A comet is a celestial body that revolves around the sun. Comets have a long tail. Early comets were described as a phonomenon. The Danish astronomer, Tycho Brahe, proved that comets were celestial bodies. A comet is made of a sharp nucleus, or center. It is also described as a "dirty snowball" because of the ice and dust on the comet.
When a comet comes toward the sun, the solar heat evaporates so that the ice on the comet brightens. When this happens, the comet has a tail that might sometimes lenghten to a point of being many millions of kilometers in space. The tail of the comet is usually turned away from the sun. Comets also have elliptical orbits.

Sometimes comets travel in a group that has nearly the same orbit. They are said to be members of a comet group. The most famous group is Ikeya-Seki of 1956. Comets have been known to be important events. The presence of a comet has also given fear of a collision between the comet and the earth. The earth has passed through the tails of comets without much effect. Although a collision of a comet with a city would destroy the city, it will probably never occur again.

Asteroids are one of the many minor planets of the solar system. The largest asteroids are Ceres, Pallas, and Vesta. The mass of an asteroid of is much less than the mass of the Moon. Few scientists now believe that asteroids are the remains of a former planet. They think that asteroids have a place in the solar system where a planet could've formed, but the gravity of Jupiter prevented it from happening. Some unusual asteroids are made of iron-nickel alloy.

Asteroids, like meteorites, can be classified into specific types. Most asteroids are related to the meteorites known as "irons". A few asteroids, such as Vesta, are probably related to the rarest meteorite class of all. It is the achondrites. These asteroids look like it has the surface composition of many lunar and terrestrial lava flows.

A meteor is a solid body that enters a planet's atmosphere from space. Brilliant meteors, consists of a radiant head and then it is followed by a tail of light, like a comet. Other meteors, called bolides, have been seen to explode. When bolides explode, they sound like thunder. Fainter meteors, called shooting stars, occur occasionally. At some times, hundreds of meteors can occur at once. These swarms are called meteor showers. Some appear annually on the same days of the year. They are called periodic showers. Others occur at different times each year. A meteor that reaches the surface of the earth or another planet is called a meteorite. A meteoroid is a solid body orbiting the sun. It will become a meteor if it enters earth's atmosphere. The majority of meteoroids are the size of grains of dust, but they can be larger without any limit.