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.

Topsy-Turvy

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. Busch's analysis found that the asteroid's shape isn't the only thing that's lumpy; its mass is also distributed in a lumpy fashion rather than evenly spread throughout the asteroid. "It may have a quite complicated internal structure," he said.

Inner Workings

It's possible the asteroid got its internal structure when a smaller body smashed into it, throwing material off. "That can fracture it, just like hitting a surface with a hammer," Busch said.

Toutatis could also have been created when two objects collided and stuck.

Or, it could have been created through the YORP effect, which explains how nothing but sunlight can cause an asteroid to start spinning. "Wind blowing on a propeller makes the propeller spin," said Dan Scheeres, an astrodynamics professor at the University of Colorado, Boulder, who's been studying Toutatis for almost two decades. And that's similar to what happens when photons hit an uneven body, making that body spin faster, he added. Scheeres thinks that a faster-spinning Toutatis could have eventually spun so fast that it began throwing off material into space, becoming two separate bodies. These bodies would have eventually slowed down and recombined, starting the process over again.

It's still just a theory, Scheeres said, but the model explains how Toutatis and other so-called contact binaries could exist.

Scheeres and Busch will be analyzing their new observations of Toutatis as it gets closer to Earth over the next week; Busch plans to refine his model of the asteroid's spin after seeing the new radar images.

But astronomers aren't the only ones who will be able to see Toutatis on its flyby. Backyard hobbyists can get in on the fun too. At its closest approach, Toutatis will be "too faint to see with the naked eye, but well within the range of a small backyard telescope," Busch said.

And no, it's not going to hit Earth, so doomsday theorists, relax.

"There may be more nonsense circulating about Toutatis than many other objects, but that's because it's a large object that makes repeated close flybys of Earth and has an interesting shape," he said.