Comets are basically dusty snowballs which orbit the Sun. They are a combination of ices, such as water, carbon dioxide, ammonia and methane, mixed with dust. These materials originated from the time that our solar system was formed. Comets have an icy nucleus surrounded by a large cloud of gas and dust (called the coma). The coma is created as the ice in the nucleus is warmed by the Sun and vaporizes. Comets can develop 2 tails as they travel closer to the Sun, a straight gas tail and a curved dust tail. The gas tail is created by the solar wind, whose magnetic fields pull the gas away from the comet's coma. The dust in the coma is not affected by magnetic fields but is vaporized by the Sun's heat, and forms a curved tail due to the comet's orbit.

The dust grains in comets absorb light from the Sun. This causes the dust grains to heat up and glow in the infrared. The infrared emission from comets can be used to get information on the nature of the dust they contain as well as on the rate at which material is being lost from the comet nucleus. Infrared studies can also tell us about the size of the dust grains in comets. Using IRAS (the Infrared Astronomical Satellite), astronomers discovered that dust from comets fills the Solar System and that comets are dustier than they were thought to be. Many of the meteors which are seen as they streak through our atmosphere are the larger pieces of this comet dust.

Comet Iras-Araki-Alcock in the infrared

Since comets contain primeval material surviving from the time of solar system formation, they offer an intriguing target for the Spitzer Space Telescope. The observatory will examine comets at large distances from the Sun. Studies will include the structure and composition of cometary dust and ices and their comparison with similar dust grains and ice particles found in other regions of the galaxy. In 2003, comet Encke will pass within 0.2 Astronomical Units (1/5th the distance between the Earth and the Sun) of Spitzer. This will provide a unique opportunity to gather infrared data that can be combined with Earth-based observations and data from the CONTOUR spacecraft - a NASA mission specifically designed to study comets.