Spotting a comet near Earth is rare enough, but an unexpected event like a flare-up is even rarer - and more spectacular. When comet Holmes suddenly flashed brightly in the sky the night of October 23, scientists at the Hopkins Applied Physics Laboratory led by Carey Lisse jumped on the opportunity to learn something about it.
Holmes is a bow tie-shaped comet consisting of thick dust layers wrapped around an icy core. The dust cloud most likely arose from a series of outbursts of rocky material from the core. These outbursts are often triggered when the comet's orbit passes close to the Sun, which partially melts its core.
Each dust particle in the cloud reflects light in a different direction. The end result is a scattering of sunlight that makes the comet look like a disco ball. Each outburst causes a temporary increase in the brightness of the comet as dust particles are ejected and settle back into the cloud.
When Holmes was discovered eight years ago, it had almost no detectable dust cloud. Over time the dust cloud has grown, a process easily observable to astronomers by measuring the brightness of light reflected off the comet's surface and dust cloud.
Researchers at APL used the Hubble Space Telescope to take a new set of images of the comet, in hopes of measuring its size and geological activity. Eight years ago Holmes was about 2.1 miles in diameter, around the size of New York's Central Park. Repeated eruptions from the core have probably decreased the size of the comet.
