Physics of Comets

Spacecraft missions like GIOTTO (see Fig. 5.3) (comet Halley) and Deep Space 1 (comet Borrelley) have shown that cometary nuclei are very dark (albedo less than 0.04). 

Comet Tempel 1 was discovered in 1867 by E. Tempel. At the time of discovery, the comet was 0.7 AU from the Earth and 1.6 AU from the Sun. Currently, Tempel 1 has an orbital period of 5.5 years and a perihelion distance of roughly 1.5 AU. With an orbital eccentricity of 0.5, Tempel 1 s orbit lies between the orbits of Mars and Jupiter. These values varied in the past and will change in the future because of close approaches with the planet Jupiter. It therefore belongs to the group of short period comets. 

Comet Tempel 1 was selected as a target for a space mission with an impactor (see Fig. 5.5). It was also expected that due to the numerous and frequent perihelion passages, its surface and upper crust might have been considerably restructured. 

The Deep Impact mission revealed many details of the properties of comet Tern-pel 1. The spacecraft arrived at comet Tempel 1 on July 4, 2005 to impact it with a 370 kg mass. The spacecraft was launched from Earth in January, 2004. The im-pactor was a battery powered spacecraft that operated only for one day. Images from the comet were obtained till few seconds before the collision. The closest approach of the main spacecraft to the comet was 500 km. The flyby spacecraft monitored the impact. 

The impact also showed that the cometary nucleus is extremely porous and that the ice was close to the surface but below a devolatilized layer with thickness of order the impactor diameter. The impact crater was about 100 m the impact itself did not change the orbital parameters of the object. The surface of the nucleus revealed layered structures, signs of outbursts, many of them correlated with the rotational period and also impact craters. A review about the results was given recently by A Hearn, 2008 [4]. 

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In the 1950 s, three major concepts have brought d isive advances to the physics of comets Oort s model of a distant cometary cloud, Biermann s solar wind theory of tail formation and dynamics, and Whipple s icy conglomerate model of the cometary nucleus. The main ideas of these concepts shall be outlined briefly. 

K. S. K. Swamy, Physics of Comets, 2nd edition (World Scientific, Singapore, 1997). 

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