Editing Comet Shoemaker–Levy 9 (section)

== Comet with a Jovian orbit ==
Orbital studies of the new comet soon revealed that it was orbiting [[Jupiter]] rather than the [[Sun]], unlike any other comet then known. Its orbit around Jupiter was very loosely bound, with a period of about 2 years and an [[apoapsis]] (the point in the orbit farthest from the planet) of {{convert|0.33|AU|e6km+e6mi|lk=in|abbr=off}}. Its orbit around the planet was highly [[Orbital eccentricity|eccentric]] (''e'' = 0.9986).<ref name="Bruton1.4" />

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]] 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>

The volume of space within which an object can be said to orbit Jupiter is defined by Jupiter's [[Hill 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 Jove-centric orbit of very high eccentricity—that is to say, the ellipse was nearly flattened out.<ref name="Chapman">{{cite journal |last=Chapman |first=C. R. |date=June 1993 |title=Comet on target for Jupiter |journal=Nature |volume=363 |issue=6429 |pages=492–493 |doi=10.1038/363492a0|bibcode = 1993Natur.363..492C |s2cid=27605268 |doi-access=free }}</ref>

The comet had apparently passed extremely close to Jupiter on July 7, 1992, just over {{convert|40000|km|abbr=on}} above its cloud tops—a smaller distance than Jupiter's radius of {{convert|70000|km|abbr=on}}, and well within the orbit of Jupiter's innermost moon [[Metis (moon)|Metis]] and the planet's [[Roche limit]], inside which [[tidal force]]s are strong enough to disrupt a body held together only by gravity.<ref name="Chapman" /> 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 fragmented comets.<ref name="Boehnhardt">{{Cite book |first=H. |last=Boehnhardt |date=November 2004 |chapter=Split comets |title=Comets II |editor=M. C. Festou, H. U. Keller and H. A. Weaver |publisher=[[University of Arizona Press]] |page=301 |isbn=978-0-8165-2450-1}}</ref>

More exciting for planetary astronomers was that the best orbital calculations suggested that the comet would pass within {{convert|45000|km|abbr=on}} of the center 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.<ref name="IAU 5800">{{cite journal |last=Marsden |first=B. G. |title=Periodic Comet Shoemaker-Levy 9 (1993e) |url=http://www.cbat.eps.harvard.edu/iauc/05800/05800.html#Item1 |journal=IAU Circular |issue= 5800 |year=1993}}</ref> Studies suggested that the train of nuclei would plow into Jupiter's atmosphere over a period of about five days.<ref name="Chapman" />