Comet

A comet is a small body in the solar system that orbits the sun and (at least occasionally)

Physical characteristics

Comets are believed to originate in a distant cloud known as the Oort cloud, after the astronomer Jan Hendrik Oort who hypothesised its existence. They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that bring them very close to the Sun.One theory says that when a comet approaches the inner solar system, radiation from the Sun causes its outer layers of ice to evaporate, but again there is no proof of this. The streams of dust and gas this releases form a huge but extremely tenuous 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. The dust and gas each form their own distinct tail, pointed in slightly different directions — dust being left behind in the comet's orbit (so that it often forms a curved tail) and the ion tail (gas) always pointing directly away from the Sun, since the gas is more strongly affected by the solar wind than dust is, and follows magnetic field lines rather than an orbital trajectory. While the solid body of the comet (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the tails can extend over 150 million km (1 Astronomical unit) or more.

Related Topics:
Oort cloud - Jan Hendrik Oort - Sun - Dust - Coma - Radiation pressure - Solar wind - Astronomical unit

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Both coma and tail are illuminated by the Sun, and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing due to ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.{{hnote|Reading Museum, scene 1}}

Related Topics:
Earth - Ionization - Telescope - Bayeux Tapestry - Norman conquest - England - 1066

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Surprisingly, cometary nuclei are among the blackest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.

Related Topics:
Black - Giotto - Comet Halley - Deep Space 1 - Comet Borrelly - Asphalt - Organic compound - Tar - Oil

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In 1996, comets were found to emit X-rays http://heasarc.gsfc.nasa.gov/docs/rosat/hyakutake.html. These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons http://www.kvi.nl/~bodewits.

Related Topics:
1996 - X-rays - Ions

~ ~ ~ ~ ~ ~ ~ ~ ~ ~