© Science/AAAS
Far out in space, between Mars and Jupiter, a group of roiling rocks has been circling the sun since the very early days of the solar system, a new study reveals.

Piecing together the planetesimals, the earliest ancestors of the asteroids, hasn't been easy because eons of collisions have broken them apart. But that has changed thanks to the discovery of a family of "dark" asteroids in the region, reported today in Science.

Unlike the brighter, highly reflective asteroid families nearby, the dark asteroids' orbits are more spread out, meaning more time has passed since the asteroids formed. By aggregating the sizes of the modern dark asteroids, researchers suggest the original dark planetesimals formed about 4 billion years ago, making this one of the oldest asteroid families in the main belt; most asteroid families are thought to have formed about 1 billion years ago.

While putting together these planetary puzzle pieces, the scientists also determined that the dark family's original planetesimals were no smaller than 35 kilometers across.

That's compelling evidence for the gravitational collapse theory of asteroid formation, which states that the space dust that made up the planetesimals smashed together almost instantaneously. In order for the theory to work, planetesimals had to be huge, and this family was one of the biggest.