SPACE

Past encounters

IN the 1950s, Dutch astronomer Jan Oort had proposed that the long-period comets are objects that fall in from a spherical cloud of icy bodies believed to extend from 10,000 to 100,000 A.U. from the sun. Astronomers estimate that the cloud, now known as the Oort cloud, contains several trillion icy bodies. Far from the sun, they are very cold, and are invisible. The gravitational influence of occasional passing stars could perturb a few of these objects to fall into the inner solar system, where the heat of the sun warms their ice and transforms them into comets. Because the Oort cloud is spherical, these long-period comets fall inward from random directions.

GAMMA

Comet Hale-Bopp.

Exploration of the small, primitive members of the solar system began in 1986, when an international flotilla of six spacecraft reached Comet Halley as it crossed the plane of the earth's orbit a month after perihelion. Two of these - the U.S. ICE spacecraft and a small Japanese craft named Sakagaki - served primarily to monitor the comet from a distance of several million kilometres, while a second Japanese spacecraft, Suisei, passed about 1 million km from the comet. The primary exploration tasks, however, were undertaken by three craft targeted for the nucleus itself.

The Soviet VEGA 1 and VEGA 2 were the first craft to arrive, on March 6 and 9, 1986. Each plunged deep into the inner atmosphere and dust cloud of the comet, passing within about 8,000 km of the nucleus. In addition to making many direct measurements of the gas and dust, they photographed the dust-shrouded nucleus. However, they could see very little beyond the bright plumes of material jetting out from the two most active regions on its surface. Both VEGA craft were severely damaged by dust impacts, losing most of their solar cells and also several instruments at the time of closest approach.

The trajectory data for the VEGA craft were provided to the ESA to allow it to target its Giotto spacecraft for an even closer encounter - just 605 km from the comet's nucleus - on March 14, 1986. Giotto also carried out many measurements of the near environment of the nucleus, confirming and extending the Soviet results. The same two prominent jets were photographed, as was the dark, lumpy surface of the nucleus, which was considerably larger (about 8 x 12 km) than what had been estimated before.

Among the most exciting discoveries was the fact that much of the dust is in the form of very small particles consisting largely of carbon and hydrocarbon compounds, rather than silicates. Thanks to the pathfinder data from VEGA, Giotto was able to image the nucleus at resolutions as high as 100m before its camera was knocked off target by impacts a few seconds before closest approach. The Giotto camera was destroyed by the impacts of Comet Halley's dust.

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