2000 March 28, SPS 1020 (Introduction to Space Sciences) - Reading: today was TNSS Ch. Chs. 5 and 24 (Comets) Read TNSS Chs. 25 and 26 (Asteroids and Meteorites) for Thursday, March 30. --------------- Comets: Visitors from the edge of the Solar System ------ and, by corollary, the beginning of the Solar System's history Comets are _primitive_, _undifferentiated_ leftovers from the origin of the Solar System. More common than Full Moons: at any given time, ~ 12 comets are observable; > 24 come and go every year. (Can follow them in the IAU Circulars: see my Big Page of Links) => Comets are named after their discoverers! Average amateur comet hunter spends ~ 1000 hr searching for each find; amateurs still get most of them. Comets are idiosyncratic and highly unpredictable: Like cats, both have tails, and do what _they_ want. "If you must bet, bet on a horse, not on a comet." -- Fred Whipple "Every comet is an education." -- John Brandt, TNSS p. 323 Astronomers have been embarrassed by comets that turned out to be much less bright and spectacular than predicted: Comet Kohoutek (1974): predicted to be m = -10 (!), but it was dusty, since it was on its first pass through inner Solar System Comet Halley (1986): I saw it, it was GREAT! => At m = +2, it was horribly harmed by city lights (light pollution). Observed since ancient times, often thought to portend plagues and wars (which were so common, anyone would make the connection). - The death of Julius Caesar in 54 B.C. was "foretold" by a comet. - The 1066 A.D. apparition of Halley's Comet is shown in the Bayeux tapestry, shortly before the Norman conquest of England. King Harold looks worried! => Comets arouse superstition even now: Halley's "Comet Pills" in 1910 Earth passed through comet's tail; some reporter made a big deal when some astronomer had told him there was cyanogen gas in it. (Of course, a comet tail makes a laboratory vacuum look crowded!) Comet Hale-Bopp and the Heaven's Gate tragedy (mass suicide in California)---in 1997! -------- It is easy to see why unsophisticated people still see comets as "signs from heaven." Back before light pollution took over the skies, they were STRIKING sights. Although usually faint, they can be BIG. They literally stretch across the sky! Examples: Comet Ikeya-Seki in 1965: "as big as a house!" (tail 40 degrees long). No telescope needed, but a dark sky was a must. Comet Hyakutake in 1996 March/April: tail streteched *over 100 degrees* across sky! Coma had m = -2 (Tell the "SEE THE COMET FREE" story) If Comet Hale-Bopp in 1997, also m = -2, had been that close, it would have been a MONSTER: m = -10, no kidding! (Like the daylight comets of 1843, 1882, and 1910 [which is often confused with Halley's, which also appeared that year]). Previous great comet (clearly visible to unaided eye): Comet West, in 1976. => On average, great comets come only about once/decade. But thanks to the nature of random numbers, this says nothing about when the next one will come: it could be tomorrow, you might never see another (19th century had a 63-year dry spell) Explain the concept of _surface brightness_: suface brightness = brightness/area on sky Although comets have large angular diameters, this says nothing about their total brightness. In fact, the largest ones can have low surface brightnesses, and so often go quite unnoticed by the general public, lost in the haze of city lights (e.g., Ikeya-Seki; IRAS-Araki-Alcock in 1983). (Tell the IRAS-Araki-Alcock story: m = +2, no tail, invisible in city because of light pollution) Coma: ----- Bright, extended atmosphere of released gases: H_2 O 65-80% by mass other molecules and ions present in coma: C_2, C_3, CO, CO_2, CH_3 OH (methanol), CH_4, hydrocarbons N_2, HCN (hydrogen cyanide), NH_3, others (How do we know this?) Nucleus: -------- Small (r ~ 1-10 km typically), solid, icy body. Releases this temporary atmosphere Composed of both ice and dust, a "dirty snowball." Light, crumbly material: comets are often seen to break apart e.g. Comet Shoemaker-Levy 9 in 1994 (later hit Jupiter); Tunguska impact thought to be part of comet Encke Tail: ----- Long streamers of gas and dust, swept away by solar wind (a "solar wind sock") Often can see 2 tails: Dust tail and ion tail Both point away from Sun (discovered by Peter Apian in 1300s!) - Dust tail: make by dust grains: ---------- Pushed out by radiation pressure (E = p c) Curved, since they follow momentum vector of comet Yellow, from reflected sunlight - Ion (or plasma) tail: made by ionized gases --------------------- Usually straight, Often 10x longer than dust tails Often show fine structure and _disconnection events_, from plasma interacting with Solar Wind: Glows bluish, from CO+ fluorescing (making emission lines, like the lights in this classroom) Thus comets contain mostly _volatiles_, along with dust => carbonaceous and silicates => Frozen balls of interstellar gas and dust; some may well pre-date the Solar System, some convincingly thought to be interlopers. (From outside!) How big is a comet nucleus? What does it look like? ---------------------------------------------------- Best image, so far: Halley's Comet, 1986, from Giotto spacecraft (European Space Agency's first planetary mission) Halley's nucleus is 16 x 8 x 8 km Hale-Bopp (1997) was unusually large at 25-40 km Chiron (seen to have tail): ~ 200 km ! Comet Tails: ----------- Halley's, Hyakutake's were > 100 million km long => Among the biggest "objects" in solar system...for a brief time (Compare with Jupiter's magnetosphere, 6 degrees in angular diameter at 4.2 AU from Earth at opposition: work it out yourself.) Dirty Snowball Model of the Cometary Nucleus: --------------------------------------------- Originated by Fred Whipple in 1950. (He's still at Harvard!) (He'd inferred it from spectra.) - A single, relatively small "snowball", roughly equal quantities of silicates (rocks) and ice - Rock and ice is well mixed: These are primitive bodies formed directly from the Solar Nebula, _not_ differentiated. - Rapid evaporation from Sun-facing side gives rise to jets of gas => Brilliantly shown by Giotto images: Spewing jets from sunlit side Peanut-shaped nucleus was coal black (albedo 4%): covered with organic material (hydrocarbons, or "tarry gunk") Cometary dust: becomes meteors Meteor showers: related to "old" comets - when Earth encounters cometary dust particles => a meteor shower DON'T CONFUSE METEORS WITH COMETS. When is the best time to see meteors? So => observe between midnight and dawn 1998 Leonid meteor shower: best I'd ever seen, Easily > 500 meteors/ hour, all over sky MUCH better the night Earth was heading INTO the stream (Nov 16-17) than when exiting (Nov 17-18)! 1999: not many here, but > 1500/hour in Canary Islands => May be back this year: stay tuned! Origin of Comets ---------------- Short-period Comets P < 200 yr, e.g. Halley's, P = 76 yr (240 BC, ...1531, 1607, 1682 Halley Observed, 1758 Halley predicted, 1835, 1910, 1986, 2061 next). Also: Comet Encke, P = 3.3 yr => Mostly seen near ecliptic Long-period Comets: P >> 200 yr, e.g. Hale-Bopp, P > 4000 yr => Orbital inclinations are random (come from all over sky) Comet orbits are often highly eccentric (e = 0.1 - 0.9) - Where do long-period comets come from? - What's the source of short-period comets, 4.6 Gyr since they were formed? Comets condense from Solar Nebula at 30 - 100 K => Neptune's distance Long-period comets: from the spherical Oort Cloud (named after Jan Oort): D = 50,000 AU, ~ 10^11 comets (number inferred from rate), Flung out there by Uranus and Neptune's gravity, in early Solar System Short-period comets: from the Kuiper Belt, D = 40 (Pluto's orbit) - 50,000 AU in a disk Leftover outer edge of the Solar Nebula => Perturbed into inner Solar System by the planets' (esp. Jupiter's) gravity Since 1st Kuiper-Belt object was discovered in 1992, over 50 have been found. In inner Solar System, comet orbits are often unstable: 1) Hit Sun: SOHO spacecraft saw about 20/year, total of > 50 2) Hit planets, e.g. Comet Shoemaker-Levy 9 hit Jupiter in 1994 Also catenae seen on Ganymede and Callisto 3) Ejected from Solar System (e.g. IRAS-Araki-Alcock in 1983, passed close to Earth) A comet may endure ~ 1000s of close passes by Sun; many break apart. Volatiles will eventually be depleted => Comet will become meteor shower From orbits: 1/2 to 1/3 Earth-approaching asteroids may be dead comets. An ambitious series of (all small) spacecraft missions is in the works: Stardust: launched 1999 February to Comet Wild To pick up dust in 2004, & return to Earth in 2006 CONTOUR: Comet Nucleus tour: 2002 launch Close flybys of 3 comets (Encke, Schwassmann-Wachmann-3 and d'Arrest), maybe also a 4th in 2006-2008. Rosetta: European Space Agency - 2003 launch, Rendezvous and Lander Mission to Comet P/Wirtanen. Champollion/Deep Space 4: - 2003 launch, NASA Lander Mission to Comet Tempel 1.