The shape of an asteroid, new research says, affects how much of it "survives" the impact without melting. (Credit: WPCLIPART)

Large asteroids typically hit Earth with enough force to vaporize the entire rock. One asteroid the size of San Francisco formed the 40-mile-wide Morokweng crater in South Africa – but puzzlingly, in 2006, scientists discovered a solid piece of the space rock the size of a bowling ball. The discovery spurred a scientific mystery: How could any of it have survived?

Planetary scientist Ross Potter of the Lunar and Planetary Institute in Houston, Texas may have an answer. In his poster Monday at the American Geophysical Union’s Fall Meeting, Potter detailed the conditions necessary for asteroid “survival,” using a model simulating the physics of shock waves rippling through an asteroid on impact. An asteroid’s shape and internal structure, he found, affect its survival rate.

An egg-shaped asteroid, Potter said, leaves more solid material behind than a round one. Shock waves bounce around within the oval shape, lessening the violent pressure that would otherwise melt or vaporize the rock. Yes, the end of the asteroid that hits the ground is still obliterated. But the spaceward end still has a chance.

Potter also showed that solid asteroids survive better than those pockmarked with Swiss-cheese-like holes. Solid rock, Potter found, holds up to stronger impacts without melting.

These results, Potter said, led him to an equation for estimating the amount of an asteroid that can survive impact, figuring in the asteroid’s speed, impact angle, shape, porosity, and composition.

But the equation doesn’t fill in all the gaps. It’s still challenging to draw conclusions about the original asteroid from the fragments remaining. Even under the best conditions, there’s not much material left to study.

“For the Morokweng impact, it’s difficult to constrain from the pieces you find how porous that particular impactor was,” Potter said.

Studies of our solar system’s asteroid belt show a wide range of possible porosities, he said. The dinosaur-era Morokweng asteroid’s could have been up to 10 percent empty space – within the ideal range for asteroid survival. A high impact angle and low speed also probably helped the asteroid survive, although the same cannot be said for the Jurassic-era dinosaurs in its path.

-Paul Gabrielsen is a science communication graduate student at UC Santa Cruz