December 10, 2012 - A giant asteroid is set to buzz Earth next week, and astronomers are already keeping their eyes on the skies—but not because 4179 Toutatis poses any danger.
Toutatis, at 2.7 miles (4.46 kilometers) long and 1.5 miles (2.4 kilometers) wide, is one of the largest asteroids that comes anywhere near Earth. But only an astronomer would consider its closest approach to be "near." When the peanut-shaped rock is at its closest to the Earth on December 12, it'll be more than 4.3 million miles (6.9 million kilometers) away, or more than 18 times the distance from the Earth to the moon.
So why are astronomers eagerly awaiting Toutatis? By figuring out what the asteroid is made of, they'll have a better picture of the early days of the solar system. And by refining a model of the asteroid's rotation, they'll get a better idea of its composition.
Michael Busch, a fellow at the National Radio Astronomy Observatory, studied radar images of Toutatis' previous passes—the asteroid approaches Earth every four years—to try to figure out how it was moving through space. "It's tumbling," Busch said. "It's spinning around its long axis, while that in turn is precessing around in a circle, like a gyroscope." Busch and his colleagues were hoping to use radar images taken in 2000, 2004, and 2008 to update a 1996 model of Toutatis' spin state. "[But] this became more complicated when we understood that [gravitational] tides were changing the spin," he said.
Every time Toutatis came close to the sun or the Earth, gravity would tug slightly on the asteroid, changing its spin by a tiny fraction. But over the years, those tugs added up. Once Busch and his collaborators were able to account for these changes, they had a much better model of its spin. And that told them how the asteroid's mass was distributed.
Toutatis is shaped sort of like a lumpy peanut, or from some angles, like a poorly built snowman. It's long and narrow, with two distinct lobes, one smaller than the other.