The Universe and Galaxies: Comets

Discovery Space: Comets

Comets are essentially chunks of dirt and ice. They have changed even less than asteroids since the beginning of the solar system. Therefore, comets can provide valuable information about their origin.

Comets are celestial bodies that are invisible until they near the sun. Some comets have short orbital periods and are thought to originate from the Kuiper belt, beyond the orbit of Neptune.

Comets with orbital periods over 200 years are believed to originate from the Oort cloud that surrounds the solar system—almost out of range of the sun’s gravitational force. These comets are so far out in space that one orbit can last up to 30 million years.

A comet awakens

The core of a comet often measures only a few miles across and is composed of ice, rock, dust, and frozen gases. When the comet penetrates the inner solar system it is warmed by the sun, the volatile components escape, and jets of gas and dust are expelled. A fog-like envelope (the coma) then forms around the core.

The coma glows because it reflects sunlight and its atoms and molecules release radiation. The pressure of sunlight and solar winds loaded with electrically charged particles blow the coma away from the sun, leading to the formation of a comet “tail.” This tail extends as a plasma tail of electrically charged molecules or as a curved dust tail.

The end of a comet

Each orbit around the sun causes the comet to lose matter from its coma until it breaks up. Some comets get so close to the sun that they vaporize (sun-grazers), while others hurtle directly into the sun.


Shooting stars that flash across the sky are actually caused by dust, rock, or metal bodies burning up in the atmosphere; they can originate from planets, asteroids, or comets. Larger chunks do not burn out completely and reach the surface of the Earth as meteorites.

There are also meteorite showers that can occur annually due to the Earth crossing a comet’s orbit and the material lost by the comet entering the Earth’s atmosphere.


Numerous asteroids and comets have already been explored. In 1986, an international armada stormed toward Halley’s comet. Galileo (NASA) passed two asteroids on its way to Jupiter in 1991 and 1993.

Stardust (NASA) visited the comet Wild 2 in 2004 and gathered dust particles that were returned to Earth. Rosetta (ESA) is currently on its way to orbit and launch a landing probe on to the comet Churyumov-berasimenko, which it will reach in 2014.