In the News this morning, it looks like they've confirmed Anthony Wesley's object, which impacted Jupiter on July 19, 2009, to be an Asteroid:
New evidence that asteroid, not comet, struck Jupiter in 2009
Quote:
A hurtling asteroid about the size of the Titanic caused the scar that appeared in Jupiter's atmosphere on July 19, 2009, according to two papers published recently in the journal Icarus.
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By piecing together signatures of the gases and dark debris produced by the impact shockwaves, an international team of scientists was able to deduce that the object was more likely a rocky asteroid than an icy comet.
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The new conclusion is also consistent with evidence from results from NASA's Hubble Space Telescope indicating the impact debris in 2009 was heavier or denser than debris from comet Shoemaker-Levy 9, the last known object to hurl itself into Jupiter's atmosphere in 1994.
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They're estimating the density at around 2.5 grams per cubic centimetres.
Cheers