Comet Halley encounters earth's space age.

Comet Halley Encounters Earth's Space Age

After visiting the earth every three-quarters of a century or so for millennia, Comet Halley has at last been met by a reception committee. First came the Soviet Union's Vega 1 spacecraft on March 6, followed three days later by Vega 2, as Japan's Suisei and Sakigake looked on from a distance. On March 13 came the most daring attendee of all, Europe's Giotto, which passed only 605 kilometers from the comet's dusty nucleus. And as the unprecedented spacecraft assemblage reported in detail to its earthbound mentors, the guest of honor more than lived up to its reputation.

Scientists had long been aware that the reception might turn out to be more of a controntation. Besides the general dust hazard, earth-based observations had already shown a number of extended "jets" spurting forth from the nucleus, as the sun's heat freed concentrations of ice, dust and gas.

Furthermore, the Venus-orbiting Pioneer spacecraft, which in January had experienced the closest view of the comet's closest approach to the sun, had revealed that the amount of water vapor coming off of the nucleus did not vary smoothly with the changing heat input. Instead, it would double or drop by half from one day to the next, with hints that the variations could be even more rapid. "It turns on so suddenly," said Jeffrey Cuzzi of the NASA Ames Research Center at Moffett Field, Calif. "Like popcorn." Commected principal investigator Ian Stewart of the University of Colorado in Boulder (who is now refining his data to see how big the changes were from hour to hour), "There's a tremendous amount riding on that Giotto camera."

But even the Vegas, aimed to miss the nucleus by larger, safer distances and intended in part to provide targeting information for Giotto, turned out to have a couple of rough rides. Vega 1 passed 8,889 km away, yet particle impacts reduced the power output from its solar panels by 45 percent. Fortunately, the panels were there merely to charge the craft's batteries, which saved the day -- day design -- when the going got heavy.

Vega 2 went closer, flying by at 8,030 km from the nucleus and suffering an 80 percent power loss. Yet officials gathered at the Space Research Institute in Moscow were puzzled. Although Vega 2 had gone closer, data from dust detectors aboard the two craft (particularly a pair of instruments designed by John Simpson and colleagues from the University of Chicago and included at Soviet invitation) seemd to indicate that Vega 1 had flown through one of the comet's jets, while Vega 2 had not. Early looks at the data, in fact, suggested that Vega 2 had been struck by three to four times fewer dust particles than its predecessor. Aided by photos from the two craft, scientists concluded that by the time Vega 2 went by, the comet's nucleus had rotated so that the jet was no longer crossing the flyby trajectory. Yet there was more damage to Vega 2's solar panels, perhaps underscoring that a few well-placed impacts can sometimes do more damage than a whole dust storm. After all, the Pioneer and Voyager spacecraft that went to Jupiter, Saturn and Uranus passed through the asteroid belt, planetary rings and radiation belts with little or no damage.

A number of the scientific instruments on the Vegas suffered in the dust bombardment, but none was destroyed on both craft. Similarly, though Vega 2's infrared spectrometer never worked at all, apparently because of a leak in its cooling system, Vega 1's reported evidence of water, carbon dioxide and hydrocarbon molecules among its early findings. It also indicated temperatures within the comet's dusty coma to be about 330K [plus-or-minus] 20K. This was higher than had been anticipated by some researchers, suggesting at least the possibility that the excess might be due to heat retained by the enveloping dust, which Space Research Institute Director Roald Sagdeev termed a "dust cocoon."

Neither the Vegas nor Giotto detected any sharp magnetic shockwave, or "bow shock," where the solar wind was "blowing" at and around the comet. This appeared to confim expectations that Halley, and perhaps any comet, lacks an intrinsic magnetic field. Vega 1'st data, in fact, seemed to show at leaast a rough similarity to the findings from the ICE spacecraft that last Sept. 11 flew through the tail of Comet Giacobini-Zinner, According to Frederick L. Scarf of TRW Inc. in Redondo Beach, Calif. ICE had not been designed to go to a comet, so it lacked comeras and many other instruments of the sort carried by the Halley fleet. It did, however, enable scientists to compare another comet in some ways with Halley, and next week ICE will pass Halley about 28 million km from its sunlit side, allowing the same set of instruments to be applied to both objects.

The Giotto encounter was indeed the spectacular that scientists had hoped for. Limited by battery capacity, it was not turned on until 4 hours 19 minutes before its 605-km closet approach to the nucleus, and 71 minutes later it took the first of about 2,000 pictures of the huge coma. The real prizes, however, were its pictures of the nucleus, taken from as close as 1,530 km.

The nucleus appeared to measure about 15 by 8 km, making it not only more irregular than expected but also considerably larger than the roughly 3- to 6-km estimates of the past. From earth-based observations, the earlier estimates had been built on an assumed reflectivity for the nucleus -- if it was shinier, a given measured brightness would mean it was smaller. But as Giotto camera-team leader Horst Keller of the Max Planck Institute in West Germany descrined it, "There's no question that the true color of the nucleus is black, absolutely black, blacker than coal, almost like velvet." This did not mean that cometary nuclei no longer seemed to be "dirty snowballs," a term coined in 1954 by astronomer Fred Whipple (who was present for the encounter at the Darmstadt, Germany, control center). Far more likely is that near the sun, the "lighter," outer areas of exposed ice are vaporized more readily by heat than are those covered by dust -- leaving a more uniform blackness.

But through they still seem to be made of ice/snow, "ball" may be a less appropriate term tna just "icy conglomorates," composed perhaps of various fragments that may even come apart occasionally and then regroup as nondescript lumps. Or, as Paul Weissman of Jet Propulsion Laboratory in Pasadena, Calif., suggested even before the spacecraft flybys, loosely bound "rubble piles."

Giotto seemed to be surviving the dust onslaught unscathed until about 2 seconds before its point of closest approach, when it was suddenly dealt a blow -- whether by dust or chunk-sized rubble was initially unclear. Scientists at first thought Giotto's mission was over, as all their data disappeared. Was Giotto crippled? Destroyed? Neither. Some 34 minutes later, is date were being picked up again, as the spacecraft's "nutation dampers" succeeded in stabilizing what had only been an abruptly induced wobble and restored continuous communication with earth.

Besides taking pictures, instruments aboard the craft measured the comet's composition (reporting carbon, oxygen, hydroxyl and other constituents) and other characteristics, and sent the researchers back to their home institutions with a treasure trove of data like none they had ever had before.

Another result was the international cooperation that the event produced among Soviet, European, American, Japanese and other scientists -- which just might be one of Comet Halley's major legacies.