A long-term comet is a periodic comet which returns to the sun every 200 years or longer. Some long-term comets have orbital periods which are measured in the thousands or millions of years.
Designation
In the Cometary Designation System, a long-term comet is named for the person who first calculated its orbit. Long-term comets are classified as C/ non-periodic comets, the same as single-apparition comets. This is why the terms “periodic” and “short-term” are sometimes used interchangeably.
The comet’s designation also includes the year in which it was discovered. If several comets were discovered that year, they are lettered and numbered in order of discovery.
Orbit
Unlike most short-term comets, which have the far ends of their orbits between the orbits of Neptune and Pluto, the aphelions of long-term comets lie far beyond the planetary regions of the solar system. Long-term comets have aphelions in the Oort Cloud, at a distance of 50,000 to 70,000 astronomical units from the sun. A single AU is the average distance of the Earth to the sun.
For example, both Comet Kohoutek and Comet West are long-period comets. The orbital period of Comet Kahoutek (C/1973 E1) is 75,000 years, while the orbital period of Comet West (C/1975 V1), whose aphelion is 70,000 AU from the sun, is roughly 6 million years. At its farthest distance from the sun, Comet West will be 1/4 of the way to Alpha Centauri, the nearest star beyond the sun.
The orbit of a long-term comet is not a simple curve. In fact, at parts of its orbit, a long-term comet may look as though its path will take it out of the gravitational reach of the sun. The proper orbit of a long-term comet cannot be calculated until its path has taken it out of the planetary regions of the solar system.
The orbits of long-term comets also have much higher range of inclinations than the orbits of short-term comets. This is because the Oort Cloud is a sphere, while the origin of most short-term comets, the scattered disc, is a wide disc mostly along the ecliptic of the solar system. Thus, while the orbits of most short-term comets are 40 degrees or less from the ecliptic, the orbits of long-term comets can be at any angle.
The difference between a single-apparition comet and a long-term comet is that long-term comets are gravitationally bound to the sun, no matter how long it takes them to return. In contrast, a single-apparition comet will only come near the sun once. After that, it will leave the solar system.
Composition
All comets are collections of dust, rocks, ice, and frozen gases which is between half a mile and 100 miles in diameter. According to NASA, a comet must be composed of at least 85% ice, which can include ice consisting of carbon monoxide, carbon dioxide, methane, and ammonia.
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
A long-term comet becomes visible when its orbit takes it close enough to the sun for part of the comet to evaporate and create a visible coma. This coma is what makes the comet bright and fuzzy-looking. At this point, a comet can be closer to the sun than the orbit of Mercury.
As the solar wind interacts with the coma, it blows part of the coma away from the sun. This is the comet’s tail. The tail can be many times larger than the comet, up to 100 million miles long, or it can be almost non-existent. The interaction between the solar wind and the comet’s tail generates X-rays as well as visible light.
Many long-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 back towards the sun.
Long-term comets which never return
Although they have periodic orbits at the time when they are observed, long-term comets have a long time to interact with other solar system bodies. All of these interactions change its orbit a little, and may disrupt it completely. Thus, even after its orbit has been calculated, no one can know for sure if a very long-term comet will return at all.
Other long-term comets vanish after coming too close to the sun. Most of a comet’s composition is water-ice and other volatile compounds, which melt and evaporate to produce the coma. Some of these compounds never return to the comet. Every comet leaves the sun much smaller than when it came in.
A long-term comet which loses most or all of its volatile materials, leaving only rock and rubble behind, are considered to be extinct. These comets look similar to rocky asteroids and other potential meteors. In fact, some meteors do come from comets, although no long-term comets can return soon enough for human history to associate a meteor shower with a former long-term comet.
However, the orbits of comets and asteroids is completely different. The orbits of most asteroids are much more spherical than the extremely elliptical orbits of comets. The orbits of asteroids also have low orbital inclinations.
The orbit is the easiest and fastest way to tell an asteroid apart from an extinct comet. This basis for telling the difference will be even more important if the new asteroid mining venture is to succeed, because profitability will depend on metal content, and comets have almost no metal content. Asteroids do not have much reflectivity, but orbital analysis alone will make spectrographic analysis much less necessary.
