Comet• A comet is an icy small Solar System body that,

when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometresacross and are composed of loose collections of ice, dust, and small rocky particles.

• The coma and tail are much larger and, if sufficiently bright, may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many different cultures.

Comet structure• The comet is

composed of three main parts: nucleus, head (formed in the front of comet) and tail. The head and tail of comet forms only when comet gets close to the Sun.

Comet nucleus• The solid, core structure of a comet

is known as the nucleus. Comet nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, some comets may have a higher dust content, leading them to be called "icy dirtballs".

Coma• The streams of dust and gas

thus released form a huge and extremely thin atmosphere around the comet called the "coma", and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.

Comet head and tail• The head and tail of comet forms

only when comet gets close to the Sun, when its nuclear radiation vaporized ice. When comet moves away from the Sun, the parts gradually disappear. The length of the tail can be even bigger than Jupiter. Tails are of two types - gas (blue) and dust (white). The most common gas tails are straight, and the dust – bent. All the tail extends to the opposite side of the sun.

Dust and gas• Comet tail is not shining,

but only reflects sunlight. Comet dust rich in carbon and silicon, and the gas is mostly water vapor. Dispelling the Sun, they make up ten million kilometers in diameter hydrogen cloud.

Discovery• The first asteroid to be discovered, Ceres, was

found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid“.

Asteroid• Asteroids are minor planets,

especially those of the inner Solar System. The larger ones have also been called planetoids. Asteroids are spherical, oblong or irregular shapes. The surface is solid, dotted with impact craters. Asteroids do not have the atmosphere. The physical composition of asteroids is varied and in most cases poorly understood. Part of the asteroids have satellites or is a binary.

Asteroids color• Asteroids become darker

and redder with age due to space weathering.However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.

Classification• Asteroids are

commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.

Near-Earth asteroids

• Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers.