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 Mercury, Venus, Earth and Mars + Asteroids So, where did all these planetary systems come from? did all these planetary systems come from? where So, 4. Planet formation Jupiter, Saturn, Uranus and Neptune Does the solar system end at Neptune?+ Pluto (a dwarf planet) !"#$%&' Comet Halley Comet Hyakutake Comet Hale-Bopp Comet West Two reservoirs of comets exist beyond the orbit of Neptune and are the sources of the comets that we see in the inner solar system !"#$%&'(#$)& •! !"#$%)&(*$&+(#$,&(-$*&%.$/*&,/)0"1$*$*)& 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 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. !"#$#%&'(&)'*+,-& •! !'",&).'/0& –!-12+"+&'(&,"#..#'%-&'(&3'*+,-& –!456555&7898& –!)'*+,-&0#-,/":+0&:;& 1<--#%$&*<--#=+&':>+3,& Trillions of Comets •! .<"$+&3.'/0&'"&-,<"& –!-'*+&+>+3,+0&("'*&-'.<"&!"#$#%&'(&)'*+,-& -;-,+*& •! !'",&).'/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 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. 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 !"#$%&'()%*& •! '"+#$,&-$."/,&0+"*%12/$3&2).&)"4/%$+5(+%*&%"&&(*%$+"2,*6& •! 782+%.&*/"9-(11*:& •! ;"*%&)"#$%*&,"&/"%&<(=$&%(21*6& •! ;"*%&)"#$%*&+$#(2/&0+">$/&0"+$=$+&2/&"4%$+&*"1(+&*.*%$#6&& &&&?&0$9&$/%$+&2//$+&*"1(+&*.*%$#3&9<$+$&%<$.&)(/&@+"9&%(21*6& A “Dirty Snowball” is a mixture of silicate dust and ice !"#$"%&'"() •! *+,-.+%)/)0&123)%("456--) –!%"-&0) –!&,.%)"7))89:;)!:9;)*8<;)=)!8>) –!0+%2) •! !"#6) –!,-"+0)"7)?6%) –!#"%2-3)89:;)!:9;)!:)) •! @6&-%) –!0+%2)=)?6%) !"#$%&'( Dimensions •! )*#+( –! !"#$%"&' –! ,-,.(/&(0"(+0#&*$*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.