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 SPACE.com. "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.
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