In the early spring of 2009, Comet Lulin appeared in the evening skies. If the sky was dark enough, this “Green Comet” could be seen without the assistance of either binoculars or telescope. Through binoculars, it appeared like a dim amorphous blob with a greenish hue. A telescope with a high-powered lens could pick up not only the comet’s head but also its two tails. Comets have always fascinated those on Earth who are fortunate to be able to see them. Streaking across the sky as they do, comets were called “stars with hair” by the Greek philosopher Aristotle. What are comets?

The nucleus of a comet is the solid part of the head. Around that nucleus and surrounding the tail is the coma, a halo of debris. Seen with the naked eye, it may seem that the comet has only one tail as it nears our planet and the sun. Actually, every comet has two tails. One of the tails is composed of gaseous vapors and points away from the sun. The other tail is composed of dust particles and ice crystals. This tail curves to correspond with the comet’s elliptical orbit.

Scientists have attempted to discover the composition of these “stars with hair”. In July 2005, they made a spacecraft impact the Tempel 1 comet hard enough for debris to be knocked off. Infrared photography showed some of the debris from the nucleus to be made of clay minerals as well as carbonates like that found in seashells. They also found silicates in crystal formations. The carbonates and clay minerals form when water is present. The silicates form when silicon heats to tremendous temperatures and then cools. The ice crystals in the tail and coma were extremely tiny, much less than a sixteenth of an inch in size. The Tempel 1 comet extended five miles with a width of three miles but most of that, about three quarters of the area, was empty space.

In January 2006, dust samples from Comet 81P/Wild 2 were analyzed. Some of the materials discovered were iron-nickel sulfide, chromium, and green olivine rock rich in magnesium and iron. There were no organic materials found.

A study of high-resolution spectrographs of Comet Boattini in 2008 showed materials scientists have seen in other comets: ice crystals, ethane, methane, methanol, formaldehyde, hydrogen cyanide, carbon monoxide, and C2H2 (a zinc finger family protein). Scientists are still attempting to analyze the composition of these celestial wonders.