Comets

Comets can be a spectacular sight. A mixture of rock and dust bound together with ice and frozen gases, comets usually stay in the deep freeze of space, orbiting the Sun in the Kuiper Belt beyond Neptune or at the edge of the Solar System in the Oort Cloud.

(Image Right: Comet, Courtesy of NASA)

Astronomers believe that comets are leftover debris from the collection of gas, ice, rocks, and dust that formed the Solar System about 4.6 billion years ago.

Short-period comets (orbits under 200 years) are probably nudged out of the Kuiper belt by the gravitational pull of the outer planets.

Long-period comets (orbits over 200 years) come from the Oort cloud, where gravitational interactions with passing stars can cause icy bodies to enter the inner Solar System and become active comets.

Features

: Parts of a comet. (Original Image courtesy of NASA.)

Comets follow elliptical orbits that bring them close to the Sun and then swing them back into space. Far from the Sun, a comet consists of a dense solid body called the nucleus (core), which is made of frozen ice, dust and gases. As it approaches the Sun, the nucleus becomes surrounded by a "cloud" of dust and gas called the coma. The solar wind drives dust particles and charged gases outward to form a double tail that can be millions of miles long.

Discover more about comets at NASA's Solar System Exploration website.

Missions

: Stardust spacecraft (Courtesy of NASA)

The Giotto mission to comet Halley by the European Space Agency (ESA) in 1986 provided the first detailed study of a comet nucleus. Launched in 2004, the Rosetta spacecraft is currently on a 10-year mission to catch and land a robot on comet Churyumov-Gerasimenko in 2014.

The ESA is spearheading this international mission, while NASA is contributing important scientific instruments to study the comet. NASA launched its first comet based mission, Stardust, in 1999. The mission returned a sample of cosmic dust back to Earth on January 15, 2006.

Find out more about comet missions from NASA.

Myths, Stories, and More

: Woodcut showing destructive influence of a fourth century comet (Courtesy of NASA/JPL)

Comets have been known since ancient times. Throughout history, people believed that these "stars with hair" as the Greeks referred to them were messengers of the gods or omens of disasters like famines, floods, or the deaths of kings.

The most famous comet, comet Halley, passes close to the Earth roughly every 76 years. Its earliest recorded sighting was in 240 B.C. in China, but Edmund Halley was the first person to recognize that the comet was periodic and returned at regular intervals. Last seen in 1986, it will return in 2062.

Learn more about comet myths and legends at Amazing Space.

Earth Matters

: View of Augsburg, Germany with the comets of 1680, 1682, and 1683 in the sky (Courtesy of NASA/JPL)

Far away from the Sun, comets did not undergo the same heating as other objects in the Solar System. As a result, they contain similar material to the original dust and gas from the Solar System's formation. Comets probably brought water and carbon-based molecules, essential ingredients for life, to our planet in collisions with young Earth. Consequently, scientists want to study clues that comets might give to the birth of the Solar System.

Learn more about why we study comets at The Planetary Society website.