The 2009 Jupiter impact event, occasionally referred to as the Wesley impact, was a July 2009 impact on Jupiter that caused a black spot in the planet's atmosphere. The spot was similar in area to the planet's Little Red Spot, approximately the size of the Pacific Ocean.[3] The impacter is estimated to have been about 200 to 500 meters in diameter.[4]
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Amateur astronomer Anthony Wesley discovered the impact at approximately 13:30 UTC on 19 July 2009 (almost exactly 15 years after the Jupiter impact of comet Shoemaker-Levy 9, or SL9). He was at his home observatory just outside Murrumbateman, New South Wales, Australia, using stacked images on a 14.5-inch (36.8 cm) diameter reflecting telescope equipped with a low light machine vision video camera attached to the telescope.[5] Wesley stated that
When first seen close to the limb (and in poor conditions) it was only a vaguely dark spot, I [thought] likely to be just a normal dark polar storm. However as it rotated further into view, and the conditions improved I suddenly realised that it wasn't just dark, it was black in all channels, meaning it was truly a black spot.[6]
Wesley sent an e-mail to others including the NASA Jet Propulsion Laboratory in Pasadena, California reporting his observations.[7]
Paul Kalas and collaborators confirmed the sighting. They had time on the Keck II telescope in Hawaii, and had been planning to observe Fomalhaut b, but they spent some of their time looking at the Jupiter impact.[8] Infrared observation by Keck and the NASA Infrared Telescope Facility (IRTF)[3] at Mauna Kea showed a bright spot where the impact took place, indicating the impact warmed a 190 million square km area of the lower atmosphere at 305 W, 57 S near Jupiter's south pole.[3]
The spot's prominence indicates that it is composed of high-altitude aerosols similar to those seen during the SL9 impact.[8] Using near-infrared wavelengths and the IRTF, Glenn Orton and his team detected bright upwelling particles in the planet's upper atmosphere and using mid-infrared wavelengths, found possible extra emission of ammonia gas.[9]
The force of the explosion on Jupiter was thousands of times more powerful than the suspected comet or asteroid that exploded over the Tunguska River Valley in Siberia in June 1908.[2] (This would be over a million times more powerful than the bomb dropped on Hiroshima.)[10]
Astronomers will further observe the impact area with a variety of instruments, including the Keck[8] and the Hubble Space Telescope's recently installed Wide Field Camera 3.[3]
The object that hit Jupiter was not identified before Wesley discovered the impact. A 2003 paper estimated comets with a diameter larger than 1.5 kilometers impact Jupiter about every 90 to 500 years,[11] while a 1997 survey suggested that the astronomer Cassini may have recorded an impact in 1690.[12]
Given the size of the SL9 impacters,[13] it is likely that this object was less than one kilometer in diameter.[2][14] Finding water at the site would indicate that the impacter was a comet,[15] as opposed to an asteroid or a very small, icy moon.[16] At first it was believed that the object was more likely to be a comet since comets generally have more planet crossing orbits.[17] At the distance of Jupiter (5.2 AU) most small comets are not close enough to the Sun to be very active, and so would be hard to detect.[17] Small kilometer-sized asteroids would also be hard to detect, however, and recent work by Orton et al. and Hammel et al. has strongly suggested the impacter was an asteroid, as it left only one impact site, did not reduce Jovian decametric radiation emission by contributing significant dust to the Jovian magnetosphere, and produced high altitude dusty debris full of silica, very different than what was produced by SL9.
As of 2011, the impacter is believed to have been an asteroid with a diameter of about 200 to 500 meters.[4]
Assuming it was an inactive comet (or asteroid) about 1 km in diameter, this object would have been no brighter than about apparent magnitude 25.[17] (Jupiter shines about 130 billion times brighter than a 25th magnitude object.)[18] Most asteroid surveys which use a wide field of view do not see fainter than about magnitude 22 (which is 16x brighter than magnitude 25).[17] Even detecting satellites less than 10 km in diameter orbiting Jupiter is difficult and requires some of the best telescopes in the world.[19] It is only since 1999 with the discovery of Callirrhoe that astronomers have been able to discover many of Jupiter's smallest moons.[20]
On June 3, 2010, Anthony Wesley discovered another impact event, smaller than the previous observed impacts.[21] The 2010 impact was then discovered to have also been captured on video by amateur astronomer Christopher Go in the Philippines.[21]
