Their average diameters usually range from 750 meters (2,460 feet) or less to about 20 kilometers (12 miles) comets are very fragile. Water probably makes up 75-80% of the volatile material in most comets. Other common ices are carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ammonia (NH3), and formaldehyde (H2CO).

A comet nucleus is small, so its gravitational pull is very weak.When they are near the Sun and active, comets have several distinct parts:

nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;

Example of nucleus X-ray of comet hyakutae:

coma:which becomes far bigger than a planet, dense cloud of water, carbon dioxide and other neutral gases sublimed off of the nucleus which means that it changes directly from a solid to a gas without becoming liquid;

hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;

dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the naked eye;

ion tail: up to 100 million km long composed of plasma caused by interactions with the solar wind.

In General

When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as "dirty snowball," since over half of their material is ice. As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.


Every comet then really has two tails, a dust tail and an ion tail.

The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. The gas molecules are torn apart by solar ultraviolet light, often losing electrons and becoming electrically charged fragments or ions. The ions interact with the wind of charged particles flowing out from the Sun and are forced back into an ion tail extending in the direction opposite to the Sun. These ions can be seen when the sunlight hits them.

As a comet ages from many trips close to the Sun, there is evidence that it loses most of its ices, or at least those ices anywhere near the nucleus surface, and becomes just a very fragile old rock in appearance, indistinguishable at a distance from an asteroid.