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Research by Dr. William F. Bottke, Dr. David Vokrouhlicky and Dr. David Nesvorny suggests that the impactor believed to have wiped out the dinosaurs and other life forms on Earth 65 million years ago can been traced back to a breakup event in the main asteroid belt.  A -- Computer modeling shows that the parent object of asteroid (298) Baptistina, which was approximately 170-kilometers in diameter with characteristics similar to carbonaceous chondrite meteorites, was disrupted 160 million years ago when it was hit by another asteroid estimated to be 60-kilometers in diameter.  B -- This impact produced what is now known as the Baptistina asteroid family, a cluster of asteroid fragments with similar orbits. The family originally included approximately 300 bodies larger than 10 kilometers and 140,000 bodies larger than 1 kilometer.  C -- (1) Following the breakup event, the newly formed fragments began to slowly migrate in space by thermal forces produced when they absorbed sunlight and re-radiated the energy away as heat. (2) Over time, this gradual spreading allowed about 20 percent of the multi-kilometer-sized fragments to drift into a nearby "dynamical superhighway" where their orbits were modifed enough to escape the main asteroid belt. (3) Eventually, these fragments were delivered to orbits that cross the Earth's path. About 2 percent of these main belt refugees hit the Earth, while a smaller fraction struck the Moon.  D -- It is probable that a large fragment from this breakup event created the 85-kilometer Tycho impact crater on the Moon 108 million years ago.  E -- It is even more likely that a still larger fragment from the Baptistina breakup created the 180-kilometer Chicxulub crater off the coast of the Yucatan 65 million years ago. The impact that produced this crater has been strongly linked to the mass extinction event that eliminated the dinosaurs.