Visitors From Outer Space
Unpredictable Shining Objects
In the past, people looked with awe and amazement at the shining objects that appeared unpredictably in the sky. When looking at a comet,They imagined a sword blazing across the sky, a sign of war and death: For others, it was a woman with her hair flowing behind her, a sign that the gods were displeased; however, in the present everybody knows what a comet is.
In modern days, scientists have discovered that a comet is composed of an icy nucleus, which is no bigger than a few kilometers. The nucleus contains tiny bits of ice and frozen gases with embedded tiny particles of rock and dust. The nucleus might contain a small rocky core. Comets are leftovers from the interstellar particles that formed the solar system 4.6 billion years ago. It is theorized that comets might have brought water and organic compounds to the inner solar system.
Short-period comets originate in the kuiper belt which is a disk-like belt of icy bodies in the trans-neptunian region. Comets from this region are pushed from time to time into orbits which bring them closer to the Sun. Their orbital periods are less than 200 years, and occasionally their appearances can be predicted. One of the best known short period comets, with an orbital period of 76 years, is comet Halley which last appeared in 1986, and will appear again in 2061.
Long period comets inhabit a region known as the Oort cloud, which lies at approximately 150,000 astronomical units (AU) from the sun. Comets from this region are less predictable and it can take as long as 30 million years before an Oort cloud comet completes one orbit around the Sun.
Glowing Tail MIllions of MIles Long
In the outer solar system, a comet remains in frozen state; however, when the comet comes into the inner solar system, it encounters the rays from the Sun which causes the nucleus of ice to change its frozen state into a gas in a process known as sublimation (the solid ice turns into a gas without becoming a liquid), A sudden flare-up of gas and dust are thrown in every direction, forming a luminous halo around the nucleus known as coma.
The solar wind causes the gas and dust particles in the coma to spread out each of them forming two distinct tails of glowing light, and pointing in slightly opposite directions. Their tails can spread for millions of miles across the night sky.
Many comets are named after their discoverers, for instance, comet Shoemaker-Levy 9 was named after Eugene and Caroline Shoemaker and David Levy. Other comets have been named after the spacecraft that have identified them, including LINEAR; SOHO; WISE, etc.
In 2004, the stardust mission from NASA successfully flew within 236 km (147 miles) from the nucleus of comet Wild 2. Samples of gas and dust were collcted. In 2009, scientists announced that the amino acid glycine was found in the particles of gas and dust collected by stardust mission.