Dark Comet Dangers: A strange comet among the Asteroids?
January 7th's announcement that the LINEAR telescope had spotted a new periodic comet wasn't all that interesting: a 20th-magnitude blip out in the asteroid belt in a benign orbit that wouldn't come anywhere near Earth. Designated P/2010 A2 (LINEAR) by the IAU's Minor Planet Center, it was just another notch on the finderscope for this discovery machine near Socorro, New Mexico, which has chalked up 77 periodic comets (and a couple hundred one-timers) since coming online in 1998. But as other observers chipped in positions over the next week, it became clear that this was an object worth watching. For one thing, the now-precise orbit was looking less like a comet's and more like an asteroid's. And images of the interloper showed a tail growing in length yet without a clearly defined head. The online chatter got more animated — just what was this, anyway?
On January 14th, Javier Licandro and others used the Nordic Optical Telescope in the Canary Islands to get a better view, and they discovered something completely unexpected: a small asteroid lay 2 arcseconds to P/2010 A2's east and was moving along with it. Moreover, the "comet" showed no central condensation and looked more like a narrow dust swarm about 110,000 miles (177,000 km) long. Licandro quickly enlisted the biggest aperture in the island's observatory complex: the Gran Telescopio Canarias. Dozens of images taken three days ago using its immense 34-foot (10.4-m) aperture confirm that the "comet" is being shadowed. It's hard not to conclude that we are watching the aftermath of a collision in the asteroid belt. But it's still too early to know for sure. Licandro and his colleagues are analyzing the GTC images carefully — and they hope to make them public soon. Meanwhile, comet specialists are hoping to observe the strange goings-on with both the Hubble and Spitzer space telescopes. Neither has been given the green light yet, but if/when that happens the observations would be made within the next few days. According to Caltech astronomer William Reach, Spitzer no longer has the ability to look deep in the infrared, but it can still record at 3.6 and 4.5 microns, where the cometary gases carbon monoxide and carbon dioxide have strong emissions. -Sky and Telescope