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NARRATOR: Asteroids collide
all the time.
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At least, we think they do.
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Close up views from spacecraft
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show asteroids to be pockmarked
with impact craters.
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But until recently,
astronomers never expected
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they'd see the recent aftermath
of a collision.
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Now, thanks to Hubble
and Swift, they have.
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In early 2010, Hubble
took a close look at the tail
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of what astronomers thought
was an unusual comet.
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The curious tail
turned out to be wreckage
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from a collision
between two small asteroids.
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On December 11, 2010,
astronomers saw
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that another asteroid,
596 Scheila,
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had also grown a tail.
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Swift's ability to see
ultraviolet light
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helped astronomers
rule out the possibility
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that they were
looking at a comet.
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None of the gases characteristic
of a comet were detected.
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These plumes
are clouds of dust -
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debris from the impact
of a smaller asteroid
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less than 100 feet across.
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The shape, evolution,
and content of the plumes
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let astronomers
reconstruct what happened.
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The smaller asteroid would have
been heading towards Scheila
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at about 11,000 miles
per hour.
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It hit at a low angle
with the force of
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at least a one hundred kiloton
nuclear bomb.
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Yet, much of the debris
fell back on to the asteroid.
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The particles that escaped
were the smallest ones.
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Easily pushed around
by sunlight,
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this fine dust
formed wispy plumes.
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When Hubble observed Scheila
two weeks later,
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the plumes were barely visible.
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Within two months
of the outburst,
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the plumes
were completely gone,
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and with them
went any sign from Earth
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that Scheila
even had a collision.
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Astronomers estimate that
somewhere in the asteroid belt,
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events like this may happen
as often as every year.
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Thanks to Hubble and Swift,
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we're just beginning
to see them.