Kuiper Belt Collision

via Space.com

Kuiper Belt object 2003 EL61, discovered in 2004 by Caltech's Mike Brown, is apparently the victim of a large collision ealry in the history of the solar system. This collision is responsible for both the body's odd shape (somewhat like a deflated football), rapid rotation (every four hours) and end-over-end spin.

"The impact spawned at least seven other rocky objects—and likely more—with diameters ranging from 6 to 250 miles (10 to 400 km). The researchers lumped the scattered objects into a family based on their matching gray color and evidence of surface water ice derived from spectral analyses."

EL61 was probably up to 20% larger than today (1500 km now) and was probalby struck by another object about half its past size at over 7000 mph. The resultant kinetic explosion would have been about the size of 10 million Hiroshima sized a-bombs.

This is the first collision discovery located in the outer solar system, the remaining 35 all being located in the asteroid belt between Mars and Jupiter.