Comet Shoemaker-Levy 9

Shoemaker-Levy redirects here. For other Shoemaker-Levy comets see List of periodic comets.
SL9 (Shoemaker-Levy)
Hubble Space Telescope
Image of Comet Shoemaker-Levy 9 fragments (total: 21), taken on May 17, 1994
Discovery
Discovered by: Carolyn Shoemaker
Eugene M. Shoemaker
David Levy
Discovery date: March 24, 1993
Orbital characteristics A
Inclination: 94.23333°

Comet Shoemaker-Levy 9 (SL9, formally designated D/1993 F2) was a comet that collided with Jupiter in 1994, providing the first direct observation of an extraterrestrial collision of solar system objects.[1] This generated a large amount of coverage in the popular media, and SL9 was closely observed by astronomers worldwide. The collision provided new information about Jupiter and highlighted its role in reducing space debris in the inner solar system.

The comet was discovered by astronomers Carolyn and Eugene M. Shoemaker and David Levy. Shoemaker-Levy 9 was located on the night of March 24, 1993, in a photograph taken with the 0.4 metre Schmidt telescope at the Palomar Observatory in California. It was the first comet observed to be orbiting a planet.

In July 1992 the orbit of Shoemaker-Levy 9 passed within Jupiter's Roche limit, and Jupiter's tidal forces acted to pull the comet apart. SL9 was later observed as a series of fragments ranging up to 2 kilometres in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and July 22, 1994, at a speed of approximately 60 kilometres per second. The prominent scars from the impacts were more easily visible than the Great Red Spot and persisted for many months.

Contents

Discovery

While conducting a program of observations designed to uncover near-Earth objects, the Shoemakers and Levy discovered Comet Shoemaker-Levy 9 (SL9) on the night of March 24, 1993 in a photograph taken with the 0.4 metre Schmidt telescope at the Palomar Observatory in California. The comet was thus a serendipitous discovery, but one that quickly overshadowed the results from their main observing program.[2]

SL9 was the ninth periodic comet (a comet whose orbital period is 200 years or less) discovered by the Shoemakers and Levy, hence its name. It was their eleventh comet discovery overall including their discovery of two non-periodic comets, which use a different nomenclature. The discovery was announced in IAU Circular 5725 on March 27, 1993.[3]

The discovery image gave the first hint that SL9 was an unusual comet, as it appeared to show multiple nuclei in an elongated region about 50 arcseconds long and 10 arcseconds wide. Brian Marsden of the Central Bureau for Astronomical Telegrams noted that the comet lay only about 4 degrees from Jupiter as seen from Earth, and that while this could of course be a line of sight effect, its apparent motion suggested that it was physically close to the giant planet.[4] Because of this, he suggested that the Shoemakers and David Levy had discovered the fragments of a comet that had been disrupted by Jupiter's gravity.

Jupiter-orbiting comet

Orbital studies of the new comet soon revealed that it was orbiting Jupiter rather than the Sun, unlike all other comets known at the time. Its orbit around Jupiter was very loosely bound, with a period of about 2 years and an apojove (furthest distance from Jupiter) of 0.33 astronomical units (49 Gm). Its orbit around the planet was highly eccentric (e = 0.9986).[5]

Tracing back the comet's orbital motion revealed that it had been orbiting Jupiter for some time. It seems most likely that it was captured from a solar orbit in the early 1970s, although the capture may have occurred as early as the mid-1960s.[6] Several other observers found images of the comet in precovery images obtained before March 24, including Kin Endate from a photograph exposed on March 15, S. Otomo on March 17, and a team led by Eleanor Helin from images on March 19.[3] No precovery images dating back to earlier than March 1993 have been found. Before the comet was captured by Jupiter, it was probably a short-period comet with an aphelion just inside Jupiter's orbit, and a perihelion interior to the asteroid belt.[7]

The volume of space within which an object can be said to orbit Jupiter is defined by Jupiter's Hill sphere (also called the Roche sphere). When the comet passed Jupiter in the late 1960s or early 1970s, it happened to be near its aphelion, and found itself slightly within Jupiter's Hill sphere. Jupiter's gravity nudged the comet towards it. Because the comet's motion with respect to Jupiter was very small, it fell almost straight toward Jupiter, which is why it ended up on a Jupiter-centric orbit of very high eccentricity—that is to say, the ellipse was nearly flattened out.[8]

The comet had apparently passed extremely close to Jupiter on July 7, 1992, just over 40,000 km (25,000 mi) above the planet's cloud tops—a smaller distance than Jupiter's radius of 70,000 km (43,000 mi), and well within the orbit of Jupiter's innermost moon Metis and the planet's Roche limit, inside which tidal forces are strong enough to disrupt a body held together only by gravity.[8] Although the comet had approached Jupiter closely before, the July 7 encounter seemed to be by far the closest, and the fragmentation of the comet is thought to have occurred at this time. Each fragment of the comet was denoted by a letter of the alphabet, from "fragment A" through to "fragment W", a practice already established from previously observed broken-up comets.[9]

More exciting for planetary astronomers was that the best orbital solutions suggested that the comet would pass within 45,000 km (28,000 mi) of the centre of Jupiter, a distance smaller than the planet's radius, meaning that there was an extremely high probability that SL9 would collide with Jupiter in July 1994. Studies suggested that the train of nuclei would plough into Jupiter's atmosphere over a period of about five days.[8]

Predictions for the collision

The discovery that the comet was likely to collide with Jupiter caused great excitement within the astronomical community and beyond, as astronomers had never before seen two significant solar system bodies collide. Intense studies of the comet were undertaken, and as its orbit became more accurately established, the possibility of a collision became a certainty. The collision would provide a unique opportunity for scientists to look inside Jupiter's atmosphere, as the collisions were expected to cause eruptions of material from the layers normally hidden beneath the clouds.[10]

Astronomers estimated that the visible fragments of SL9 ranged in size from a few hundred metres to at most a couple of kilometres across, suggesting that the original comet may have had a nucleus up to 5 km across—somewhat larger than Comet Hyakutake, which became very bright when it passed close to the Earth in 1996. One of the great debates in advance of the impact was whether the effects of the impact of such small bodies would be noticeable from Earth, apart from a flash as they disintegrated like giant meteors.[11] Other suggested effects of the impacts were seismic waves travelling across the planet, an increase in stratospheric haze on the planet due to dust from the impacts, and an increase in the mass of the Jovian ring system. However, given that observing such a collision was completely unprecedented, astronomers were cautious with their predictions of what the event might reveal.[10]

Impacts

Jupiter in Ultraviolet (about 2.5 hours after R's impact)

Anticipation grew as the predicted date for the collisions approached, and astronomers trained terrestrial telescopes on Jupiter. Several space observatories did the same, including the Hubble Space Telescope, the ROSAT X-ray observing satellite, and significantly the Galileo spacecraft, then on its way to a rendezvous with Jupiter scheduled for 1995. While the impacts took place on the side of Jupiter hidden from Earth, Galileo, then at a distance of 1.6 AU from the planet, was able to see the impacts as they occurred. Jupiter's rapid rotation brought the impact sites into view for terrestrial observers a few minutes after the collisions.[12]

Two other satellites made observations at the time of the impact: the Ulysses spacecraft, primarily designed for solar observations, was pointed towards Jupiter from its location 2.6 AU away, and the distant Voyager 2 probe, some 44 AU from Jupiter and on its way out of the solar system following its encounter with Neptune in 1989, was programmed to look for radio emission in the 1–390 kHz range.[13]

HST images of a fireball from the first impact appearing over the limb of the planet

The first impact occurred at 20:13 UTC on July 16, 1994, when fragment A of the nucleus slammed into Jupiter's southern hemisphere at a speed of about 60 km/s.[1] Instruments on Galileo detected a fireball which reached a peak temperature of about 24,000 K, compared to the typical Jovian cloudtop temperature of about 130 K, before expanding and cooling rapidly to about 1500 K after 40 s. The plume from the fireball quickly reached a height of over 3,000 km.[14] A few minutes after the impact fireball was detected, Galileo measured renewed heating, probably due to ejected material falling back onto the planet. Earth-based observers detected the fireball rising over the limb of the planet shortly after the initial impact.[15]

Astronomers had expected to see the fireballs from the impacts, but did not have any idea in advance how visible the atmospheric effects of the impacts would be from Earth. Observers soon saw a huge dark spot after the first impact. The spot was visible even in very small telescopes, and was about 6,000 km (one Earth radius) across. This and subsequent dark spots were thought to have been caused by debris from the impacts, and were markedly asymmetric, forming crescent shapes in front of the direction of impact.[16]

Over the next 6 days, 21 distinct impacts were observed, with the largest coming on July 18 at 07:33 UTC when fragment G struck Jupiter. This impact created a giant dark spot over 12,000 km across, and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (600 times the world's nuclear arsenal).[17] Two impacts 12 hours apart on July 19 created impact marks of similar size to that caused by fragment G, and impacts continued until July 22, when fragment W struck the planet.[18]

Observations and discoveries

Chemical studies

Brown spots mark impact sites on Jupiter's southern hemisphere.

Observers hoped that the impacts would give them a first glimpse of Jupiter beneath the cloud tops, as lower material was exposed by the comet fragments punching through the upper atmosphere. Spectroscopic studies revealed absorption lines in the Jovian spectrum due to diatomic sulfur (S2) and carbon disulfide (CS2), the first detection of either in Jupiter, and only the second detection of S2 in any astronomical object. Other molecules detected included ammonia (NH3) and hydrogen sulfide (H2S). The amount of sulfur implied by the quantities of these compounds was much greater than the amount that would be expected in a small cometary nucleus, showing that material from within Jupiter was being revealed. Oxygen-bearing molecules such as sulfur dioxide were not detected, to the surprise of astronomers.[19]

As well as these molecules, emission from heavy atoms such as iron, magnesium and silicon was detected, with abundances consistent with what would be found in a cometary nucleus. While substantial water was detected spectroscopically, it was not as much as predicted beforehand, meaning that either the water layer thought to exist below the clouds was thinner than predicted, or that the cometary fragments did not penetrate deeply enough.[20]

Seismic waves

As predicted beforehand, the collisions generated enormous seismic waves which swept across the planet at speeds of 450 km/s and were observed for over two hours after the largest impacts. The waves were thought to be travelling within a stable layer acting as a waveguide, and some scientists believed the stable layer must lie within the hypothesised tropospheric water cloud. However, other evidence seemed to indicate that the cometary fragments had not reached the water layer, and the waves were instead propagating within the stratosphere.[21]

Other observations

A sequence of Galileo images, taken several seconds apart, showing the appearance of the fireball of fragment G on the dark side of Jupiter

Radio observations revealed a sharp increase in continuum emission at a wavelength of 21 cm after the largest impacts, which peaked at 120% of the normal emission from the planet. This was thought to be due to synchrotron radiation, caused by the injection of relativistic electrons into the Jovian magnetosphere by the impacts.[22]

About an hour after fragment K entered Jupiter, observers recorded auroral emission near the impact region, as well as at the antipode of the impact site with respect to Jupiter's strong magnetic field. The cause of these emissions was difficult to establish due to a lack of knowledge of Jupiter's internal magnetic field and of the geometry of the impact sites. One possible explanation was that upwardly accelerating shock waves from the impact accelerated charged particles enough to cause auroral emission, a phenomenon more typically associated with fast-moving solar wind particles striking a planetary atmosphere near a magnetic pole.[23]

Some astronomers had suggested that the impacts might have a noticeable effect on the Io torus, a torus of high-energy particles connecting Jupiter with the highly volcanic moon Io. High resolution spectroscopic studies found that variations in the ion density, rotational velocity, and temperatures at the time of impact and afterwards were within the normal limits.[24]

Post-impact analysis

A reddish, asymmetric ejecta pattern

One of the surprises of the impacts was the small amount of water revealed compared to prior predictions.[25] Before the impact, models of Jupiter's atmosphere had indicated that the break-up of the largest fragments would occur at atmospheric pressures of anywhere from 30 kilopascals to a few tens of megapascals (from 0.3 to a few hundred bar),[20] with some predictions that the comet would penetrate a layer of water and create a bluish shroud over that region of Jupiter.[11]

Astronomers did not observe large amounts of water following the collisions, and later impact studies found that fragmentation and destruction of the cometary fragments in an 'airburst' probably occurred at much higher altitudes than previously expected, with even the largest fragments being destroyed when the pressure reached 250 kPa (2.5 bar), well above the expected depth of the water layer. The smaller fragments were probably destroyed before they even reached the cloud layer.[20]

Longer-term effects

The visible scars from the impacts could be seen on Jupiter for many months. They were extremely prominent, and observers described them as more easily visible even than the Great Red Spot. A search of historical observations revealed that the spots were probably the most prominent transient features ever seen on the planet, and that while the Great Red Spot is notable for its striking colour, no spots of the size and darkness of those caused by the SL9 impacts have ever been recorded before.[26]

Spectroscopic observers found that ammonia and carbon sulfide persisted in the atmosphere for at least fourteen months after the collisions, with a considerable amount of ammonia being present in the stratosphere as opposed to its normal location in the troposphere.[27]

Counterintuitively, the atmospheric temperature dropped to normal levels much more quickly at the larger impact sites than at the smaller sites: at the larger impact sites, temperatures were elevated over a region 15,000–20,000 km wide, but dropped back to normal levels within a week of the impact. At smaller sites, temperatures 10 K higher than the surroundings persisted for almost two weeks.[28] Global stratospheric temperatures rose immediately after the impacts, then fell to below pre-impact temperatures 2–3 weeks afterwards, before rising slowly to normal temperatures.[29]

Frequency of impacts

A chain of craters on Ganymede, probably caused by a similar impact event. The picture covers an area approximately 190 km (120 mi) across.

SL9 is not unique in having orbited Jupiter for a time; at least two other comets (82P/Gehrels and 111P/Helin-Roman-Crockett) are known to have been temporarily captured by the planet.[30] Cometary orbits around Jupiter are generally unstable, as they will be highly elliptical and likely to be strongly perturbed by the Sun's gravity at apojove (the furthest point on the orbit from the planet).

By far the most massive planet in the solar system, Jupiter can capture objects relatively frequently, but the size of SL9 makes it a rarity: one post-impact study estimated that comets 0.3 km in diameter impact the planet once in approximately 500 years and those 1.6 km in diameter do so just once in every 6000 years.[31]

There is very strong evidence that comets have previously been fragmented and collided with Jupiter and its satellites. During the Voyager missions to the planet, planetary scientists identified 13 crater chains on Callisto and three on Ganymede, the origin of which was initially a mystery.[32] Crater chains seen on the Moon often radiate from large craters, and are thought to be caused by secondary impacts of the original ejecta, but the chains on the Jovian moons did not lead back to a larger crater. The impact of SL9 strongly implied that the chains were due to trains of disrupted cometary fragments crashing into the satellites.[33]

Jupiter as a "cosmic vacuum cleaner"

The impact of SL9 highlighted Jupiter's role as a kind of "cosmic vacuum cleaner" for the inner solar system. The planet's strong gravitational influence leads to many small comets and asteroids colliding with the planet, and the rate of cometary impacts on Jupiter is thought to be between two thousand and eight thousand times higher than the rate on Earth.[34] If Jupiter were not present, the probability of asteroid impacts with the Solar System's inner planets would be much greater.

The extinction of the dinosaurs at the end of the Cretaceous period is generally believed to have been caused by the impact event which created the Chicxulub crater, demonstrating that impacts are a serious threat to life on Earth. Astronomers have speculated that without Jupiter to mop up potential impactors, extinction events might have been much more frequent on Earth, and complex life might not have been able to develop.[35] This is part of the argument used in the Rare Earth hypothesis.

References

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  2. ↑ Marsden, Brian G. (July 18, 1997)). "Eugene Shoemaker (1928-1997)". Jet Propulsion Laboratory– National Aeronautics and Space Administration. Retrieved on 2008-08-24.
  3. ↑ 3.0 3.1 "D/1993 F2 Shoemaker-Levy 9". Gary W. Kronk's Cometography. Cometography.com (1994). Retrieved on 2008-08-08.
  4. ↑ Marsden, Brian G. (March 26, 1993). "Comet Shoemaker-Levy (1993e)". Harvard–Smithsonian Center for Astrophysics. Retrieved on 2008-08-08.
  5. ↑ Burton, Dan (July 1994). "Frequently Asked Questions about the Collision of Comet Shoemaker-Levy 9 with Jupiter". Queston 2.4. Department of Physics and Astronomy, Stephen F. Austin State University. Retrieved on 2008-08-20.
  6. ↑ Landis, R.R. (1994). "Comet P/Shoemaker-Levy's Collision with Jupiter: Covering HST's Planned Observations from Your Planetarium". Proceedings of the International Planetarium Society Conference held at the Astronaut Memorial Planetarium & Observatory, Cocoa, Florida, July 10–16 1994. Students for the Exploration and Development of Space. Retrieved on 2008-08-08.
  7. ↑ Benner, L.A.; McKinnon, W.B. (March 1994). "Pre-Impact Orbital Evolution of P/Shoemaker-Levy 9". Abstracts of the 25th Lunar and Planetary Science Conference, held in Houston, TX, March 14–18, 1994: 93. http://adsabs.harvard.edu/abs/1994LPI....25...93B. Retrieved on 2008-08-24. 
  8. ↑ 8.0 8.1 8.2 Chapman, Clark R. (June 1993). "Comet on target for Jupiter" (PDF). Nature 363: 492–493. http://www.nature.com/nature/journal/v363/n6429/pdf/363492a0.pdf. Retrieved on 2008-08-25. 
  9. ↑ Boehnhardt, H. (November 2004). "Split comets" in Comets II. M.C. Festou, H.U. Keller, H.A. Weaver: 301, University of Arizona Press. Retrieved on 2008-08-25. 
  10. ↑ 10.0 10.1 Burton, Dan (July 1994). "Frequently Asked Questions about the Collision of Comet Shoemaker-Levy 9 with Jupiter". Queston 1.4. Department of Physics and Astronomy, Stephen F. Austin State University. Retrieved on 2008-08-20.
  11. ↑ 11.0 11.1 Burton, Dan (July 1994). "Frequently Asked Questions about the Collision of Comet Shoemaker-Levy 9 with Jupiter". Queston 1.5. Department of Physics and Astronomy, Stephen F. Austin State University. Retrieved on 2008-08-20.
  12. ↑ Yeomans, D.K.; P.W.Chodas (December 1993). "PERIODIC COMET SHOEMAKER-LEVY 9 (1993e)". IAU Circulars (Cambridge, Massachusetts: Smithsonian Astrophysical Observatory) 5909. http://www.cfa.harvard.edu/iauc/05900/05909.html#Item2. Retrieved on 2008-08-27. 
  13. ↑ Williams, David R.. "Ulysses and Voyager 2". Lunar and Planetary Science. National Space Science Data Center. Retrieved on 2008-08-25.
  14. ↑ Martin, Terry Z. (September 1996). "Shoemaker-Levy 9: Temperature, Diameter and Energy of Fireballs". Bulletin of the American Astronomical Society 28: 1085. http://adsabs.harvard.edu/abs/1996DPS....28.0814M. Retrieved on 2008-08-24. 
  15. ↑ Weissman, P.R.; Carlson, R. W.; Hui, J.; Segura, M.; Smythe, W. D.; Baines, K. H.; Johnson, T. V.; Drossart, P.; Encrenaz, T.; Leader, F.; Mehlman, R. (March 1995). "Galileo NIMS Direct Observation of the Shoemaker-Levy 9 Fireballs and Fall Back". Abstracts of the Lunar and Planetary Science Conference 26: 1483. http://adsabs.harvard.edu/abs/1995LPI....26.1483W. Retrieved on 2008-08-24. 
  16. ↑ Hammel, H.B. (December 1994). "The Spectacular Swan Song of Shoemaker-Levy 9" in 185th AAS Meeting. 26: 1425, American Astronomical Society. 
  17. ↑ Bruton, Dan (February 2006). "Frequently Asked Questions about the Collision of Comet Shoemaker-Levy 9 with Jupiter". Question 3.1. Texas A&M University. Retrieved on 2008-08-20.
  18. ↑ Yeomans, Don; Chodas, Paul (March 18, 1995). "Comet Crash Impact Times Request". Jet Propulsion Laboratory, NASA. Retrieved on 2008-08-26.
  19. ↑ Noll, K.S.; McGrath, M.A.; Weaver, H.A.; Yelle, R.V.; Trafton, L.M.; Atreya, S.K.; Caldwell, J.J.; Barnet, C.; Edgington, S. (March 1995). "HST Spectroscopic Observations of Jupiter Following the Impact of Comet Shoemaker-Levy 9". Science 267 (5202): 1307–1313. doi:10.1126/science.7871428. http://www.sciencemag.org/cgi/content/abstract/267/5202/1307. Retrieved on 2008-08-24. 
  20. ↑ 20.0 20.1 20.2 Hu, Zhong-Wei; Chu, Yi; Zhang, Kai-Jun (May 1996). "On Penetration Depth of the Shoemaker-Levy 9 Fragments into the Jovian Atmosphere". Earth, Moon and Planets 73 (2): 147–155. http://adsabs.harvard.edu/abs/1996EM&P...73..147H. Retrieved on 2008-08-24. 
  21. ↑ Ingersoll, A. P.; Kanamori H. (April 1995). Waves from the collisions of comet Shoemaker-Levy 9 with Jupiter. 374. pp. 706–708. doi:10.1038/374706a0. http://www.nature.com/nature/journal/v374/n6524/abs/374706a0.html. Retrieved on 2008-08-21. 
  22. ↑ Olano, C. A. (August 1999). "Jupiter's Synchrotron Emission Induced by the Collision of Comet Shoemaker-Levy 9". Astrophysics and Space Science 266 (3): 347–369. doi:10.1023/A:1002020013936. http://www.springerlink.com/content/t07q22w438p75864/. Retrieved on 2008-08-21. 
  23. ↑ Bauske, Rainer; Combi, Michael R.; Clarke, John T. (November 1999). "Analysis of Midlatitude Auroral Emissions Observed during the Impact of Comet Shoemaker–Levy 9 with Jupiter". Icarus 142 (1): 106–115. doi:10.1006/icar.1999.6198. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45GMF9Y-19&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=127ddc17d4270f1197e041727ed54af5. Retrieved on 2008-08-20. 
  24. ↑ Brown, Michael E.; Moyer, Elisabeth J.; Bouchet, Antonin H.; Spinrad, Hyron (1995). "Comet Shoemaker-Levy 9: No Effect on the Io Plasma Torus". Geophysical Research Letters 22 (3): 1833–1835. http://www.agu.org/pubs/crossref/1995/95GL00904.shtml. Retrieved on 2008-08-24. 
  25. ↑ Loders, Katharina; Fegley, Bruce (1998). "Jupiter, Rings and Satellites". The Planetary Scientist's Companion. Oxford University Press. pp. 200. ISBN 0-195-11694-1. http://books.google.com/books?id=qTZAyOF8J4wC&dq=Comet+Shoemaker-Levy+9,+water,+impact,+small+amount&source=gbs_summary_s&cad=0. 
  26. ↑ Hockey, T.A. (1994). "The Shoemaker-Levy 9 Spots on Jupiter: Their Place in History". Earth, Moon and Planets 66: 1–9. ISSN 0167-9295. 
  27. ↑ McGrath, M.A.; Yelle, R. V.; Betremieux, Y. (September 1996). "Long-term Chemical Evolution of the Jupiter Stratosphere Following the SL9 Impacts". Bulletin of the American Astronomical Society 28: 1149. http://adsabs.harvard.edu/abs/1996DPS....28.2241M. Retrieved on 2008-08-24. 
  28. ↑ Bézard, B. (October 1997). "Long-term Response of Jupiter's Thermal Structure to the SL9 Impacts". Planetary and Space Science 45 (10): 1251–1271. doi:10.1016/S0032-0633(97)00068-8. http://www.ingentaconnect.com/content/els/00320633/1997/00000045/00000010/art00068. Retrieved on 2008-08-08. 
  29. ↑ Moreno, R.; Marten, A.; Biraud, Y.; Bézard, B.; Lellouch, E.; Paubert, G.; Wild, W. (June 2001). "Jovian Stratospheric Temperature during the Two Months Following the Impacts of Comet Shoemaker-Levy 9". Planetary and Space Science 49 (5): 473–486. doi:10.1016/S0032-0633(00)00139-2. http://www.ingentaconnect.com/content/els/00320633/2001/00000049/00000005/art00139. Retrieved on 2008-08-08. 
  30. ↑ Tancredi, G.; Lindgren, M.; Rickman, H. (November 1990). "Temporary Satellite Capture and Orbital Evolution of Comet P/Helin-Roman-Crockett". Astronomy and Astrophysics 239: 375–380. 
  31. ↑ Roulston, M.S.; Ahrens T.J. (March 1997). "Impact Mechanics and Frequency of SL9-Type Events on Jupiter". Icarus 126 (1): 138–147. http://www.ingentaconnect.com/content/ap/is/1997/00000126/00000001/art05636. Retrieved on 2008-08-26. 
  32. ↑ Schenk, Paul M.; Asphaug, Erik; et al. (June 1996). "Cometary Nuclei and Tidal Disruption: The Geologic Record of Crater Chains on Callisto and Ganymede". Icarus 121 (2): 249–24. doi:10.1006/icar.1996.0084. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45MH0V4-39&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=c4d68cb03502bc7bb34491b532c564c3. Retrieved on 2008-08-08. 
  33. ↑ Greeley, R.; Klemaszewski, J. E.; Wagner, L.; et al. (2000). "Galileo views of the geology of Callisto". Planetary and Space Science 48: 829–853. doi:10.1016/S0032-0633(00)00050-7. http://adsabs.harvard.edu/abs/2000P%26SS...48..829G. 
  34. ↑ Nakamura, T.; Kurahashi, H. (February 1998). "Collisional Probability of Periodic Comets with the Terrestrial Planets – an Invalid Case of Analytic Formulation". Astronomical Journal 11: 848. http://adsabs.harvard.edu/abs/1998AJ....115..848N. Retrieved on 2008-08-08. "For Jupiter-interacting comets of greater than 1 km diameter, a Jupiter impact takes place every 500–1000 yr, and an Earth impact every 2–4 Myr.". 
  35. ↑ Wetherill, G.W. (1994). "Possible Consequences of Absence of "Jupiters" in Planetary Systems". Astrophysics and Space Science 212: 23–32. http://www.ncbi.nlm.nih.gov/pubmed/11539457. Retrieved on 2008-08-08. 

See also

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