An image of Itokawa, an asteroid photographed by the Japanese Hayabusa mission in 2005, which researchers suspect has a composition similar to Apophis's.

An image of Itokawa, an asteroid photographed by the Japanese Hayabusa mission in 2005, which researchers suspect has a composition similar to Apophis's.


Avalanche on an asteroid, due to close pass with Earth

On Friday the 13th in April 2029, an asteroid named Apophis is expected to pass, with luck, within a hair’s breadth of Earth. The space rock, about as wide as three football fields, won’t do any damage to Earth—it’s predicted to pass at a safe distance of at least 35,000 kilometers—but the reverse may not be true. A new study finds that the near miss could trigger tiny avalanches on Apophis.

“It’s a difficult topic, but this paper looks to me like one of the best jobs done on Apophis to date,” says William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who was not involved in the study.

If asteroids pass close to Earth, they begin to experience the effects of our planet’s gravity. Just like the moon pushes and pulls the oceans, creating the tides, asteroids are susceptible to tidal forces from our planet. To judge what effect this will have on Apophis, scientists need to know what it’s made of. Their best guess is based on photos taken by a Japanese spacecraft named Hayabusa, which took detailed pictures of an Apophis-sized asteroid named Itokawa. Those images revealed that the asteroid wasn’t a solid mass of rock spinning through space, but rather a giant clump of debris held together loosely by gravity. “You look at the [Hayabusa] pictures and you’re like, ‘Uh, that’s a pile of rocks, dude.’ It’s very likely that Apophis is similar,” says astrophysicist Derek Richardson of the University of Maryland, College Park.

To show that Earth’s gravity could cause some of these rocks to tumble, Richardson and his colleagues developed a computer model that allowed them to place virtual sand piles across the surface of a model asteroid with roughly the same dimensions as Apophis. By factoring in the gravity from the asteroid, the tidal force from Earth, centrifugal force caused by the asteroid’s rotation, inertial forces, and other effects, the team was able to predict how the particles on the surface of the asteroid would behave on approach. The results confirm that Earth's tidal forces would be strong enough to cause tiny avalanches on the asteroid, the team reported online ahead of print in Icarus.

Astronomers aren’t in for much of a show, however. The avalanches are so small that they’d be hard to see even if you were standing on Apophis’s surface, Richardson says. They would also be incredibly sluggish. “They’d move very slowly because the gravity is so weak on this asteroid. You could literally have lunch before the avalanche stopped moving,” he says. “This is a very gentle, distant encounter.”

Still, the perturbations might be enough to keep Apophis looking young. Most asteroid aging is caused by exposure to the sun, and therefore affects only exposed surfaces. Like a spa treatment, the tiny avalanches exfoliate the surface particles and replace the aged-out layer with fresh ones from below.

To know for certain whether this happens, astronomers will need to observe Apophis during its 2029 pass. Scientists age asteroids by surveying infrared light; older, weathered rocks tend to appear more reddish in these observations. If a high-quality spectrum can be obtained during the approach, it should be possible to compare the color of Apophis before and after its flyby, Bottke says. “When [Apophis] makes its pass, we’ll get to run the experiment. Whether it’s all solved or not we’ll have to find out.”

“Through this paper, we’re hoping to encourage observers to make this observation,” Richardson says. This might be their only chance, too; Apophis is expected to return again in 2036, but estimates have it missing by a whopping 9100 Earth radii—well outside of the range of Earth’s tidal influence.

*Correction, 29 October, 12:31 p.m.: The width of Apophis, in relation to the size of a football field, has been corrected.

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