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 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.

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.

Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.