Even a casual fan of "Star Wars" may have a distorted view of what an "asteroid belt" looks like. In "The Empire Strikes Back" the Millennium Falcon dodges constantly to avoid space rocks while outrunning the Empire's Star Destroyers. In reality, asteroids between Mars and Jupiter are spaced hundreds of thousands of miles apart. It's rare for them to actually crash into each other, but when they do, that could produce a spurt of dust.
"The result of such a collision depends on the relative sizes, velocities and internal strengths, and range from catastrophic destruction to a simple impact crater," Agarwal wrote in an email. "In any case, the collision will likely create a dust cloud, and it is this cloud that we can observe from Earth and which gives a temporary comet-like appearance to the impacted object."
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One example of this was 596 Scheila, which was smacked by a much smaller asteroid in 2010 and created two temporary dust clouds. The scar on the surface remained visible afterwards, as the light curve -- the change of total brightness in a telescope -- had changed, Agarwal said.