Space and Time
Comet Borrelly as seen by Deep Space One
Comets are dirty-ice leftovers from the formation of our solar system around 4.6 billion years ago. They are among the least-changed objects in our solar system. As such, they may tell us important clues about the formation of our solar system. We can predict the orbits of many of them, but not all.
Around a dozen 'new' comets are discovered each year. Short-period comets are more predictable because they take less than 200 years to orbit the Sun. Most come from a region of icy bodies beyond the orbit of Neptune. These icy bodies are variously called Kuiper Belt Objects, Edgeworth-Kuiper Belt Objects, or trans-Neptunian objects.
Each comet has only a tiny solid part, called a nucleus, often no bigger than a few kilometres across. The nucleus contains icy chunks and frozen gases with bits of embedded rock and dust. At its centre, the nucleus may have a small, rocky core. As a comet nears the Sun, it begins to warm up. The comet gets bright enough to see from Earth while its atmosphere - the coma - grows larger. The Sun's heat causes ice on the comet's surface to change to gases, which fluoresce (glow) like a neon sign. 'Vents' on the Sun-warmed side may release fountains of dust and gas for tens of thousands of kilometres. The escaping material forms a coma that may be hundreds of thousands of kilometres in diameter.
Comet Halley, seen on its last visit to our skies in 1986
The pressure of sunlight and the flow of electrically charged particles, called the solar wind, blow the coma materials away from the Sun, forming the comet's long, bright tails. These are often seen separately as straight tails of electrically charged ions and an arching tail of dust. The tails of a comet always point away from the Sun.
Most comets travel a safe distance from the Sun itself. Comet Halley comes no closer than 89 million kilometres from the Sun, which is closer to the Sun than Earth is. However, some comets, called sun-grazers, crash straight into the Sun or get so close that they break up and vaporize.
Impacts from comets played a major role in the evolution of the Earth, primarily during its early history billions of years ago. Some believe that they brought water and a variety of organic molecules to Earth.
Unlike the other small bodies in the solar system, comets have been known since ancient times. There are Chinese records of Comet Halley going back to at least 240 BC. The famous Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an appearance of Comet Halley. The arrival of a comet was often seen as a bad omen in days gone by. Today, we understand them more and can appreciate them for their fleeting beauty.
Images, information and videos courtesy of NASA.