Image of Comet Kohoutek taken by a team from the University of Arizona at the Catalina observatory on January 11, 1974. (NASA Copyright Free)
A comet is a small body that orbits the Sun. Because comets are made of frozen ice and gas mixed with dust and rock, they are often described as "dirty snowballs". They range in size from less than a mile to more than 20 miles in diameter. Some, like Halley's Comet, are on regular, predictable, elliptical orbits around the Sun. Others come close to the Sun only once before either getting shot out of the solar system or breaking up.
Below is a fairly typical optical light image of a comet:
Image of Comet Kohoutek taken by a team from the University of Arizona at the Catalina observatory on January 11, 1974. (NASA Copyright Free)
The main features to notice are its bright head and trailing tail. The nucleus is the "dirty snowball" that travels around the Sun. When the nucleus gets close to the Sun, however, Sunlight vaporizes ice in the nucleus, creating a bright coma, which seen above as the bright "head" of the comet.
Also, as the comet approaches the Sun, it develops two tails. One is a dusty tail from dust carried off by escaping gases. This dusty tail always points opposite the direction of the comet's travel, and shines due to reflected Sunlight. The second tail is an ion tail, and appears bluish. This ion tail always points away from the Sun, because it is ultimately caused by the solar wind. (The ion tail is not resolved in the above image.)
On March 27, 1996, the ROSAT X-ray observatory turned its telescope on Comet Hyakutake. Researchers had reason to believe that X-rays may be produced by a comet, but they didn't realize how strong the signal would be. In fact, the X-ray emission was about 100 times stronger than even the most optimistic estimates!
ROSAT X-ray image of Comet Hyakutake from March 26, 1996. This is the first image of X-rays from a comet.
In astronomy, X-rays are usually associated with very, very hot regions like the Sun or supernova remnants. So, how can we see X-rays from comets — big, dirty, snowballs? One thing to notice in the above ROSAT image is that the X-rays are concentrated on the Sunward side of the comet's nucleus. It may not be a big surprise, then, that the X-rays are in some way related to the solar wind.
A small fraction of the solar wind is made up of ions — atoms that have either gained or lost electrons leaving them with an electric charge. When these particles fly past a comet, some of them interact with the comet material. A neutral atom (one with no electric charge) in the comet might find that an electron has suddenly jumped ship to an ionized atom in the passing solar wind. When this happens, the newly ionized atom will emit an X-ray in an attempt to minimize its energy. These are the X-rays that we see. In July 1999, Chandra looked at Comet LINEAR, and found strong evidence supporting this theory.
Physicists at Lawrence Livermore National Laboratory recreated conditions similar to those experienced by a comet interacting with the solar wind. Their results bolster the case for the above described X-ray emission mechanism. By understanding this emission process, scientists can start to better understand the solar wind.
