Humans may be justifiably nervous when an asteroid passes very close to Earth, but a new study has found that the encounter leaves the asteroid pale and shaken as well.

UH Manoa astronomers Schelte Bus and Alan Tokunaga are members of an international team who set out to understand why some asteroids have colors different from those of others.

In particular, they wanted to know why is it that most asteroids have a comparatively dark surface-the result of slow weathering by interplanetary sunlight-but others have a paler color characteristic of fresh, unweathered rocks?

By looking very carefully at the various asteroids' orbits around the sun, the team noticed that all those with pale colors had passed very close to Earth, while those with dark colors had not.

How can a near-miss with Earth affect an asteroid's color?

"We now suspect that most asteroids are loose conglomerations of rocks and boulders, rather than strong, monolithic objects," said astronomer Bus.

"When one of these rock piles passes close to Earth, it is shaken by the rapidly changing pull of Earth's gravity. Landslides on the asteroid cause the dark weathered areas to be covered by fresh, lighter colored rocks. Hence the asteroid's color, after the encounter, will appear paler than before."

"The more we can learn about what holds an asteroid together, the better chance we have to reduce or eliminate damage to Earth," added Tokunaga.