This was a comet that broke apart, colliding with Jupiter and providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a large amount of coverage in the popular media, and the comet 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.
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| "Shoemaker-Levy 9 on 1994-05-17" by NASA, ESA, and H. Weaver and E. Smith (STScI) - http://hubblesite.org/newscenter/archive/releases/1994/26/image/c/ (direct link). Licensed under Public Domain via Wikimedia Commons |
The comet was discovered by astronomers Carolyn and Eugene
M. Shoemaker and David Levy. Shoemaker–Levy 9, at the time captured by and
orbiting Jupiter, was located on the night of March 24, 1993, in a photograph
taken with the 40 cm (16 in) Schmidt telescope at the Palomar Observatory in
California. It was the first comet observed to be orbiting a planet, and had
probably been captured by the planet around 20 – 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 forces had acted to pull apart the comet. The comet was later observed as
a series of fragments ranging up to 2 km (1.2 mi) in diameter. These fragments
collided with Jupiter's southern hemisphere between July 16 and July 22, 1994,
at a speed of approximately 60 km/s (37 mi/s) or 216,000 km/h (134,000 mph).
The prominent scars from the impacts were more easily visible than the Great
Red Spot and persisted for many months.
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.
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. The relatively low levels of water were later confirmed by Galileo's
atmospheric probe, which explored Jupiter's atmosphere directly.
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