million meters. The nucleus is shaped like a gigantic bowling pin, with a length of about 8 kilometers and a width of roughly half that size. A dark veneer of material covers most of the nucleus, reﬂecting only 4 percent of the incident sunlight on average. Rugged terrain is found on both ends of the nucleus, while bright smooth plains are present in the middle. Jets of gas and dust shot out from all sides of the comet’s nucleus as it rotated, producing a ﬂow of ions that was not centered on the nucleus. 14.2 Comets The Deep Impact mission slammed a projectile into comet Tempel 1 and studied the material expelled in order to analyze the composition of the comet

This allowed analysis of fresh material below the dark crust Comet Tempel 1

Date: 6 Sep 2005

The impact site has the highest resolution because images were acquired until about 4 sec from impact or a few meters from the surface.

Arrows a and b point to large, smooth regions. The impact site is indicated by the third large arrow.

The scale bar is 1 km and the two arrows above the nucleus point to the sun and the rotational axis of the nucleus.

Impact Site Bull's Eye!

Date: 4 Jul 2005

This image shows the initial ejecta that resulted when NASA's Deep Impact probe collided with comet Tempel 1 at 10:52 p.m. Paciﬁc time, July 3 (1:52 a.m. Eastern time, July 4) . Name Dimensions Density km g/cm3

Halley's Comet 15!"!8!"!8 0.6

Tempel 1 7.6 x 4.9 0.62

19P/Borrelly 8"4"4 0.3

81P/Wild 5.5"4.0"3.3 0.6

Densities are less than water ice - must be porous

The albedos (reﬂectivity) are low - a few percent - comet nucleii are black! 14.2 Comets

The Stardust mission flew through the tail of comet Wild-2, gathering dust particles in detectors made of aerogel and returning them to Earth for analysis !"#$%&'"(( )*''*+,( -.../0112(

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So, where did all these planetary systems come from? KEPLER mission is searching for habitable planets

National Aeronautics and Space Administration

NASA Ames Research Center/W. Stenzel (OSC)/Artist’s Concept Stenzel (OSC)/Artist’s Center/W. NASA Ames Research

Kepler Mission: A Search for Habitable Planets www.nasa.gov Kepler in Brief

A Nutshell Description of the Kepler Mission

Why?

The Kepler Mission is a NASA Discovery Program for detecting potentially life-supporting planets around other stars. All of the extrasolar planets detected so far by other projects are giant planets, mostly the size of Jupiter and bigger. Kepler is poised to ﬁnd planets 30 to 600 times less massive than Jupiter.

How?

By a method known as the transit method of planet ﬁnding. When we see a planet pass in front of its parent star it blocks a small fraction of the light from that star.When that happens, we say that the planet is transiting the star. If we see repeated transits at regular times, we have discovered a planet! From the brightness change we can tell the planet size. From the time between transits, we can tell the size of the planet's orbit and estimate the planet's temperature.These qualities determine possibilities for life on the planet.

What?

The Kepler satellite has a 0.95-meter diameter telescope that is a photometer having a ﬁeld of view a bit over 10 degrees square (and area of sky the size of about two open hands). It is designed to continuously and simultaneously monitors brightnesses of 100,000 stars brighter than 14th magnitude in the constellations Cygnus & Lyrae.

To detect an Earth-size planet, the photometer must be able to sense a drop in brightness of only 1/100 of a percent. This is akin to sensing the drop in brightness of a car's headlight when a fruitﬂy moves in front of it! The photometer must be spacebased to obtain this precision.

When?

Kepler was launched in March 2009. Kepler in Brief Kepler in Brief

A Nutshell Description of the Kepler Mission AKepler Nutshell in Brief Description of the Kepler Mission

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Kepler was launched in March 2009. Kepler in Brief

A Nutshell Description of the Kepler Mission

Why?

How?

What?

When?

Kepler was launched in March 2009. KEPLER star ﬁeld

National Aeronautics and Space Administration Milky Way photo by Carter Roberts Carter by photo Way Milky

The Kepler Mission Star Field www.nasa.gov Astronomers see more planets than stars in galaxy

By SETH BORENSTEIN AP Science Writer Published: Wednesday, January 11, 2012 at 4:04 p.m.

Last Modiﬁed: Wednesday, January 11, 2012 at 4:04 p.m.

The more astronomers look for other worlds, the more they ﬁnd that it's a crowded and crazy cosmos. They think planets easily outnumber stars in our galaxy and they're even ﬁnding them in the strangest of places.

Keep watching this space

Enlarge

This handout illustration provided by San Diego State University, shows a newly discovered planet, called Kepler 35, that circles not one but two stars. Scientists thought this type of two- sun system _ made famous as the home planet of the ﬁctional Luke Skywalker _ is too unstable to support planets. But so far they’ve found three of these planets with two suns, showing that planets seem to be everywhere. The study is in this week’s journal Nature. (AP Photo/Lynette R. Cook, San Diego State University)

And they've only begun to count.

Three studies released Wednesday, in the journal Nature and at the American Astronomical Society's conference in Austin, Texas, demonstrate an extrasolar real estate boom. One study