A distant look at Comet Halley.

A distant look at Comet Halley

Out of sight to the naked eye but not out of astronomers' minds or the reach of Earth-based scientific instruments and telescopes, Comet Halley remains under intense scrutiny. Last April and May, astronomers from the European Southern Observatory (SEQ), using the Danish telescope at La Silla, Chile, captured images of the comet, then about 1,250 million kilometers from Eart or roughly where the orbit of Saturn lies. The comet was so faint that the scientists needed to combine more than 60 images obtained over 19 nights, an exposure totaling nearly 12 hours, to see details of the comet's structure.

The resulting picture is the most detailed ever obtained of a comet so far from the sun. The comeths avocado-shaped nucleus appears as a small, bright point, which varies in brightness, reflecting rotation of the nucleus. This core appears to one side of a relatively bright, asymmetric region called the inner coma, about 120,000 kilometers across. The somewhat darker outer coma, is at least 300,000 kilometers across. The shape and density of the inner coma indicate the comet's nucleus is still releasing dust, which is pushed away from the comet by the solar wind.

"The comet is still active, even at this distance," says ESO's Richard M. West, of Garching, West Germany. ESO astronomers expect to make further observations in early 1989 and in 1990, when Comet Halley will be 1,900 million kilometers away. "Not only may such observations provide a firmer basis for the determination of the rotation $(period$) of the nucleus, they also carry cometary research further into the outer reaches of the solar system than ever before attempted," he says.