Why Study Comets?
Comets are fascinating bodies that have both thrilled and terrified people through the generations. The phenomena of comets are spectacular, but what they are essential treasure troves that can tell us about the early formation of the solar system.
As the solar system formed, tiny fractions survived and are now observed as asteroids and comets, the only bodies still in existence that preserved solid materials involved in the solar system’s formation over 4.567 billion years ago.
The Origin of Comets:
Comets are thought to have come from the Oort cloud believed to lay roughly 50,000 AU from the Sun. The Oort cloud is believed to be comprised of two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices such as water, ammonia and methane. Astronomers believe that the matter comprising the Oort cloud formed closer to the Sun, and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System’s evolution.
A Comets Journey:
As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since about half of their material is ice. When a comet approaches within a few Astronomical Units (1 AU equals approximately 150 million kilometers) of the Sun, the surface of the nucleus begins to warm, and volatile material on the comet evaporates. The evaporating gases carry small grains with them, forming the comet's coma of gas and dust.
When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce.