People on the earth knew about the comets from thousands of years. The first recording of Halley's Comet was made as early as 240 BC. But people weren't aware where these comets came from and the reason behind their strange tail. But they did fear that their appearance is a sign that something bad is going to happen.
We can classify comets into two kinds- Short period and Long period.
- Short period comets are those which have an orbit period of less than 200 years. Such comets were thought to be originating from a place called Kuiper Belt which lies beyond Neptune. The dwarf planet Plato itself is a part of this Kuiper Belt.
- Long period comets have an orbit period of more than 200 years. These Comets were thought to be originating from Oort cloud which lies on the boundaries of the solar system.
The core centre of the Comet is called the Nucleus. Nucleus consists of rock and dust along with the frozen particles of methane, water and ammonia. The atmosphere that surrounds the Comet is called Coma. When the Comets get closer to sun, the frozen particles get melted and form a broom like shape called Tail. The Tail of a Comet can be more than million kilometres in length.
The orbit of a Comet around the sun is highly elliptical... which means that they won't orbit in a circular shape. Scientists have been able to predict the orbits of some Comets and expect their next arrival towards the sun. Such Comets were known as Periodic Comets. Non-Period Comets are those whose orbit can't be determined by humans till now.
According to an estimation, there are more than 5,000 Comets in the solar system. But people are aware of just a few popular Comets such as Halley's Comet, which can be visible from earth for every 75-76 years. Halley was last seen in 1986 and would be visible again in 2061.
Earth was hit by many Comets in the past. But that was a long time ago. Some scientists doubt that such collisions might have brought water to earth. Even though there is no proof to support such arguments, scientists believe that those collisions might have certainly paved way for the atmosphere that could support life. As for now, there doesn't seem to be any danger of a Comet colliding with earth.