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Comet or Asteroid?

by David Jewitt

The arrows point to the tail of 133P/Elst-Pizarro that extends about 200,000 miles, comparable to the distance between Earth and the Moon. Other objects in the image are stars and galaxies smeared by the motion of the telescope while tracking 133P.

The distinction between comets and asteroids at first appears clear-cut. Comets are ice-rich bodies that evaporate in the heat of the Sun. They show distinctive, expanding atmospheres and tails, and vary wildly in brightness as they zip through the inner solar system. On the other hand, asteroids are just rocks. Most orbit the Sun in the asteroid belt between Mars and Jupiter.

This clear difference between the comets and the asteroids turns out to be, at least in some cases, an illusion. IfA graduate student Henry Hsieh, working with SIRTF Fellow Yan Fernández and me, has uncovered a startling object that has characteristics of both asteroids and comets. The object is known as 133P/Elst-Pizarro, after its two discoverers. It orbits in the asteroid belt between Mars and Jupiter, and appears to be a member of a family of asteroids having similar orbits. In fact, it was first known only as asteroid number 7968, reflecting its lack of cometlike characteristics. However, images taken with the University of Hawaii 2.2-meter (88-inch) telescope on Mauna Kea reveal a spectacular, if skinny, tail of dust extending behind the object in the plane of its orbit. Object 133P is clearly ejecting matter like a comet, even though its orbit is recognizably asteroidlike. Observations in late 2002 showed that the tail persisted for months, and must have been composed of tiny (0.01 of a millimeter) dust grains ejected over a long period.

So what is it? Hsieh and collaborators have explored several possibilities. 133P could be what it appears to be, a comet. In this case, its asteroidlike orbit must have been produced in an unusual way. Dynamical simulations by other scientists suggest that it is very difficult for the large, elongated orbit of a comet to be changed into the more nearly circular orbit of 133P. Forces in addition to gravity, perhaps due to an asymmetric ejection of mass from the comet nucleus, may have triggered this transformation. Another possibility is that 133P is a true asteroid, as its orbit suggests, but one in which ice has for some reason been exposed on the surface. Perhaps a recent collision with another asteroid has revealed a patch of ice that continues to sublimate in sunlight.

Scientific interest in 133P lies partly in the ambiguity of its true nature, but also in the possible implications for the comets and asteroids. If unusual dynamics can propel comets into asteroidlike orbits, should we expect to find other comets lurking in the asteroid belt? Where are they? If some asteroids really contain near-surface ice, then what is the fundamental difference between the comets and asteroids? Object 133P could be telling us that the difference between comets and asteroids is only one of degree. Rocky (and metallic) asteroids that formed close to the Sun accumulated matter at high temperatures that precluded the inclusion of water ice. Objects far from the Sun formed at low temperatures and incorporated water ice. These are the comets, the dirty snowballs (or icy dirtballs?) familiar to us through the dramatic appearances of comets like Halley, Hale-Bopp, and Hyakutake. In between, small amounts of ice were incorporated into otherwise rocky asteroids. For the most part, this ice remains buried and hidden from view. Sometimes, as in the (so far) unique case of 133P, ice near the surface is heated and produces a dusty trail.

So, what at first seemed obvious turns out to be a real puzzle. Is object 133P an asteroid that turned on, or a comet that got lost, or perhaps something even stranger that we have yet to figure out?

For more information, see http://www.ifa.hawaii.edu/~hsieh/ast_elstpiz.html.