Rocks that go bump in the night.

etween the orbits of Mars and Jupiter lie the remains of the early, violent history of the Solar System. These are asteroids, rocky bodies that range in diameter from a few hundred metres to just under 1,000 km. Over the past century, astronomers have realized that asteroids (and comets, their icy cousins originating further out in the Solar System) are the building blocks of planets. On page 720 of this issue, Nesvornyánd colleagues 1 report their discovery of an asteroid family that offers unprecedented insight into the dynamics of asteroid collisions — and hence into how the planets of the Solar System formed. For planets to grow in size from their humble beginnings, they must be continually bombarded by smaller bodies for tens to hundreds of millions of years. This process still happens today: not only do these small bodies continue to strike the Earth, with effects ranging from spectacular fireballs to mass extinctions, but they continue to bump into each other as well. In the past these bumps were relatively gentle, which allowed the first protoplanets to form. But as giant planets such as Jupiter came into existence, the gravitational fields they generated stirred up the smaller bodies. This energy increase made them more likely, if they collided, to break apart than grow, with the result that only the largest protoplanets survived to form planets while the rest were ground down to small fragments. Nesvornyánd colleagues 1 have uncovered the aftermath of one such energetic collision. What makes their discovery unique and exciting is that a large-scale break-up event has, for the first time, been precisely dated. Piecing together the fragments, Nesvornyét al. find that a 25-km asteroid in the Outer Main Belt between Mars and Jupiter broke up in collision 5.8 million years ago — a mere blink of an eye in the 4.5-billion-year lifetime of the Solar System. Theorists studying planet formation would like nothing better than to smash two asteroids together and see what happens. Many questions could be answered. Under what conditions does an asteroid break apart? What happens to the debris? How do pre-existing fractures affect the outcome? Could a 'doomsday' asteroid threatening Earth be stopped by brute-force methods such as a nuclear blast? Of course, full-scale experiments are not possible, so planetary scientists rely on much smaller laboratory experiments as well as numerical simulations to predict collision outcomes 2,3. But nature has provided …