Peering into Halley's Heart

By Joseph Wisnovsky

As the Giotto spacecraft and Halley's comet raced toward each other last week at a closing speed of 155,000 m.p.h., the tension at the European Space Agency's control center in Darmstadt, West Germany, became palpable. Images of the cornet, relayed to the center at intervals of four seconds, loomed larger and larger on television screens, finally yielding by far the best look yet at an elongated shape near Halley's heart. It was the comet's nucleus. Giotto Investigator Wolfgang Schmidt, giving a play-by-play description of the images, could not contain his excitement. "It's obvious that there is structure on this nucleus," he exclaimed. "Mountains, hills, craters--incredible detail!" Suddenly the image froze and then, only two seconds before the spacecraft's closest approach to the comet, the screens went blank.

One of the worst fears of the international Giotto team, it seemed, had been realized. Fully aware of the dangers of meeting any outsize dust particles at the tremendous speed of encounter but determined to get a closeup look, the scientists had aimed the spacecraft to swoop only 338 miles from Halley's dust-shrouded nucleus. That, according to Roger Bonnet, ESA's director of scientific programs, was like playing Russian roulette: "You may survive, but one shot will kill you."

It was indeed a gamble. In the last seconds before the encounter, Giotto ran into what one scientist described as a "wall of dust the size of grains of sand." The spacecraft's protective dust shields were peppered with particles at a rate of 100 impacts a second, a bombardment that swung its antenna out of alignment with a tracking station in Australia. That brought communications to a halt. But before the blackout, Giotto relayed more than 2,000 images of Halley's back to earth, plus a torrent of data from the ten on-board instruments.

At the control center, cheers rang out and champagne corks popped. Then came the bonus. Half an hour after the screens blacked out, Giotto's signals were picked up again; except for the camera, all of its instruments were still working.

Although scientists will be interpreting Giotto's data for months to come, Horst Keller, principal investigator of the camera team, announced some preliminary results. Halley's nucleus appeared to be 9.4 miles long and at least 2.5 miles wide, he said, and the surface, "velvet black and very irregular, with an indentation in the middle, like a peanut or a potato." On one side of the nucleus were what appeared to resemble nozzles, spewing out one minor and two major jets of gas and dust. Keller was puzzled by the blackness of the nucleus, which suggested that there is little or no ice on its surface. Astronomer Fred Whipple, whose concept of a comet as a "dirty snowball" was apparently confirmed by earlier findings of the Soviet Vega probes, offered a possible explanation: the comet's surface might be covered by densely packed, extremely small particles embedded in an icy mantle.

Giotto's grand finale was preceded by the flybys of the second Soviet Vega and two Japanese craft. Early in the week, Vega 2 passed 5,125 miles from the comet's nucleus, sending back 700 pictures and confirming that the nucleus was solid. But the dust clouds encountered by the craft disabled nearly half of its solar panels and two of its experiments.

A day earlier, Japan's Suisei probe passed Halley's at a distance of 94,000 miles. Images from Suisei's ultraviolet camera, relayed to the Institute of Space and Astronomical Science in Tokyo, showed that Halley's huge hydrogen coma (radius: 6 million miles) appeared to brighten and darken in a cycle of 53 hours, which seemed to confirm other observations that the nucleus rotates about once every two days. The Japanese were surprised when Suisei recorded hits by two cometary dust particles, each more than a millimeter in diameter. Said Project Manager Tomizo Itoh: "It was quite a discovery that dust particles that size were traveling that far away from the nucleus."

As Sakigake, the other Japanese craft, passed 4.4 million miles from the comet, it detected sporadic "plasma waves" that were described by Hiroshi Ohya of Tohoku University as being "akin to the electric waves of lightning." The waves, which originated in a region between 250,000 miles and 625,000 miles from the comet's core, apparently result from the interaction of solar wind with the fluctuating outer edge of the cometary coma.

At week's end, as Halley's comet continued on its journey back toward the outer reaches of the solar system, not to return to earth's vicinity until 2061, many of its secrets had been stripped away. And more would be unlocked as researchers dug into the huge volume of data transmitted by the international flotilla of spacecraft. It had easily been, as one scientist said, "the greatest week cometary science has ever had or is likely to have for some time." --By Joseph Wisnovsky. Reported by James L. Graff/Darmstadt and Yukinori Ishikawa/Tokyo

With reporting by James L. Graff/Darmstadt, Yukinori Ishikawa/Tokyo