Narrator: This is Science Today. To better understand asteroids, scientists must have some idea about their basic composition and structure. Mark Hammergren, a planetary scientist at the Lawrence Livermore National Laboratory, has recently discovered that very elongated asteroids are weaker in centrifugal force than circular ones. This finding supports a theory that most asteroids are not solid chunks of rock, as formerly thought - but rather loosely held piles of rubble.

Hammergren: It's a theory that's growing in popularity among asteroid researchers - that asteroids could be rubble piles, that they might be held together - not by material strength, but only by gravity.

Narrator: This can give researchers an idea of how asteroids have changed since the formation of the solar system over four and a half billion years ago.

Hammergren: And also it helps us figure out how we're going to deal with these things if one ever happens to come towards the Earth because how you deal with a solid object is different than how you deal with a loosely-held pile of rubble.

Narrator: For Science Today, I'm Larissa Branin.