Third time's a charm
The granddaddy of the new comet class is a body named Elst-Pizarro whose double life mystified astronomers. Elst-Pizarro, found in 1979, was first classified as an asteroid; it's number 7968. Then, in 1996, discoverers Eric Elst and Guido Pizarro using the European Southern Observatory's 1m telescope in La Silla, Chile, found it emitting dust like a comet. Thus began asteroid 7968's "dual citizenship" as Comet 133P/Elst-Pizarro.
Although Elst-Pizarro's puzzling activity disappeared between 1997 and 2001, Hsieh and Jewitt saw dust emission in 2002. Since 2003, Elst-Pizarro has appeared steadfastly asteroidal.
The second main-belt comet identified is Comet P/2005 U1, discovered in October 2005 by Michael Read with the Spacewatch 0.9m telescope on Kitt Peak, Arizona. There's no doubt the object is a comet, but its orbit lies squarely in the main asteroid belt.
Then, a month later, Hsieh and Jewitt witnessed asteroid 118401 emitting dust.
"The discovery of the other main-belt comets shows that 133P/Elst-Pizarro is not alone in the asteroid belt," Jewitt says. "Therefore, it is probably an ordinary although icy asteroid and not a comet from the outer solar system that has somehow had its comet-like orbit transformed into an asteroid-like one. This means that other asteroids could have ice as well." The study was published online
by ScienceExpress March 23 and is scheduled to appear in the journal Science
"If there were only one, it could be a fluke, but they've got three," says Brian Marsden, director of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "There's support in numbers, isn't there? Clearly, the whole thing has to be investigated further."
Traditionally, astronomers envisioned asteroids as marching through the asteroid belt on nearly circular orbits, while comets move every which way, taking eccentric paths often tilted steeply to the plane in which the planets move. "The line between comet and asteroid is no longer clearly drawn," says Donald Yeomans, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "The old rules probably don't apply."