Halley's on View

By Joseph Wisnovsky

As Halley's comet began its swing around the far side of the sun in early January, it disappeared from view, not to be visible from earth again until after it emerged from the sun's glare in mid-February. Unfortunately for earthbound observers, it was during that unseen passage that Halley's put on its most dramatic display so far. As the comet neared its Feb. 9 perihelion, its closest approach to the sun (about 55 million miles), the searing solar rays caused increasing amounts of material to evaporate from its icy surface. Eventually the comet's enveloping gas cloud, or coma, grew to more than 12.5 million miles across, temporarily becoming the largest object in the solar system.

In fact, Halley's spectacular show did not go entirely unobserved. Last week scientists at NASA's Ames Research Center in California proudly displayed computercolored ultraviolet images of Halley's hydrogen coma as it appeared between Feb. 2 and 5, and described the changes in the comet during its most active period.

The data used to create the image had arrived in California by an unusual route. Looking for ways to view Halley's comet at perihelion, Ames scientists had hit upon the idea of using the Pioneer 12 spacecraft, which has been orbiting Venus since December 1978, surveying the planet with an array of instruments. Around the time of Halley's perihelion, they realized, Venus--and thus Pioneer--would be in position to have a direct view of the comet. Late in December the scientists ordered the spacecraft to pivot 90 degrees and point its ultraviolet scanner at the comet. It has been transmitting Halley's data to Ames ever since. Says Ian Stewart, chief scientist of the Pioneer- Halley's project: "It's a gift from the gods, being in the right place at the right time." Pioneer has also determined the rate at which Halley's loses water as its icy surface is evaporated by solar radiation. Early in January, with the comet 93 million miles from the sun, the loss was about 12 tons a second. A month later, as it approached perihelion, the loss increased to 40 tons a second, and has since varied between 30 and 70 tons. Stewart estimates that the loss of surface ice causes Halley's to shrink 20 ft. to 30 ft. in diameter each time it passes the sun. At that rate, he says, the comet's nucleus, now about four miles in diameter, will swing close to the sun hundreds of times before Halley's disappears forever.

Pioneer will continue to observe Halley's, measuring water loss and looking for oxygen, carbon, sulfur and other elements in the coma's gases, until March 6, when the sun will begin blocking the Venusian view of the comet. On that day, however, the first of an international flotilla of spacecraft will take over Halley's vigil. The Soviet probe Vega 1 will fly through the coma, passing within 6,000 miles of the nucleus. It will be followed by another Soviet craft, two Japanese probes, and the European Space Agency's Giotto, which will make the most daring pass of all. On March 13, Giotto will swoop within about 300 miles of Halley's nucleus and--if it survives the encounter --transmit the first close-up pictures of a comet's nucleus ever seen by man.

With reporting by Paul A. Witteman/San Francisco