Sunlight Splits Asteroids into Pairs
The main belt is between the orbits of Mars and Jupiter, and contains countless asteroids.
Credit: diagram — Minor Planet Center; image — NASA/Johns Hopkins University Applied Physics Laboratory

Asteroids often come in pairs, with the two objects spinning around each other. Now scientists say sunlight could be the cause of these binary boulders.

A new study suggests energy from the sun can spin up a single asteroid until it ejects material that becomes a separate satellite.

Astronomers first discovered these strange asteroid pairs 15 years ago, and have been puzzled about what causes them. Now scientists have created a computer model that matches what they see.

"So far our results match the properties of binary asteroids quite well," said astronomer Kevin Walsh of the Observatoire de la Cote D'Azur in Nice, France. Walsh led the study when he was a graduate student at the University of Maryland, working with his advisor Derek Richardson and Patrick Michel of the Cassiope?e, University of Nice-Sophia Antipolis, in Nice.

Lots of them

Binary asteroid systems are surprisingly common ? they seem to make up about 15 percent of near-Earth asteroids, or those that come near our planet's orbital path around the sun. (Most asteroids orbit in a belt between Mars and Jupiter and are too far from Earth for detailed measurements).

In order to probe how these binary setups are spawned, the astronomers modeled asteroids as "rubble piles," or chunks of rock held together by gravity, and watched what happened over time. The thinking is that many asteroids are not solid objects but rather loose conglomerations prone to rearrangement and pulling apart.

When sunlight hits one of these piles, the material absorbs some of the radiation and then re-emits it at a slightly different angle, giving itself a small push in angular momentum, Walsh said. This change can cause the asteroid to spin up slightly in what's called the YORP effect (named for the scientists who discovered it: Yarkovsky-O'Keefe-Radzievskii-Paddack).

The YORP effect is also known to slow down the spin of some asteroids and tweak the orbits of others, sending them flying in slightly different directions, and occasionally toward another planet, such as Earth.

Over time, as an asteroid spins up, its shape will become more spherical with an increasingly circular equator where mass begins to build up. Eventually, if the rock spins fast enough, the force outward from the rotation will overpower the gravitational pull inward and material will eject along the equator to form a new small satellite. The two asteroids then circle each other in binary pairs.

"Ours was the first model that could actually simulate an asteroid being spun up to its maximum spin rate by the YORP effects," Walsh told "We made the first step and showed YORP can make systems that look exactly like what we observe."

Not the only way

The team compared their model's predictions to observations of a binary asteroid called KW4 made by astronomers using the Arecibo radio telescope in Puerto Rico.

"It's by far the best observation we have of any single asteroid," Walsh said. "Globally, observations suggest that nearly all the small near-Earth binary systems are similar to this KW4 system. We recreate its shape pretty closely with a lot of our simulations."

Though the scientists say the YORP effect isn't the only way to create binary systems ? they could also be formed by impacts or by tidal forces from passing another large body ? they think the process they've modeled accounts for most of the known asteroid pairs.