Short-term comets are periodic comets which return to the sun every 200 years or less. In the Cometary Designation System, a short-term comet is named for the person who first calculated its orbit, and is classified as a P/ periodic comet. This is why the terms “periodic” and “short-term” are sometimes used interchangeably.
All periodic comets, including short-term comets, have highly elliptical orbits which bring them close to the sun at regular intervals. This is important for observation and determining orbit because comets only have their classic comas and tails when they are nearest the sun. Without its tail, a comet is just another cosmic object which is extremely hard to spot, even with the best telescopes.
Over 250 short-term comets are currently known. However, some of these have been destroyed by collision or too-close approach to the sun. Some have also been lost to continued observation, because their orbits are not yet known.
Most short-term comets have the far ends of their orbits, or aphelions, between the orbits of Neptune and Pluto. This region is divided into 2 parts: the Kuiper belt and the scattered disc. Because Kuiper belt objects are usually stable in their orbits, short-term comets originate from the less stable scattered disc. In turn, these objects may have been previously pulled in from the inner Oort Cloud.
The Kuiper belt and its associated scattered disc are generally flat regions along the plane of the Earth’s and other planets’ orbits. The orbits of scattered disc objects do not have inclinations of higher than 40 degrees from the ecliptic. For this reason, short-term comets also have fairly flat orbits, unlike long-term comets, which can come in towards the sun from any angle.
Halley’s Comet is one of the comets which originates in the scattered disc. Its inclination is just 18 degrees from the ecliptic. However, Halley’s Comet, unlike most scattered disc objects, has a retrograde orbit.
A second group of short-term comets is the Jupiter family. After they are pulled into the inner solar system, these comets never go farther from the sun than the orbit of Jupiter. As a result, these comets have orbital periods of less than 20 years. The orbits of most Jupiter family comets are usually inclined less than 15 degrees from the plane of the solar system. Encke’s Comet is an example of a Jupiter family comet.
The perihelion of a short-term comet’s orbit is its point of closest approach to the sun. Halley’s Comet goes inside the orbit of Venus. Other short-term comets frequently have perihelions inside the orbit of Mercury. The closest sungrazing comets may come within a few thousand miles of the sun’s surface.
A short-term comet is a dirty snowball made mostly of water-ice and rock. Other materials can include frozen ammonia, methane, or carbon dioxide. Comets which have made previous approaches to the sun have proportionately more rocky material, because they have lost part of their ice and other volatile materials to evaporation.
Many comets contain some organic material, such as ethanol, formaldehyde, or hydrogen cyanide. Even amino acids, the basic building blocks of life, have been observed in comets. Current NASA research proposes that DNA components may also be found in comets.
Coma and tail
As a short-term comet approaches the sun, frozen volatile materials sublimates into gaseous form. The resulting cloud of dust and ionized gas which surrounds the nucleus is known as the coma. Although the nucleus of a comet is rarely more than 30 miles across, the coma can be larger than Jupiter.
When the comet comes closer to the sun, the solar wind pushes part of the gaseous coma away from the nucleus to form the comet’s tail. This is why a comet’s ionized tail always points away from the sun. In a great comet, the tail may be longer than the distance of Earth to the sun.
Most short-term comets also form a second, much fainter tail, which is made up of non-ionized dust from the coma. This tail points back along the comet’s orbit. When the comet is leaving the sun, this dust tail may point towards the sun.
The coma and tail are much brighter than the frozen comet was. Some of the brightness is because of reflected light. Some of it is also produced directly by the comet’s ionized gases.
Short-term comets which never return
Although they have periodic orbits at the time when they are observed, many short-term comets never return a second time. Many short-term comets vanish after coming too close to the sun. After all, nearly all comets are made mostly of water-ice, which melts and then evaporates. Comets which survive a close encounter with the sun leave the region much smaller than when they came in.
After a few encounters with the sun, a short-term comet may be left with few or no volatile materials, leaving only rock or rubble behind. These extinct comets look similar to rocky asteroids, except that their orbit is usually much more elliptical. In some cases, the remaining volatile materials may be locked within the nucleus of the comet by a thick crust. The crust gives protection against evaporation until it is disrupted. These comets are dormant rather than extinct, because damage to the crust could bring them back to life. Roughly 6% of Near Earth Objects (NEOs) are believed to be extinct or dormant comets.
Many comets, such as Comet Shoemaker-Levy in 1994, collide with Jupiter or other solar system bodies. Comets which escape collision still cross the orbits of the giant planets and are constantly affected by their gravity. Some of these comets are captured by a planet’s gravity and become its satellites. Others are flung out of the solar system, never to return.
The Cometary Designation System accounts for these possibilities by classifying new comets as X/ if a meaningful orbit cannot be computed. The classification C/ is given to comets which have been confirmed as non-periodic or which have not yet been observed to return. Comets which no longer exist are classified as D/. For example, Comet Shoemaker-Levy is now designated as D/1993 F2.
A few short-term comets do not vanish completely when they die. In their afterlives, they come back as meteor showers. The annual Perseid meteor shower consists of remnants of Comet Swift-Tuttle which travel along the same orbit as the parent comet.
Comet Swift-Tuttle itself has a small chance of colliding with either the Earth or the Moon in one of its future orbits, although its closest approaches won’t happen until the year 4479. This could be similar to how water from other comets may live on as lunar ice, or even in the Earth’s oceans. Some astrobiologists have hypothesized that these comets may even have been the source of life on earth.