page_title: Comet Shoemaker–Levy 9
section_title: Impacts
hierarchical_section_title: Comet Shoemaker–Levy 9 / Impacts
caption_attribution_description: A fireball appears over the limb of Jupiter after the impact of fragment A
context_page_description: Comet Shoemaker–Levy 9 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. 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 possible role in reducing space debris in the inner Solar System.
  The comet was discovered by astronomers Carolyn and Eugene M. Shoemaker and David Levy in 1993. Shoemaker–Levy 9 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 46 cm 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 20–30 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.
context_section_description: 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. Although the impacts took place on the side of Jupiter hidden from Earth, Galileo, then at a distance of 1.6 AU (240 million km; 150 million mi) 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.
  Two other space probes 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 (390 million km; 240 million mi) away, and the distant Voyager 2 probe, some 44 AU (6.6 billion km; 4.1 billion mi) 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 and make observations with its ultraviolet spectrometer.
  The first impact occurred at 20:13 UTC on July 16, 1994, when fragment A of the nucleus entered Jupiter's southern hemisphere at a speed of about 60 km/s (35 mi/s). Instruments on Galileo detected a fireball that reached a peak temperature of about 24,000 K (23,700 °C; 42,700 °F), compared to the typical Jovian cloudtop temperature of about 130 K (−143 °C; −226 °F), before expanding and cooling rapidly to about 1,500 K (1,230 °C; 2,240 °F) after 40 seconds. The plume from the fireball quickly reached a height of over 3,000 km (1,900 mi). 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.
  Despite published predictions, astronomers had not expected to see the fireballs from the impacts and did not have any idea in advance 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 even in very small telescopes, and was about 6,000 km (3,700 mi) (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.
  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 12,000 km (7,500 mi) across, and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (600 times the world's nuclear arsenal). 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.
caption: Hubble Space Telescope images of a fireball from the first impact appearing over the limb of the planet
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