Guest Author - Barbara Melville

Comets are essentially leftovers from the solar system’s formation, originating from the Oort Cloud and Kuiper Belt. Despite such a benign-sounding description, comets make for interesting study and viewing, with a fascination leading back to antiquity. Halley’s comet is historically the most significant, observed in ancient China and portrayed in the Bayeux Tapestry.

Comets are small objects that reflect light from the Sun. They range from a few kilometers to tens of kilometers in diameter, and consist of a nucleus (center) of ice, rock, dust and gases. When close to the Sun, the nucleus becomes surrounded by a water and gas cloud called the “coma”. The nucleus and coma are sometimes referred to as the “head” of the comet, however, when not near the Sun, the nucleus is the comet.

Like the coma, the tails form following sublimation, where a solid becomes a gas without entering a liquid state. Put simply, when a comet approaches the Sun, its solids vaporise into gases, which then stream out to create what we perceive as the tail. These tails can be huge – several million kilometers in length – and always face away from the Sun due to the affect of solar winds.

But comets don’t keep their beautiful tails forever. After several hundred passes by the Sun, comets will lose most (if not all) of their icy, gaseous materials. At this stage they may shatter, but some keep going. In his article Comets on the Nine Planets website, author Bill Arnett suggests that some of what we consider to be asteroids may have previously been comets.

Are comets a threat to us? Potentially, however, there are currently no Near Earth Objects to be concerned about, and astronomers are learning more about comets every day. The NASA Stardust program, for example, designed a craft to photograph and gather samples from comet Wild 2. The ESA Rosetta craft is currently on a similar mission to comet Churyumov-Gerasimenko.

For more information, NASA’s Lunar and Planetary science site has an excellent section on comets, including images, mission details and FAQs.

Sources

Comet, World Book at NASA, NASA website, accessed: 28th December 2008, author: Yeomans, D.K.

Comets, Nine Planets website, accessed: 28th December 2008, author: Arnett, B.

Stardust, NASA’s Jet Propulsion Laboratory website, accessed: 28th December 2008, author: Sorensen, G.

Rosetta, ESA website, accessed: 28th December 2008, no authors specified.