First near-Earth triple asteroid found

Contributed by
Feb 14, 2008
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Another cool bit of astronomy news: astronomers have discovered the first triple asteroid known to get near the Earth. Asteroid 2001 SN263 was thought to be a typical single body that has an orbit that sometimes brings it near us. This made it a target for mapping using the giant 300 meter Arecibo radio telescope in Puerto Rico. They can actually get three-dimensional maps of asteroid using the dish. By sending pulses of what is essentially radar at an asteroid and timing how long it takes the pulses to reach the asteroid and return to Earth, a rough map of the rock can be made -- a pulse that takes slightly longer to get there and back means you've bounced it off a part of the asteroid that is slightly farther away. Think of the astronomers as bats, the telescope as the bat's sonar, and the asteroid as a mosquito, and you'll get the idea.

When astronomers took a shot at SN263, they got a big surprise: they got three separate returns, indicating that a big rock is being orbited by two smaller ones. The main mass is roughly spherical and 2 kilometers across. The smaller moons are about half that size. The observations were made when SN263 was about 11 million kilometers from Earth.

Other triplet asteroids are known (like 87 Sylvia), but this is the first one known that gets near Earth. This is a pretty interesting find. The orbits of the moonlets will allow the densities of the objects to be determined, which can yield insight into the formation mechanism. One possible formation explanation is a slow collision with another asteroid causing the main body to fracture. Or did the system form all at once? The densities of the three bodies can support or rule out different mechanisms (for example, a very low density main mass is hard to fracture).

Plus, the stability of the system needs to be understood; if the orbits are unstable, that means it formed this way recently (well, astronomically recently). Did this object form in the asteroid belt and get sent inward by some gravitational encounter? Or did it form in its current orbit?

All of these questions are interesting scientifically, but they have real-world implications as well. The more we know about near-Earth asteroids, the better! If you disagree, I suggest going to your local natural history museum and checking out the dinosaur bones. Look around. See how all the dinosaurs on display are dead, and no live ones are around?

Yeah. They didn't know anything about asteroids. But we do, and we need to learn more.