On the 19th July 2009, between 9 and 11 UTC, a small celestial body crashed into Jupiter's atmosphere. The first to notice was an Australian amateur astronomer, Anthony Wesley, who immediately spread the news to professional researchers, amongst them, Glenn Orton, an astronomer at NASA's Jet Propulsion Laboratory.
Orton, in collaboration with Leigh Fletcher (Oxford University, U.K.) and other researchers, immediately observed the traces of the collision with the Infrared Telescope Facility (Mauna Kea, Hawaii), and also managed to gain observing time with the two Gemini telescopes and the VLT.
From the data collected in the following weeks, it has been possible to reach some unexpected conclusions, recently made public in two articles published in the journal Icarus.
Straight after the collision it was thought that further, smaller impacts might follow, as happened in 1994 when Jupiter was hit by a cometary nucleus. There were, in fact, a lot of similarities between the traces observed by Orton's team and those left by the impact of comet Shoemaker-Levy 9: dark marks and ammonia in the atmosphere, with stratospheric heating near the impact site.
More careful analysis revealed, however, that in 2009 the debris did not rise as high into the atmosphere, almost certainly because it was heavier, and that the heating (3-4 Kelvin) was limited to the lower stratosphere rather than the high stratosphere, as was the case for comet S-L 9.
Further, the more recent impact produced traces of hydrocarbons, silicates and silicas, but not carbon monoxide, something which points to a rocky composition, poor in water. An asteroid then, rather than a comet.
The conclusion is that a small asteroid of about 200-500 metres in diameter, with a density of around 2.5 g/cm3,
penetrated deep into the Jovian atmosphere, producing an explosion with
an energy release equivalent to 5 gigatons of TNT. The energy of the
impact converted some of the silicon component of the asteroid into
spewing this and other by-products back along the path of the asteroid,
towards external space.
By determining which orbits would be compatible with the impact
geometry, Orton and colleagues found that the orbit must have been
similar to the chaotic orbit of asteroid 2005 TS100,
that has had several near misses with the giant planet. These results
add to mounting evidence that not only Jupiter family comets risk