Some pictures of famous comets:

Comets - from the Greek kome, meaning “hair”. Only visible when far from the Sun due to reflected light. As they near the Sun, comets emit light of their own.

Comets have highly elliptical orbits. They develop tails as icy matter in the comet becomes heated and sublimes away. A comet’s tail always points away from the Sun.

If a comet survives its close approach to the Sun (some are completely broken apart or crash into the Sun), it continues to extreme distances from the Sun.

The orbits extend far beyond Pluto, perhaps 50,000 A.U.’s. Most take hundreds of thousands to millions of years to orbit the Sun. A “short” period comet is a comet with an orbit less than 200 years.

Comets orbit at all inclinations and orientations, both prograde and retrograde. For each comet we see, there are many more so far from the Sun that they are invisible from Earth.

There may be a huge cloud of comets, beyond the orbit of Pluto, called the Oort Cloud. Most comets spend their entire lives in the cloud.

Occasionally the gravity of a passing star “kicks” a comet into an orbit that brings it close to the Sun.

Halley’s Comet - Edmund Halley realized that this comet visits every 76 years and predicted its reappearance in 1758.

He did not live to see his prediction proved correct, but the comet was named in his honor. Sightings of Halley’s comet have been traced back to 240 B.C.

The tail of Halley’s comet can reach almost one A.U. in length, stretching tens of degrees across the sky. The 1986 visit was not good for viewing from Earth, but spacecraft did visit it at this time.

The main solid body of a comet is called the nucleus. It is typically only a few kilometers in length.

The Sun’s heat causes the nucleus to form a diffuse coma of dust and evaporated gas. The coma can measure as much as 100,000 km in diameter (almost as large as Jupiter).

An invisible hydrogen envelope surrounds the coma and stretches millions of km into space.

The tail stretches almost an A.U. The tail and the coma are the only parts visible from Earth. Most of a comet’s light comes from the coma.

Comets are of two types, distinguished by their tails:Type I and Type II.

Type I (ion, or plasma) tails: very straight, made of glowing, linear streams.

Type II (dust) tails: broad, diffuse, gently curved, only reflects light.

Many comets have both types mixed. Comet Kahoutek (1975) was a highly publicized flop because its large dust tail scattered the light from its ion tail.

The tail of a comet always is directed away from the Sun as it is produced by the solar winds. Ions in the type I tail are more influenced by the solar winds, so they are always directed in a straight line from the Sun.

The dust particles of the type II tail are heavier, so they have more of a tendency to follow the comet’s orbit, making them slightly curved.

In 1986, a number of spacecraft visited Halley’s comet. Vega 2(Russian) went through the tail, and Giotto(European) moved within 600 km of the nucleus(this damaged Giotto’s camera). They each imaged Halley’s nucleus.

Halley’s nucleus is irregular, potatoe-shaped and is almost jet black. Jets of matter are expelled from small areas on the sunlit side. These jets are what causes the nucleus to rotate once every 53 hours.

Comets have masses ranging from 1015 to 1019 g (much like small asteroids), but a comet’s mass decreases over time.

Comets that move within 1 A.U. of the Sun typically lose 107 grams of material every second. That is a loss of 10 tons of cometary material for every second the comet spends near the Sun.

Halley’s comet will be gone in about 40,000 years.

Sun-grazing comets may break apart when close to the Sun or may even plunge into the Sun.

Cometary nuclei are believed to be composed of dust particles trapped within a mixture of methane, ammonia, and ordinary water ice. “Dirty snowballs” is the term coined by comet expert Fred Whipple.

Fred Whipple also said, “Comets are like cats, they both have tails, and they do whatever they want!”

Eugene Shoemaker's passion was Astrogeology. He dreamed of going to the Moon. Credited with inventing the branch of Astrogeology within the U.S. Geological Survey, his contributions to the field and the study of impact craters, lunar science, asteroids, and comets are legendary. Though his own career as an astronaut/geologist was sidelined by a health problem, he helped train the Apollo astronauts in geology and the investigation of the lunar surface.

Seen here at Meteor Crater, Arizona in the mid 1960s, Shoemaker was killed in a tragic car accident in July 1997. He is survived by his wife and professional colleague, Carolyn, and children. In a fitting tribute conceived by a former student, Eugene Shoemaker's ashes were placed on-board the Lunar Prospector spacecraft which has now successfully reached a polar mapping orbit around the Moon. After completing its scientific mission, the spacecraft will ultimately impact the lunar surface.

The Lunar Prospector crashed to the Moon’s surface in late 1999.

Gene Shoemaker traveled to the Moon as he wished. He ultimately became what he had spent his life studying, a crater.