BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Comet Observing

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in February, 1994.]

To the ancients comets were "hairy stars" (from which the name "comet" derives) and were considered bad omens. We now know that they are debris from the formation of the solar system. Analysis of comets reveals the composition of the original solar nebula. The source of comets is the Oort Cloud (named for Danish astronomer Jan Oort), a spherical shell of some 10^12 to 10^13 comets orbiting the Sun at a distance of 50,000 AU (astronomical units; the distance from the Sun to the Earth), about 1000 times the farthest distance of the planet Pluto. From time to time a comet is diverted out of the Oort cloud by close passage of another body, and it travels into the Solar System, perhaps to become an observed comet. If the comet's orbit is further perturbed by passing close to a planet (Jupiter is usually the culprit), it can become a "short period comet" like Comet Enke, which has an orbital period of 3.3 years. Otherwise it remains a "long period comet" whose orbit may extend almost all the way back out to the Oort cloud, like Comet Kohoutek, which has an estimated period of 80,000 years.

Comets are composed of a conglomeration of rock, dust and ices, including water and methane ice. The prevailing model of comets for many years has been Fred Whipple's "dirty snowball" model, but that has recently been challenged by observations suggesting that comets may be more closely related to asteroids than previously thought. In any case, as a comet approaches the Sun, the volatile ices begin to evaporate. The resulting gas, combined with dust from the comet, form the "coma" that we observe. The coma usually begins to appear when the comet is about 3 AU from the Sun. The body of the comet itself (referred to as the "nucleus") may only be 0.1 to 10 km in diameter, while the coma can expand to 10^5 to 10^6 km. The evaporation of ice from the comet can also produce forces that alter the orbit of the comet. Consequently, comet orbits are notoriously difficult to predict accurately.

As the comet gets closer to the Sun, the ejected material may form tails. Often there are two separate tails: The gas or "ion" tail projects from the comet directly away from the Sun, regardless of the true motion of the comet. The gas is driven by the solar wind and more or less ignores the comet's orbital motion. The dust tail, on the other hand, lies approximately along the orbit of the comet, because the particles of dust respond more to gravitational effects than to the solar wind. Often the tails are distinctly separate, and are usually different colors.

For the amateur astronomer there are essentially two major observational "events" associated with comet observing: "recovery" of known comets, and "discovery" of new comets. The predicted return of a known comet is often inaccurate, and being the first to observe a returning periodic comet is still a significant achievement. Over 600 periodic comets are known. Although only a handful attain naked-eye brightness, many become interesting telescopic objects and can be located with the help of positional information published in magazines and ephemerides.

Discovering a comet is still a very real possibility for the amateur, but significant dedication and perseverance are required. Many comets are now discovered by professionals using photographic techniques, but about half are still discovered by amateurs; on average about five per year. Since a newly discovered comet is named for the first person to observe it, there is substantial motivation for comet hunting. Only about 300 to 400 people have discovered comets in all of history, and only 150 or so have found more than one. The record is still held by Jean Louis Pons, the doorkeeper at the Marseille observatory (I don't know if this makes him a professional?), who discovered 37 comets between 1801 and 1837. More recently, George Alcok in England, Kaoru Ikeya in Japan, and William Bradfield in Australia (among others) have achieved notoriety for their comet discoveries. On average it takes about 300 hours of observing, with good technique, to discover a comet.

Comet hunters typically use telescopes in the 6-inch to 12-inch aperture range, and employ very low magnification. The best technique is to sweep consistent areas of the sky, usually in the early morning. It is important that the observer become familiar with all the non-stellar objects in their sweeping area, lest they be mistaken for comets. Charles Messier was a comet hunter who compiled the Messier Catalog of "M" objects to help others avoid misidentification. If you think you have found a new comet, you must report the find immediately by telegram to the Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. Good hunting!