Earth causes asteroids to shake apart

Earthlings casting a wary eye for rogue asteroids may be comforted to learn that our planet is not a sitting duck.

New research published in this week's Nature shows Earth's gravity triggers ground-shifting quakes on asteroids passing close by.

The findings may not only help scientists deflect an Earth-bound asteroid, but also provide fresh insights into the connections between asteroids and meteorites.

Scientists made the discovery by comparing differences in light reflected off asteroids that have breezed by Earth with those that orbit farther away. Though they are made of the same materials, the asteroids that encounter Earth's gravity have fresh surfaces that are noticeably less weathered by the space environment.

Scientists believe the resurfacing is due to slow-falling landslides, triggered by tidal forces from Earth.

"Asteroids get sunburned out there by the light from the Sun, the radiation from the Sun," says Dr Dan Durda, a planetary scientist with the Southwest Research Institute.

Over time, the weathering causes a change in an asteroid's spectra - the breakdown of reflected light into its component wavelengths.

Backtracking the orbits of 95 near-Earth asteroids, scientists determined that over the past 500,000 years, 75 of them had passed closer to Earth than the Moon, which is about 385,000 kilometres away. The 75 asteroids included 20 bodies with spectra of fresh surface materials.

Most notably, there were no freshly surfaced asteroids among those that hadn't had recent close encounters with Earth. The team then did some math and showed that Earth's gravitational muscle could be strong enough to trigger asteroid-quakes as far as about one-quarter of the way to the Moon - less than 100,000 kilometres.

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