 HaleBopp The diffuse tail is made up of dust and gas removed from the head of the comet by light pressure. The structured ionic tail has been swept away from the head by the solar wind. |
In 1932 Ernst Öpik, an Estonian astronomer, proposed that comets originate in an orbiting cloud at the outermost edge of the solar system. In 1950 the idea was revived by Dutch astronomer Jan Hendrik Oort, because comets are destroyed by several passes through the inner solar system and must therefore be in recent orbits. According to Oort the cloud provides a continual supply of new comets. The number required places the total mass of the Oort cloud at many times the mass of Earth.
For most of the 20th century comets were imagined to be 'dirty snowballs'. The evidence for that conclusion was indirect but compelling, because the tails of comets were known to contain dust and water molecules. Recently (2005) a probe was deliberately crashed into the nucleus of a small comet called Tempel 1. Our ideas have (as usual) been modified. The average density of Tempel 1 was found to be half that of water. The comet head appears to be loosely packed with holes, rather than a closely packed snowball. The surface is soft, but firm enough to retain the marks of ancient impacts.
Comets have very highly elliptical orbits and spend most of the time in the outer reaches of the solar system far from the Sun. When, at long intervals, they do briefly approach the Sun, they are warmed and develop two tails.
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HaleBopp The diffuse tail is made up of dust and gas removed from the head of the comet by light pressure. The structured ionic tail has been swept away from the head by the solar wind. |
Great comets light up the night sky. They are occasional visitors and have not been seen for a century. Lesser comets look fine in time exposure photographs, but are a small faint smudge about the head to the naked eye. There have been several of these in the last 50 years, including Halleys comet in 1986 which was, at this visit, a huge disapointment.
HaleBopp is here photographed at perihelion [closest approach to the sun]. The two tails are clearly separated and look quite different. Many comets are in known orbits so their return is predictable but this one, like all the large comets, arrived unannounced. Amateur astronomers with binoculars and small telescopes are usually the first to spot the new arrivals as faint smudges. Hale and Bopp shared the discovery of this one.
The image at right was compiled by Don Davis from observations he made in 3/24/96 of comet Hayakutake with 9x36 binoculars [tail] and a 10 inch Dobsonian telescope [head]. The dust and ion tails are superimposed.
McNaught was (in 2007) the brightest comet for many decades. The spectacular tail ion was visible in long exposures. The tail shows structure that is thought to be the effect of gusts in the solar wind. The Comet is here photographed in the early evening from the southern hemisphere, as it begins its long journey away from the sun.
Image from ...
Astronomy Picture of the Day Feb 12 2007
The SOHO project has lead to the discovery of many comets which were previously unknown. The comet called 96P/Macholz1 has a period of only five years.
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96P/Macholz1  The comet 96P/Macholz1 is shown one eighth of the distance from Earth to the Sun on the 8/1/ 2002. The tail is pointing away from the Sun and is shown here as a foreshortened image. The orbit is elliptical. |
Direct sunlight is blocked by the disk to the right of the comet and the solar wind is seen streaming in to space, millions of kilometers above the surface of the Sun, marked by the white circle.
Sun grazers
The SOHO satellite has to date [2001] imaged more than 200 Sun grazing comets.
 A Sun-grazing comet |
They are thought to have all originated from the breakup of a single large comet that passed very close to the sun some 2000 years ago. They pass within 50 000 km of the photosphere and must all have a very short remaining life.