Comets
They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed.
When they are near the Sun and active, comets have several distinct parts:
Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto. By far the most famous comet is Comet Halley,that can be seen every 75-76 years.
Meteor shower sometimes occur when the Earth passes through the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.
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.
- Coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;
- 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 unaided eye.
- Ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.
Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto. By far the most famous comet is Comet Halley,that can be seen every 75-76 years.
Meteor shower sometimes occur when the Earth passes through the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.