14. Comets

Spacecraft glimpse the comet nucleus

An international flotilla of six spacecraft, belonging to four space agencies, flew by Comet Halley in March 1986, to examine the gas and dust in the vicinity of the comet and to photograph its nucleus.

Fig. .. 

The European spacecraft Giotto obtained the best images, with the highest resolution. The surface of the nucleus was charcoal black, reflecting only about 4 percent of the sunlight that falls on it. Bright jets were spewing gas and dust from the cometís sunlit side, but there wasnít much white ice in site.

The images obtained with the camera on Giotto showed that the nucleus of Comet Halley has an elongated, irregular shape with dimensions of 16 x 8.5 x 8.2 thousand meters, about the size of Paris or Manhattan. For a mass density about that of water, this volume corresponds to a mass of about 1015 kilograms or 1000 billion tons. A varied, lumpy topography was seen, with craters, valleys, hills, and mountains.

Fig. .. 

We now know that Comet Borrelly also has a black heart, with a nucleus that is just as dark and unreflective as that of Comet Halley. The Deep Space 1 spacecraft was directed toward this comet after completing its primary mission of flight-testing an ion engine and 11 other advanced technologies. On 22 September 2001, the spacecraft whizzed by Comet Borrelly at a distance of just 2.2 million meters, revealing an irregular chunk of rock and ice, about 8 thousand meters long and perhaps 4 thousand meters wide. It is covered with a dark, carbon-rich slag that reflects only about 4 percent of the incident sunlight, on average; comparable to the reflectivity of the powdered toner used in laser printers. The surface of this nucleus also has a rugged terrain, with mountains, valleys, deep fractures and smooth rolling terrain where jets of gas and dust have apparently polished the surface. Some of the jets vastly exceed the nucleus in length, resulting in an asymmetric coma that was offset from the center of the nucleus by up to 2 million meters.

Fig. .. 

A cometís nucleus also rotates. Typical rotation periods are a few hours to a few days. Observations of Comet Halley, for example, indicate that it rotates around its longest axis once every 7.4 days, and that it wobbles about its shortest axis once every 2.2 days or 53 hours. As the nucleus rotates, new regions turn to face the Sun, heat up and become active, while others face away from the Sun and momentarily turn off their activity.

Fig. .. 

The gas and dust streaming off the sunlit side of a comet nucleus initially heads toward the Sun, before being swept back into the comet tails. The expelled material pushes the comet in the opposite direction, making it arrive sooner or later than expected. A similar recoil effect explains the darting action of a small balloon when it is released, as well as the forward thrust of a rocket engine. The thrust of the rocket-like, comet jets either pushes the comet along in its orbit or slows it down.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University