1. A small body made of ice, dust, and rocks which orbits the sun.
2. A collection of gas, dust, and volatile ice that travel around the sun, generally in very eccentric orbits.

The source of such bodies may be the oort cloud (cloud of comets).

3. A body, probably resembling a "dirty snowball", between 0.1 and 100 kilometers across, which travels through the solar system in an ecliptical orbit of random inclination to the ecliptic.

A comet grows a tail if it goes close enough to the sun.

Large and exceptionally active comet, which in March, 1997, made its closest approach to Earth since 2000 B.C., coming within 190 million kilometers or 118 million miles.

It has a diameter of approximately 40 kilometers or 25 miles and an extensive gas coma. Hale-Bopp has three tails: one consisting of dust particles, one of charged particles, and a third of sodium particles.

Comet Hale-Bopp was discovered in July, 1995, by two amateur U.S. astronomers, Alan Hale and Thomas Bopp.

The comet that orbits the sun about every 76 years, named for Edmond Halley, who calculated its orbit.

It is the brightest and most conspicuous of the periodic comets and recorded sightings go back more than 2,000 years.

It travels around the sun in the opposite direction to that of the planets and its orbit is inclined at almost 20° to the main plane of the solar system and ranges between the orbits of Venus and Neptune.

Halley's comet is expected to reappear in 2061.