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A Collision in the Asteroid Belt

14 October 2010

Credit: NASA, ESA, and D. Jewitt (UCLA)

NASA's Hubble Space Telescope (HST) has acquired the first images of a probable asteroid collision. The images show a comet-like trail of material, extending from a bizarre X-shaped object.
In January 2010, astronomers began using HST, to track the object for five months. They thought they had spotted a fresh asteroid collision, but were surprised to realize the collision occurred in early 2009.

“We expected the debris field to expand dramatically, like shrapnel flying from a hand grenade," said astronomer David Jewitt of the University of California, who is a leader of the HST observations. "But what happened was quite the opposite. We found that the object is expanding very, very slowly."

The peculiar object, termed P/2010 A2, was found cruising around the asteroid belt, a zone populated by millions of small rocky objects, between the orbits of Mars and Jupiter. It is estimated that collisions of small asteroids occur once a year. When the asteroids collide, they eject dust into interplanetary space. Until now, however, astronomers have only made models, to calculate the frequency of these collisions and the amount of dust emitted.

Detecting colliding asteroids is difficult because large impacts are rare, while small ones, such as the one that produced P/2010 A2, are very faint. The two asteroids involved in the P/2010 A2 event were not observed before the collision, because they were too dim to be noticed. The collision itself was not recorded, due to the asteroids' location relative to the Sun. About 10 or 11 months later, in January 2010, the Lincoln Near-Earth Research (LINEAR) Program Sky Survey discovered the comet-like tail resulting from the collision. However, only sophisticated HST glimpsed the X pattern, yielding clear evidence that something different from the emission of gas from a comet had occurred.

Although the HST images give strong evidence for an asteroid collision, Jewitt says he still does not have enough information to exclude other interpretations for the peculiar object. In one such explanation, a small asteroid's rotation increases, due to solar radiation, and loses mass, giving rise to the comet-like tail.

"These observations are important because we need to know where the dust in the Solar System comes from, and how much of it comes from colliding asteroids as opposed to 'outgassing' comets," Jewitt said. "We also can apply this knowledge to the dusty debris disks around other stars, because these are thought to be produced by collisions between unseen bodies in the disks. Knowing how the dust was produced will yield clues about those invisible bodies."

The HST images, taken in the period between January and May 2010, with an advanced camera aboard the telescope, reveal a point-like object about 100 meters across, with a long, streaming dust tail behind the X pattern. Particles in the tail are estimated to vary from about 1 mm to 3 cm in size.

The 100-meter-wide object in the HST image is the remnant of a slightly larger precursor body. Astronomers think a smaller asteroid, perhaps 3 to 5 meters across, smashed into the larger one. The two probably collided at high speed, about 17,000 km/h, which vaporized the small asteroid and removed material from the larger one.

Jewitt estimates that the impact took place in February or March 2009, and was as powerful as the explosion of a small atomic bomb. Solar radiation then swept the debris behind the remnant asteroid, forming a comet-like tail. The tail contains enough dust to make a ball 20 meters in diameter, most of it flung from the larger body by the explosion.

"Once again, Hubble has revealed unexpected phenomena occurring in our celestial 'back yard," said Eric Smith, Hubble Program scientist at NASA Headquarters. "Though it's often Hubble's deep observations of the universe or beautiful images of glowing nebulae in our Galaxy that make headlines, observations like this of objects in our own Solar System remind us how much exploration we still have to do locally."
Astronomers plan further Hubble observations next year, to examine the object again. Jewitt and his colleagues hope to see how far the dust has been swept back by the Sun's radiation and how the mysterious X-shaped structure has evolved.

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