What are comets???

Comets are balls of dirty ice from the outer solar system that follow elliptical orbits with high eccentricities, so that they are near to the Sun for only a small portion of their lives. As a comet comes near to the Sun at perihelion, the outer layers heat up and turn to gas, causing a coma (halo) and a tail to form. Very close
to the Sun, the tail of a comet splits into two pieces, an ion or plasma tail and a dust tail. While both tails point away from the Sun, the dust tail curves ‘‘back’’ along the orbit, while the plasma tail is swept straight away from the Sun by the solar wind.

These tails can be as long as 1 AU, making comets the largest objects in the solar system. However, comet tails are extremely diffuse; comet tails are more perfect vacuums than any we can make on Earth. The entire mass of a comet is less than 1 billionth the mass of the Earth.The nucleus of a comet is a few kilometers across, and contains lots of water ice and carbon dioxide ice. This nucleus is surrounded by the coma—this is the‘‘head’’ of the comet. The coma can be over 1 millionkm across. The coma shines both by reflected sunlight, and by the transitions of excited atoms and molecules in the gas.

Parts of a comet

Comets can be divided into two types—long-period and short-period comets. This distinction is not quite as arbitrary as it sounds, since there are two different reservoirs for comets in the solar system.

1. LONG PERIOD COMET
The long-period comets come from the Oort cloud, a swarm of comets 50,000–100,000 AU from the Sun. These comets
have been in the Oort cloud since the solar system formed, and contain material that has remained the same since before the Sun formed. The Oort cloud is approximately spherical in shape, although there is probably a denser region near the plane of the solar system. An ice ball leaves the Oort cloud to become a comet when a star passes nearby (within 3 light years), and changes the ice ball’s orbit. The passage of a star slows the ice ball, so that it no longer has enough energy to maintain its orbit. These objects fall into long elliptical orbits around the Sun. It is rare for such an event to happen; about 10 stars per million years pass close enough to change the orbits in the Oort cloud. Each star may affect several ice balls, however. There are probably trillions of icy balls in the Oort cloud.

2.SHORT PERIOD COMET
Short-period comets have periods less than 200 years, and originate in the Kuiper belt. The Kuiper belt is located just outside the orbit of Neptune, between about 30 and 50 AU from the Sun. These comets are distributed in a flat ring on the ecliptic. Extrapolating from known Kuiper belt objects indicates that there are probably about 70,000 comets in the Kuiper belt larger than 100km across. Each time a comet passes near the Sun, it sheds some of its mass, which remains in the orbital path. Eventually, the comet disintegrates entirely, unless, of course, it runs into the Sun, a planet, or receives a gravitational ‘‘assist’’ out of the solar
system during one of its orbits.

Kuiper Belt