Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Comets, like asteroids, are leftovers from the formation of the Solar System. Asteroids are leftover rocky material from the warm inner part of the Solar System and comets are relics from the cold outer part.

Comets that orbit in the outer Solar System generally go unnoticed. But sometimes comets make close passages by the outer planets. They may be flung outward into orbits that may take thousands of years to travel, or they may be diverted into the inner solar system.

Orbit of Halley's Comet

The orbit of Halley's Comet is pretty typical. It has an eccentricity of .967, meaning an extremely elongated ellipse with the Sun very close to one end. Orbits like this are Kepler's Second Law on steroids. Halley's Comet spends fully half its time beyond Neptune, tracing out  narrow but extremely long wedges, then whips through the same arc in the inner Solar System in about two years.

Comet capture

The diagram at left shows how a comet can be captured into a shorter orbit. If it passes by Jupiter and is slowed down by Jupiter's gravity, it falls in toward the Sun. It will travel back out about to where its original orbit was disturbed, that is, somewhere near Jupiter's orbit. Jupiter has a large family of comets captured this way, with aphelia close to Jupiter's orbit. The other gas giant planets also have comet families, though smaller than Jupiter's. Halley's Comet has been proposed as a member of Neptune's family.

Comet captures are rarely as simple as depicted here. Usually there are multiple encounters involved.

Comet Flung outward On the other hand, comets can also be flung into longer orbits. In this diagram, a comet originally orbiting in the outer Solar System gets a gravity assist from Neptune and is propelled into a much longer period orbit.

When a comet approaches the Sun, it begins to vaporize (below). An envelope of thin gas called the coma surrounds the tiny solid core, the nucleus. Sunlight pushes the gas away to form a tail. Solid grains of dust also are blasted off. There are actually two tails. The ion tail, made of charged atoms, is easily pushed by sunlight. The ions are rapidly pushed away from the comet, so the ion tail is usually pretty straight and pointed directly away from the Sun. The ion tail is usually bluish because tiny objects like atoms scatter blue light most effectively, and also the atoms and molecules are excited and emit blue light. The dust tail is usually brighter and since it is simply reflected sunlight, tends to be yellowish. The dust particles are accelerated more slowly by sunlight, and often becomes fanned or curved as the comet moves relative to the escaping dust. Really bright comets commonly have a tail that appears to consist of thin streamers or striae. These probably mark major eruptions from the surface, or the breakoff of chunks of the nucleus. They are probably, in effect, micro-comets.

Sunlight reflected off the coma and tail may make the comet bright enough to be visible to the naked eye. The material evaporated into the coma and tail is permanently lost, so all comets eventually die. Halley's Comet is estimated to have lost about ten meters of material from its surface during its 1985-86 encounter. If it's ten kilometers wide (it's elongated) that means it can survive about another 500 passes (38,000 years) before it disappears. More likely it will break up before then.

Comets can be wildly unpredictable. Comet Kohoutek in 1973 looked like a exceptionally promising comet, but it turned out to be much fainter than predicted and for a time became a synonym for an over-hyped disappointment. On the other hand, Comet Holmes had been plodding uneventfully through the inner solar system since its discovery in 1892. In October 2007 it suddenly brightened by a million times, going from something visible only through a large telescope to naked eye visibility. As its coma expanded, Comet Holmes briefly became larger in diameter than the Sun, though the mass of the coma was negligible. Many comets have been observed to break up and some have simply disappeared.

The dust shed by comets continues to orbit the Sun. If the orbit intersects the earth's orbit, we sweep through the dust stream every year and experience a meteor shower.

Comets were mysteries, often dreaded as portents of disaster, until the 1600's. In 1682, Edmund Halley showed that one particular comet returned to the inner solar system every 76 years. He predicted that the comet would return in 1758, which proved correct, and the comet was named Halley's Comet in his honor. This was a radical discovery because Halley's Comet was the first new object orbiting the Sun ever found in the Solar System. It also travels three times as far from the Sun as Saturn, then the most distant planet known. Halley effectively tripled the size of the Solar System. Halley's Comet had a rather unfavorable return in 1985-6, and most people missed it, but it did become the first comet ever visited by a spacecraft. It is due to have a much better appearance in 2061. Halley's Comet travels beyond the orbit of Neptune; in fact, because of Kepler's Second Law, Halley's comet spends almost half its time beyond the orbit of Neptune. Halley's comet was sighted in 2003 at a distance of 4.2 billion kilometers from the Sun, by far the most distant observation ever made of a comet. When it reaches aphelion in 2023, it should still be detectable using the same technology. For the first time since it was first sighted in 240 B.C., it is possible to observe Halley's comet all the way around its orbit.

Comets with periods less than 200 years are called "short period." Comet Ikeya-Zhang, discovered in 2002, was later determined to be a rediscovery of a comet observed in 1661, and holds the record for the longest period comet ever observed at two appearances. It is predicted to return in 2362. Short period comets probably originate in an outer icy asteroid belt, the Kuiper Belt, beyond Neptune.

Long period comets with periods of thousands of years or more are believed to originate in the Oort Cloud, a vast spherical swarm of comets extending a light year or more from the Sun. Oort cloud comets probably formed in the outer solar system and were flung outward by encounters with the outer planets. Comet Hale-Bopp, which put on a stunning show in 1997, is a long period comet. Its period was about 4200 years, but its 1997 passage through the Solar System shortened it to about 2400 years because of the gravitational effect of Jupiter. For perhaps the first time in human history, millions of people were able to enjoy the spectacle of a bright comet without being haunted by fear of what the comet portended (although even then, there were those who did).

Comets are traditionally named after their discoverers. One of the most prolific comet hunters was Charles Messier in the 1700's. He became frustrated by objects in the sky that looked like comets but were not, so he compiled a list to help avoid confusion. Ironically, nobody remembers Messier's 21 comets, but his list of objects has immortalized him. Messier's list is a catalog of the most spectacular deep sky objects in the heavens. Under a newly established system, comets are now identified by a number, a letter code, and the discoverer's name. Halley's Comet is 1P/halley. The P stands for "periodic."

After the end of the Apollo Program, Congress heeded the demands to "take all that money being spent on space and use it for problems here on earth." Are you curious what was done with the money? Me too.

So when Halley's Comet returned, the U.S. did not launch an imaging mission. The European Space Agency and the Soviet Union did. The ESA mission was named Giotto, after the 14th century Italian artist who worked an image of Halley's Comet into one of his paintings.

Above is the best image returned by Giotto. The nucleus is a dark peanut about the size of Manhattan Island silhouetted against a background of gas and dust illuminated by the sun. Two jets of water vapor are brightly illuminated. The ESA hoped to route Giotto on to another comet, but it was so badly sandblasted by this encounter it was no longer usable. Below are two composite images of the nucleus. Given its shape, no astronomer would be surprised to see the nucleus break up at some point in the future.
Soviet Vega image of Comet Halley The Soviet spacecraft were Vega 1 and 2. Vega comes from the Russian Venera-Galley or "Venus-Halley" (Russian often uses G to approximate our H). The spacecraft used gravity assist from Venus to reach the comet and dropped landers en route and, remarkably, balloon probes. This is the most detailed Vega image.

Automated solar observatories frequently discover comets. Most prolific has been SOHO (Solar and Heliospheric Observatory) which has discovered over 1500, more than all other comet hunters put together. Amateurs routinely scan the SOHO site looking for new ones. Many are tiny chunks that seem to have resulted from the breakup of a large comet long ago.

In this series of images, a fairly bright comet makes a very close pass by the Sun (masked by a white disk) and vaporizes entirely as a result.

In 2001, Comet 19P/borelly was visited by NASA mission Deep Space 1. It has a highly elongated nucleus 8 kilometers long.
Comet 81P/Wild 2 only joined the short period comets in 1974 after a close encounter with Jupiter.
In 2004 the comet was visited by NASA's Stardust mission, which imaged the 3.3 by 4.0 by 5.5 kilometer object. The nucleus is pockmarked by steep sided pits, which may be craters or a result of gas venting.

In 1995, the NASA probe Deep Impact made a close approach to Comet 9P/Tempel. It ejected a copper impactor to strike the nucleus. The nucleus is about 7.6 by 4.9 kilometers.

Why copper? Because copper is not typically found in comets, making it easy to separate cometary material from spacecraft debris.

The impactor relayed images to the flyby spacecraft.
The last image before impact. The oval at bottom is a circular depression.
POW!! Direct hit! The impact was equivalent to five tons of dynamite. About 250 million kilograms of water and 10 to 25 million kilograms of dust were blasted off. The comet continued to vent water vapor for almost two weeks.
The comet's dusty plume illuminated brightly by sunlight. The mission was a victim of its own success. The ejecta was far dustier than expected, making it hard to view the impact crater. Since the mass and energy of the impactor was precisely known, a good view of the impact crater would have given valuable information on the mechanical properties of the nucleus.
Comet Holmes after its spectacular outburst in 2007.

Schumacher-Levy 9

In 1994, for the first time in human history, we witnessed a comet impacting a planet. A year earlier, the comet hunting team of Gene and Carolyn Schumacher and David Levy had spotted a bizarre comet. At first glance, instead of a fuzzy round coma, it appeared as a straight line. Closer inspection showed a large number of tiny comets in a row. Calculating its orbit backward, the team discovered it had recently had a close encounter by Jupiter, actually made a couple of orbits as a temporary satellite, then was flung out again into a new orbit. Tidal stresses during the encounter broke the comet apart. Even more exciting, the comet was predicted to impact Jupiter in the summer of 1994.

Comet Schumacher-Levy 9 As seen by large telescopes, the comet consisted of a long string of tiny comets, some of which broke apart themselves. Others dissipated completely.
Unfortunately, the impacts happened on the side of Jupiter away from earth, though they were viewed by the still distant Galileo probe. Some later impacts, however, were close enough to the edge of Jupiter that the rising fireball after impact could be seen. In the sequence at left one fireball rises, expands, and flattens out over a period of a few minutes.
Infrared image of Jupiter One way to view the impacts was to observe Jupiter in infrared, at a wavelength absorbed by Jupiter's atmosphere. At that wavelength, most of the sunlight striking Jupiter is absorbed, but the impacts radiated infrared at all wavelengths because they were still hot. Here we see three clusters of impacts, as well as temperature differences in Jupiter's cloud belts.
Infrared image of Jupiter Here a brand new impact has just rotated into view, so hot that it blinds the detector. Three older and somewhat cooler impacts are visible.
In visible light, the impacts made dark smudges as large as the Earth. The chemistry of the dark coloration is still poorly understood.

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Created 11 July 2008, Last Update 11 January 2020