Comets can be described as small, celestial bodies in our Solar System that orbit the Sun. One distinguishing characteristic of comets is that they exhibit a visible coma (atmosphere) or a tail, when they are close enough to the Sun. This coma or tail can be attributed to the effects of solar radiation on their nucleus. There are various types of comets in the space, with different orbital periods that can range from a few years to hundreds of thousands of years. Then, there are other comets, which pass through the inner Solar System only once, after which they are thrown out into interstellar space.
More Information on Comets
The name 'comet' has been derived from the Latin term 'cometes', which in turn has been taken from Greek word 'komç' (meaning "hair of the head"). It was Aristotle who first used the term 'komçtçs' for comets, describing them as "stars with hair", mainly because of their hair-like tail. Comets originate in the outer Solar System. From the outer reaches, they are thrown inwards, towards the Sun, because of gravitational perturbations from the outer planets or due to collisions. Till June 2008, 3,535 known comets were reported by astronomers.
Types of Comets
Comets are classified on the basis of the length of their orbital period. The longer the period, the more elongated will be the ellipse.
Short-period comets are those comets whose orbital periods are less than 200 years. These comets can be seen orbiting, more-or-less, in the ecliptic plane, in the same direction as the planets.
The orbits of long-period comets are extremely eccentric (elongated) and they have orbital periods, ranging from 200 years to thousands or even millions of years. However, they remain gravitationally bound to the Sun.
Single-apparition comets are quite similar to long-period comets, with the exception of their parabolic or hyperbolic trajectories. Their trajectories do not let them enter the solar system after they have passed the Sun once.
The term periodic comet can be used in context of comets that have a periodic orbit, irrespective of whether they are short-period or long-period comets. It alternative use is in context of short-period comets, exclusively.
Talking in the literal sense, non-periodic comet is the same as single-apparition comet. However, some astronomers take the term to mean only long-period comets that are not "periodic".
Main-belt comets are the comets that are found orbiting in more circular orbits within the asteroid belt. This type of comets have been discovered by astronomers recently only.