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By now you've probably heard that on September 10, Jupiter got whacked by an asteroid or comet again. It was seen directly by amateur astronomer Dan Peterson, and video was taken by George Hall, who kindly posted a fairly awesome a four-second clip on Flickr:
[You may need to refresh this page to see it.]
You can read more about it at Scientific American.
I tweeted about this shortly after it happened (I was returning from the UK and didn't have time to write about it here), and got a lot of people wondering how big the flash was. On the video it looks like it's thousands of kilometers across! But that's not real; the size you see is due mostly to the optics of the telescope and video camera Hall used (plus other factors like atmospheric blurring). Given the brightness I guessed the object was a kilometer or so across, but I heard a radio interview the other day (I missed the guest's name; sorry) where an astronomer said it may have been only 10 meters across - the size of a large truck.
I was surprised at first when he said that, but then realized something critical: Jupiter is a lot bigger than Earth.
The energy released by an asteroid upon impact depends primarily on two things: how big it is and how fast it's moving. For a given asteroid, that means it'll explode with far more energy if it hits Jupiter, because Jupiter's gravity is much stronger than Earth's, and pulls the rock in faster. Simply because of this, impacts on Jupiter can be greater than 20 times more energetic than on Earth. There are other factors (like orbital speeds, what direction the asteroid was moving, and so on), but in general and pound for pound Jupiter impacts are bigger than on Earth.
A 10-meter rock hitting the Earth will release roughly as much energy as a 0.1 megaton bomb, whereas on Jupiter that same rock will release about 2 megatons. A rock twice that size will have 8 times the mass (volume increases with the cube of the radius) so even if it were 20 meters across, the explosion could've been in the 15 - 20 megaton range, which is starting to get to the size of the largest nuclear weapons ever detonated on Earth.
So yeah, that's a lot of power. It doesn't take a big rock to make a bright flash when you've got Jupiter pulling the strings.
And the big planet gets hit a lot. The last one seen was on August 20, 2010, and it got whacked in June 2010 as well as in July 2009 (not to mention the ferocious series of impacts in 1994 from comet Shoemaker-Levy 9). That's pretty close to one biggish impact seen per year, and remember we only see half of Jupiter at a time, and it's not observed constantly! So the real rate is probably far higher.
I also got a lot of people asking why we call it an impact when Jupiter has no solid surface. That's because the rock will still explode as it rams through Jupiter's dense atmosphere; I wrote an explanation of this for an earlier impact. The 1908 Tunguska event here on Earth was an air blast, as well.
It's amazing we can see these planetary at all, but that's due to the digital revolution: amateur astronomers take video of Jupiter and other objects to maximize the number of frames they get, which they then can combine into amazing images. A nice side-effect from this is the collection of rapidly-taken data providing long coverage of the planets, which means significantly increasing the odds of seeing something like this. That is precisely why we're seeing more impacts now than ever before. They've always been happening, we're just a whole lot better at seeing them.
Which is a sobering thought. The Universe is worth investigating, if only for our own self-interest.