Many faces. Like so many other asteroids, Ida 243 is irregularly shaped and loosely composed of rubble.

Earth Can Shake Up Asteroids, Too

It isn't exactly payback for taking out the dinosaurs, but Earth's gravity seems to disrupt the surfaces of asteroids that pass close by, researchers have found. The findings should improve our understanding of the composition of these tiny planetary bodies and might help scientists find ways to fend off asteroids that could strike Earth.

For many years, the popular conception of asteroids--the thousands of planetary fragments that cloud the inner solar system between the orbits of Mars and Jupiter--was that they consisted of solid rock or metal. Astronomers explained their potatolike shapes as the result of frequent collisions with their neighbors. But more-recent observations have shown that asteroids actually resemble piles of gravel held together very weakly by gravity. So weakly in fact that they sometimes temporarily split in two, recongealing after thousands of years.

Now, research suggests that near-Earth asteroids (NEAs)--those whose paths take them near or across Earth's orbit--are so fragile that their surfaces roil anytime they encounter our planet's gravity. The idea arose because of a curious observation. Most asteroids show a slight reddish cast caused by the constant bombardment of solar-wind particles, a process called space weathering. Yet the surfaces of nearly half of all NEAs appear fresh, as scientists term it, with no effects of space weathering.

One possible reason for the fresh surfaces, a team reports tomorrow in Nature, is that those asteroids had their surfaces disturbed by past close gravitational encounters with Earth. So the researchers retraced the orbit of every known NEA that didn't exhibit space weathering. "Each and every fresh asteroid, when we traced it backward for 500,000 years, had passed very close to Earth"--within 100,000 kilometers, versus millions of kilometers for the other NEAs-- says planetary scientist and co-author Richard Binzel of the Massachusetts Institute of Technology in Cambridge.

What seems to happen, Binzel explains, is that even a gentle gravitational pull from a planetary encounter is enough to churn the asteroid's weathered surface. "It's kind of like if you could shake [up] a dirty snowbank," he says. "The result would be a lot of fresh snow ending up back on top and making the surface fresh and bright again."

Binzel says that he and colleagues are looking forward to an encounter with Apophis in 2029. That's when the 270-meter-wide asteroid is expected to pass within 35,000 kilometers of Earth--closer than the orbit of some telecommunications satellites. The flyby "should be a very interesting case," he says. Space agencies might be able to place instruments on the asteroid's surface that could "tell us how potentially hazardous asteroids like Apophis are put together," he says. "And learning how they are put together would help us to figure out how to deflect these objects, in the event one was ever found to be on a collision course with Earth."

Space scientist William Bottke of the Southwest Research Institute in Boulder, Colorado, says the findings reveal much about the physical evolution of asteroids. Learning more about how space weathering and close encounters with Earth can alter their appearance, he explains, which could make it easier to match meteorites that land on Earth with the asteroids where they originated. And that can make meteorites recovered on the ground as valuable as those studied in space.

Posted in Space