Perhaps like no other objects that can be observed in the night sky, comets captivate our imaginations. The comets chronicled throughout recorded history have evoked our fears and dreams by the light of their mysterious beauty. Incidentally, comets were once completely obscured in a veil of myth and superstition; it is only within the past 250 years or so that science has revealed factual truth about these heavenly messengers.

In 1705, Edmund Halley calculated the orbit of a comet which was later named in his honour, using Isaac Newton's newly formulated laws of motion. Halley proposed that the comets of 1531, 1607, and 1682 were the same object, and came up with a mathematical model that described the comet's orbit. In 1758, when the comet appeared again, Halley and Newton's work was verified by observation. Nearly overnight, the evil omen myths borne from our geocentric misconceptions were being dispelled by hard scientific evidence.

During the most recent apparition of Halley's Comet in 1986, scientists from different countries sent an armada of spacecraft to photograph and study the comet. What we discovered did nothing less than revolutionize our way of thinking. It became clear that Comet Halley contained all the organic ingredients necessary to sustain life, including carbon, hydrogen, oxygen, and nitrogen.

Interestingly, the comet's organic molecular proportions were virtually identical to the organic matter found in humans. One of the most intriguing theories in science today deals with the role of comets in the early Solar System and their relation to the beginnings of life on Earth. Today, research continues with several missions to comets currently in progress or in the planning stages.

Equally as fascinating as the origins of life in our Solar System is the suggestion that a comet several miles in diameter may have been responsible for the impact off the coast of Mexico's Yucatan Peninsula, some 65 million years ago. The consensus is that this impact marked the end of the Cretaceous era as well as the reign of the dinosaurs. We cannot be absolutely certain this object was a comet rather than an asteroid. However, in analyzing the size and population statistics of asteroids and comets, the data seems to favour that the intruder was, with some degree of certainty, a comet. Extinction level events caused by such impacts have occurred on Earth periodically over large time scales, and chances are this will likely occur again, sometime in the future. This may be a sufficient reason in itself to consider amateur observations useful!

Composition

Comets contain four general components:

1. A solid nucleus of ices, organic molecules, and dust
2. A coma of ionized gases and dust around the nucleus with varying condensation
3. A dust tail
4. An ion tail

Observation

Observing comets in many ways is unique to other aspects of amateur astronomy, as they display physical characteristics very different from asteroids. When cometary ices interact with solar radiation as a comet approaches perihelion in its orbit about the Sun, the frozen gases sublimate (bypass their liquid state) through jets in the nucleus, quite often unexpectedly. Since these out-gassings are highly unpredictable due to a number of factors, such as the solar wind and the varying composition of comets, amateur observations in some cases can be valuable and will contribute to the advancement of cometary astrophysics.

The Oort Cloud

Jan Oort understood from Kepler's Second Law that comets spend most of their orbital time period near their greatest distance from the Sun, due to their highly elliptical orbits. Kepler's Law requires that an object in an elliptical orbit has to move slower around aphelion or greatest distance from its parent body. Moreover, Oort noted that most comets have not repeated their orbits within recent human history. These particular comets tended to approach the Sun from any direction, which suggested their unique location differing from the disc-shaped orbital plane of the Sun and planets. It became reasonable to assume the presence of countless other comets within this hypothetical population, for every one of these comets was observed travelling through the inner Solar System. Thus it is now widely accepted that a spherical halo of comet nuclei enshrouds our Solar System at a distance of 50,000 to 150,000 astronomical units. This extremely remote population of comets is known as the Oort Cloud, and it contains an estimated 100 billion comet nuclei that originated in the primordial solar nebula, along with the planets, some 4.6 billion years ago. The Oort Cloud comets were probably a result of gravitational interactions, mainly with Jupiter and the Sun, that expelled them into the frozen depths of space, far beyond the planets, near the edge of the Solar System. The Oort Cloud cannot be directly observed, even by the most powerful telescopes, due to the small size of the component objects and their very low light reflecting properties.

The Kuiper Belt

Gerald Kuiper hypothesized a second concentration of comets, residing approximately in the plane of our Solar System, just beyond the orbits of Neptune and Pluto at distance of 30 to100 astronomical units. This population of comets is known as the Kuiper Belt. Although small objects at 30 to 100 astronomical units are generally too faint to be observed directly, recent observations by the ten-metre Keck Telescope at Mauna Kea, Hawaii, have produced images of the Kuiper Belt's largest and nearest icy bodies, at a distance of 35 to 45 astronomical units. It is interesting to note that Pluto and its moon Charon are two objects that originated in the Kuiper Belt.