Comet Shoemaker–Levy 9: Difference between revisions

| image = [[File:Shoemaker-Levy 9 on 1994-05-17.png|400px|Hubble Space Telescope]]

'''Comet Shoemaker–Levy 9''' ([[Astronomical naming conventions#Comets|formally designated]] '''D/1993&nbsp;F2''') was a [[comet]] that broke apart in July 1992 and collided with [[Jupiter]] in July 1994, providing the first direct observation of an extraterrestrial [[collision]] of [[Solar System]] objects.<ref name="NASA2005">{{cite web |url=http://nssdc.gsfc.nasa.gov/planetary/comet.html |title=Comet Shoemaker–Levy 9 Collision with Jupiter |access-date=August 26, 2008 |publisher=[[National Space Science Data Center]] |date=February 2005 |archive-url=https://web.archive.org/web/20130219011148/http://nssdc.gsfc.nasa.gov/planetary/comet.html |archive-date=February 19, 2013 |url-status=dead }}</ref> This generated a large amount of coverage in the popular media, and the comet was closely observed by [[astronomer]]s worldwide. The collision provided new information about Jupiter and highlighted its possible role in reducing [[space debris]] in the [[Solar System#Inner planets|inner Solar System]].

The comet was discovered by astronomers [[Carolyn S. Shoemaker|Carolyn]] and [[Eugene Merle Shoemaker|Eugene M. Shoemaker]], and [[David H. Levy|David Levy]] in 1993.<ref name="IAU 5725" /> Shoemaker–Levy 9 (SL9) had been captured by Jupiter and was orbiting the planet at the time. It was located on the night of March 24 in a photograph taken with the {{convert|46|cm|in|abbr=on}} [[Schmidt camera|Schmidt telescope]] at the [[Palomar Observatory]] in [[California]]. It was the first active comet observed to be orbiting a planet, and had probably been captured by Jupiter around 20 to 30 years earlier.

Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, the orbit of Shoemaker–Levy 9 passed within Jupiter's [[Roche limit]], and Jupiter's [[tidal force]]s had acted to pull apart the comet apart. The comet was later observed as a series of fragments ranging up to {{convert|2|km|abbr=on}} in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and 22, 1994 at a speed of approximately {{convert|60|km/s|0|abbr=on}} (Jupiter's [[escape velocity]]) or {{convert|216000|km/h|mph|abbr=on}}. The prominent scars from the impacts were more easily visible than the [[Great Red Spot]] and persisted for many months.

While conducting a program of observations designed to uncover [[near-Earth object]]s, the Shoemakers and Levy discovered Comet Shoemaker–Levy 9 on the night of March 24, 1993, in a photograph taken with the {{convert|0.46|mcm|ft|abbr=on}} [[Schmidt camera|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.<ref name="Marsden1">{{cite web |url=http://www2.jpl.nasa.gov/sl9/news81.html |title=Eugene Shoemaker (1928–1997) |access-date=August 24, 2008 |last=Marsden |first=Brian G. |date=July 18, 1997 |publisher=[[Jet Propulsion Laboratory]]}}</ref>

Comet Shoemaker–Levy 9 was the ninth periodic comet (a comet whose orbital period is 200 years or less) discovered by the Shoemakers and Levy, [[naming of comets|hencethence 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 26, 1993.<ref name="IAU 5725">{{cite journal |last= Marsden |first=B. G.|url=http://www.cbat.eps.harvard.edu/iauc/05700/05725.html#Item1 |title=Comet Shoemaker-Levy (1993e) |journal=IAU Circular |volume=5725 |year=1993 }}</ref>

Tracing back the comet's orbital motion revealed that it had been orbiting Jupiter for some time. It is 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.<ref name="Landis">{{cite web |url=http://www.seds.org/sl9/landis.html |title=Comet P/Shoemaker–Levy's Collision with Jupiter: Covering HST's Planned Observations from Your Planetarium |access-date=August 8, 2008 |last=Landis |first=R. R. |year=1994 |work=Proceedings of the International Planetarium Society Conference held at the Astronaut Memorial Planetarium & Observatory, Cocoa, Florida, July 10–16, 1994 |publisher=[[Students for the Exploration and Development of Space|SEDS]]|archive-url = https://web.archive.org/web/20080808142753/http://www.seds.org/sl9/landis.html |archive-date=August 8, 2008 }}</ref> 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, [[Satoru Otomo|S. Otomo]] on March 17, and a team led by [[Eleanor Helin]] from images on March 19.<ref name="D1993">{{cite web|url=http://cometography.com/pcomets/1993f2.html |title=D/1993 F2 Shoemaker–Levy 9 |access-date=August 8, 2008 |year=1994 |work=Gary W. Kronk's Cometography |url-status=dead |archive-url=https://web.archive.org/web/20080509145539/http://cometography.com/pcomets/1993f2.html |archive-date=May 9, 2008 }}</ref> An image of the comet on a Schmidt photographic plate taken on March 19 was identified on March 21 by M. Lindgren, in a project searching for comets near Jupiter.<ref>{{Cite journal|last=Lindgren|first=Mats|date=August 1996|title=Searching for comets encountering Jupiter. Second campaign observations and further constraints on the size of the Jupiter family population.|journal=Astronomy and Astrophysics Supplement Series|volume=118|issue=2|pages=293–301|doi=10.1051/aas:1996198|bibcode=1996A&AS..118..293L|doi-access=free}}</ref> However, as his team were expecting comets to be inactive or at best exhibit a weak dust coma, and SL9 had a peculiar morphology, its true nature was not recognised until the official announcement 5 days later. 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]].<ref name="Benner">{{cite journal |last1=Benner |first1=L. A. |date=March 1994 |title=Pre-Impact Orbital Evolution of P/Shoemaker–Levy 9 |journal=Abstracts of the 25th Lunar and Planetary Science Conference, Held in Houston, TX, March 14–18, 1994 |volume=25 |page=93 |bibcode=1994LPI....25...93B |last2=McKinnon |first2=W. B.}}</ref>

Two other space probes made observations at the time of the impact: the [[Ulysses (spacecraft)|''Ulysses'' spacecraft]], primarily designed for [[Sun|solar]] observations, was pointed towardstoward Jupiter from its location {{convert|2.6|AU|e6km+e6mi|abbr=unit}} away, and the distant ''[[Voyager 2]]'' probe, some {{convert|44|AU|e9km+e9mi|abbr=unit}} 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&nbsp;[[kHz]] range and make observations with its ultraviolet spectrometer.<ref name=williamsnasa>{{cite web |url=http://nssdc.gsfc.nasa.gov/planetary/others.html |last=Williams|first=David R.|title=Ulysses and Voyager 2 |access-date=August 25, 2008 |work=Lunar and Planetary Science |publisher=[[National Space Science Data Center]]}}</ref>

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.<ref name="Weissman">{{cite journal |last1=Weissman |first1=P.R. |date=March 1995 |title=Galileo NIMS Direct Observation of the Shoemaker–Levy 9 Fireballs and Fall Back |journal=Abstracts of the Lunar and Planetary Science Conference |volume=26 |page=1483 |bibcode=1995LPI....26.1483W |last2=Carlson |first2=R. W. |last3=Hui |first3=J. |last4=Segura |first4=M. |last5=Smythe |first5=W. D. |last6=Baines |first6=K. H. |last7=Johnson |first7=T. V. |last8=Drossart |first8=P. |last9=Encrenaz |first9=T.|author9-link=Thérèse Encrenaz|last10=Leader |first10=F. |last11=Mehlman |first11=R. |display-authors=9 }}</ref>

Despite published predictions,<ref name="Boslough" /> astronomers had not expected to see the fireballs from the impacts<ref name="Fizzle">{{cite journal |last=Weissman |first=Paul |date=July 14, 1994 |title= The Big Fizzle is coming |journal=Nature |volume=370 |issue=6485 | pages=94–95 |doi=10.1038/370094a0|bibcode = 1994Natur.370...94W |s2cid=4358549 |doi-access=free }}</ref> and did not have any idea how visible the other atmospheric effects of the impacts would be from Earth. Observers soon saw a huge dark spot after the first impact; the spot was visible from Earth. 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.<ref name="Hammel">{{Cite conference |last=Hammel |first=H.B. |date=December 1994 |title=The Spectacular Swan Song of Shoemaker–Levy 9 |conference=185th AAS Meeting |publisher=American Astronomical Society |volume=26 |pages=1425 |bibcode=1994AAS...185.7201H}}</ref>

Over the next six 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 {{cvt|12000|km|disp=or}}<ref>{{cite web |title=Remembering Comet Shoemaker-Levy 9's Impact on Jupiter, 23 Years Ago This Week |url=https://www.americaspace.com/2017/07/17/remembering-comet-shoemaker-levy-9s-impact-on-jupiter-23-years-ago-this-week/ |website=AmericaSpace |access-date=12 January 2022 |date=17 July 2017}}</ref> (almost one [[Earth radius#Derived quantities: diameter, circumference, arc-length, area, volume|Earth diameter]]) across, and was estimated to have released an energy equivalent to 6,000,000&nbsp;[[TNT equivalent|megatons of TNT]] (600 times the world's nuclear arsenal).<ref>{{cite web |first=Dan |last=Bruton |url=http://www.physics.sfasu.edu/astro/sl9/cometfaq2.html#Q3.1 |title=What were some of the effects of the collisions? |work=Frequently Asked Questions about the Collision of Comet Shoemaker–Levy 9 with Jupiter |publisher=[[Stephen F. Austin State University]] |date=February 1996| |access-date=January 27, 2014 |archive-date=August 28, 2021 |archive-url=https://web.archive.org/web/20210828080844/http://www.physics.sfasu.edu/astro/sl9/cometfaq2.html#Q3.1 |url-status=dead }}</ref> Two impacts 12&nbsp;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.<ref>{{cite web |first=Don |last=Yeomans |author2=Chodas, Paul |url=http://www2.jpl.nasa.gov/sl9/impacts4.html | title=Comet Crash Impact Times Request |publisher=[[Jet Propulsion Laboratory]] |date=March 18, 1995 |access-date=August 26, 2008}}</ref>

As well as these [[molecule]]s, emission from heavy [[atom]]s such as [[iron]], [[magnesium]] and [[silicon]] waswere detected, with abundances consistent with what would be found in a cometary nucleus. Although a substantial amount of water was detected spectroscopically, it was not as much as predicted, 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.<ref name="Hu" />

As predicted, the collisions generated enormous waves that swept across Jupiter at speeds of {{convert|450|m/s|0|abbr=on}} 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 thought the stable layer must lie within the hypothesised [[troposphere|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]].<ref name="Ingersoll">{{cite journal |last1=Ingersoll |first1=A. P. |date=April 1995 |title=Waves from the collisions of comet Shoemaker–Levy 9 with Jupiter |journal=Nature |volume=374 |issue= 6524|pages=706–708 |doi=10.1038/374706a0 |pmid=7715724 |last2=Kanamori |first2=H|bibcode = 1995Natur.374..706I |s2cid=4325357 }}</ref>

''[[Voyager 2]]'' failed to detect anything with calculations, showing that the fireballs were just below the craft's limit of detection; no abnormal levels of UV radiation or radio signals were registered after the blast.<ref name="williamsnasa" /><ref name=Ulivi449>{{cite book |last1=Ulivi |first1=Paolo |last2=Harland |first2= David M |date=2007 |title=Robotic Exploration of the Solar System Part I: The Golden Age 1957–1982 |publisher=Springer |page=449 |isbn=9780387493268 }}</ref> ''[[Ulysses (spacecraft)|Ulysses]]'' also failed to detect any abnormal radio frequencies.<ref name=williamsnasa />

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 {{convert|15000|to|20000|km|abbr=on}} wide, but dropped back to normal levels within a week of the impact. At smaller sites, temperatures {{convert|10| K-change}} (10 '''°'''C; 18 '''°'''F) higher than the surroundings persisted for almost two weeks.<ref name="Bézard">{{cite journal |last=Bézard |first=B. |date=October 1997 |title=Long-term Response of Jupiter's Thermal Structure to the SL9 Impacts |journal=Planetary and Space Science |volume=45 |issue=10 |pages=1251–1271 |doi=10.1016/S0032-0633(97)00068-8 |bibcode=1997P&SS...45.1251B}}</ref> Global stratospheric temperatures rose immediately after the impacts, then fell to below pre-impact temperatures 2–3&nbsp;weeks afterwards, before rising slowly to normal temperatures.<ref name="Moreno">{{cite journal |last1= Moreno|first1=R.|date=June 2001 |title= Jovian Stratospheric Temperature during the Two Months Following the Impacts of Comet Shoemaker–Levy 9|journal=Planetary and Space Science |volume=49 |issue=5 |pages=473–486 |doi=10.1016/S0032-0633(00)00139-2 |bibcode=2001P&SS...49..473M |last2= Marten |first2= A |last3= Biraud |first3= Y |last4= Bézard |first4= B |last5= Lellouch |first5= E |last6= Paubert |first6= G |last7= Wild |first7= W}}</ref>

SL9 is not unique in having orbited Jupiter for a time; five comets, (including [[82P/Gehrels]], [[147P/Kushida–Muramatsu]], and [[111P/Helin–Roman–Crockett]]), are known to have been temporarily captured by the planet.<ref name="Ohtsukaetal2008">{{cite journal| bibcode = 2008A&A...489.1355O| title = Quasi-Hilda comet 147P/Kushida–Muramatsu. Another long temporary satellite capture by Jupiter| last1 = Ohtsuka| first1 = Katsuhito| journal = Astronomy and Astrophysics| volume = 489| issue = 3| date = October 2008| pages = 1355–1362| doi = 10.1051/0004-6361:200810321| last2 = Ito| first2 = T.| last3 = Yoshikawa| first3 = M.| last4 = Asher| first4 = D. J.| last5 = Arakida| first5 = H.|arxiv = 0808.2277 | s2cid = 14201751}}</ref><ref>{{cite journal|last1=Tancredi|first1=G. |date=November 1990 | title=Temporary Satellite Capture and Orbital Evolution of Comet P/Helin–Roman–Crockett |journal=[[Astronomy and Astrophysics]] |volume=239 |issue=1–2 |pages=375–380 |bibcode = 1990A&A...239..375T|last2=Lindgren|first2=M.|last3=Rickman|first3=H. }}</ref>