Secret of Twin Asteroid Birth Revealed

Asteroids are changeable worlds that can split into pieces, creating two smaller space rocks with separate paths around the sun, a new study finds. The process can happen non-destructively — just add sunlight and lots of time.

The space rock discovery comes from an analysis of 35 so-called "divorced asteroid pairs" by an international team of astronomers. First discovered only two years ago, divorced asteroid pairs are space rocks that take similar — but separate — paths around the sun, and have come very close together at some point in the last million years. Their origins remained a mystery, until now.

"Asteroids aren't just static boulders floating around in space," said study co-author Daniel Scheeres of the University of Colorado, Boulder. "They're constantly evolving over time."

The research team, led by astronomer led by Petr Pravec of the Astronomical Institute in the Czech Republic, used several telescopes around the world to make the asteroid find. They determined the sizes of these asteroids by measuring their relative brightness, and studied the spin rate of each pair with a technique known as photometry.

"It was clear to us then that just computing orbits of the paired asteroids was not sufficient to understand their origin," Pravec said in statement. "We had to study the properties of the bodies." [Photos of asteroids in space.]

The research is detailed in the Aug. 26 issue of the journal Nature.

Twin asteroid birth predicted

The asteroids scrutinized in the study were all on the small side, averaging less than 6 miles (10 km) wide. Researchers found that all of the asteroid pairs analyzed shared a specific size relationship: The smaller one was always less than 60 percent as big as its companion. These measurements fit precisely with a theory Scheeres developed in 2007, which postulated a way that divorced asteroid pairs could form.

Scheeres' theory addressed the nature and destiny of "binary asteroid pairs" — asteroids that orbit each other as they zoom around the sun. Unlike "divorced pairs," the two asteroids share an overall path, orbiting together.

Binary asteroid pairs are fairly common in the solar system. One way they can form, astronomers think, is via some long-term solar heating. If an asteroid is small — less than 6 miles or so in diameter — the sun can help break it apart. Solar radiation blasting one side can cause the space rock to spin faster and faster over millions of years.

"When sunlight shines on asteroids, it can spin them up like a propeller," Scheeres told SPACE.com.

Most known asteroids in the solar system are concentrated in a region between the orbits of Mars and Jupiter called the asteroid belt, which is about 200 million miles from the sun. But some also extend into the inner solar system as near-Earth asteroids. Astronomers estimate there are 1 million asteroids larger than 0.6 miles (1 km) wide, in the solar system. NASA's infrared WISE space telescope has discovered more than 25,000 previously unknown asteroids in the last six months.

More observations needed

Many asteroids are thought to be "rubble piles," rocky bits and pieces held together by each other's tenuous gravity. If the solar-induced spin gets fast enough, a chunk on an asteroid's end can split off.

In binary asteroid pairs, the theory goes, this chunk sticks with the bigger, "parent" asteroid, and the two rotate around each other. But Scheeres' calculations predicted that the "baby" can break free if it's less than 60 percent as big as the parent. The result would be two space rocks that take slightly different paths around the sun — a divorced asteroid pair.

The new study's findings confirmed that theory and should help astronomers understand how asteroids form and develop — and give them further confidence that many of their theories and models represent reality.

"It's one thing to do the math and predict these things," Scheeres said. "It's another to actually go out and observe them."

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