Comet, definition and study. A-Z index of Cognitio.

A comet is a relatively small celestial body, similar to an asteroid but predominantly composed of ice. In the Solar System, the orbit of the comets extends beyond Pluto’s.

Solar system, definition and study. A-Z Index of Cognitio.

Solar system

Comets entering the inner system, and thus becoming visible from Earth, are frequently characterized by elliptical orbits. Earth, definition and study. A-Z index of Cognitio.

Comets are thought to be the residues left by the condensation of the nebula from which the Solar System was formed: the peripheral areas of that nebula would have been cool enough to allow the water to be in solid form (rather than as gas). Comet-cores may vary in size from hundreds of meters to fifty and more kilometers and consist of rock, dust and water ice and other substances commonly found on the Earth in the gaseous state such as carbon monoxide, carbon dioxide, methane, and ammonia. When a comet approaches the inner solar system, the heat of the Sun sublimates its outer layers of ice.Sun, definition and study. A-Z index of Cognitio.

Currents of dust and gases produce a large but rarefied atmosphere around the nucleus called the hair, while the force exerted on the hair by the sun’s radiation pressure and above all the solar wind leads to the formation of a huge tail pointing in the opposite direction to the Sun.

Atmosphere, definition and study. A-Z index of Cognitio.


Most comets follow elongated elliptical orbits that lead them to approach the Sun for short periods and stay in the farthest areas of the Solar System for the rest. Comets are usually classified according to the length of their orbital period. Comets have a relatively short life. Repeated passages near the Sun gradually remove the volatile elements, until the queue can no longer form, and only the rock material remains. If this is not tied enough, the comet can simply vanish into a cloud of dust. If instead, the rocky core is consistent, the comet is now an inert asteroid, which will no longer be affected.

Asteroid, definition and study. A-Z index of Cognitio.


The core of each comet continuously loses matter, which goes to form the tail. The heaviest part of this material is not pushed away by the solar wind but remains in an orbit similar to the original one. Over time, the orbit described by the comet is filled with small, but very large, particle swarms, and grouped into clouds that originate in a core activity period. When Earth crosses the orbit of a comet at a cloud, the result is a swarm of falling stars, such as the famous “tears of St. Lawrence” (August 10), or numerous smaller and lesser-known swarms. The question of what the comets were, whether atmospheric or interplanetary objects, remained unresolved for a long time. Astronomers were limited to recording their appearance, but attempts to explain were also speculations. The turning point began in the 16th century. In those years, Tycho Brahe tried to find themselves beyond the Moon’s orbit, and thus far beyond the Earth’s atmosphere.Moon, definition and study. A-Z index of Cognitio.

Seven articles published in the Science magazine by a team of international scientists, including seven Italians, announce the discovery of Wild 2 comet dust particles of long organic molecules, of amino acid precursors such as DNA.Wild 2, definition and study. A-Z index of Cognitio.

The Stardust probe, having traveled 4.6 billion kilometers in about seven years, captured a hundred grains each smaller than a millimeter.

Comet, definition and study. A-Z index of Cognitio.



To open the video  click on the image, good view from your Alessandro Brizzi.Comet, definition and study. A-Z index of Cognitio.