Remnant of Solar System’s Birth : Halley’s Comet Still Holds Mysteries for Scientists

The spaceships are gone now, yet some of the mysteries remain.

The last opportunity for at least three quarters of a century to get a close look at Halley’s comet has passed, and five sophisticated robots are now wandering around the solar system with nowhere in particular to go.

Whether any will be useful again remains to be seen.

HalleY gave them QuitE a struggle, even sending dust particles blitzing more than 5,000 miles to hit the two Soviet probes that had stayed what was presumed to be a safe distance away. Many scientists thought that Europe’s Giotto was on a suicide mission in its sweep to within 375 miles of the comet’s nucleus Thursday night. The 10-foot-tall probe sustained extensive damage as tiny dust particles ripped through with such fury that they tore off the thermal blankets that protect the craft’s instruments from the heat of the sun.

During the days and weeks ahead, Soviet, European and Japanese scientists will have to decide if they want to pay the price for manpower and equipment to track and control the probes, or if perhaps the equipment should be left to drift through the solar system as dying embers of the Battle of Halley, 1986.

Much of the data sent back by the probes amounted simply to confirmation of what had already been known. A large poster plastered on the walls of the European Space Operations Center here features a painting of what scientists thought Halley would look like if they could get a close look.

The poster bears such a striking similarity to the most dramatic of the color photographs sent back by Giotto that some spectators were wondering if Halley came from Central Casting for a script that had already been written.

However, the fact that the poster was so near the mark shows what has happened in cometary science in the last four or five years. Ray Newburn of Pasadena’s Jet Propulsion Laboratory, co-chairman of the International Halley Watch, said comets have been watched more during the last few years than “in all the history of cometology.”

Solar Backwater

“Halley triggered it all,” Newburn said in an interview here. “Comets were the solar system’s backwater until Halley came along.”

The reappearance of the comet once every 76 years has ensured its place in history, although this year its performanCe wAs quite short of star quality.

The Earth even passed through the tail of Halley in the comet’s apparition of 1910. In some past appearances, the comet has come close enough to the Earth that its tail stretched from horizon to horizon, adding volumes to the folklore about the comet.

Giotto added another chapter to that.

Data sent back by Giotto showed that Halley, which at times has been the brightest object in the skies, has perhaps the darkest center in the solar system.

That such a bright object should in fact have such a dark soul is perhaps more a mystery for poets than for scientists, but even the stiffest of academicians who were here for the encounter embraced the findings like small children with new toys.

Much Older Carbon

To the scientists, the dark mantle meant that the surface of Halley is covered with carbon that is probably much older than that commonly found on Earth, dating back to the origin of the solar system.

“It may be older than anything we’ve seen,” Newburn said.

That is why scientists have shown such interest in Halley. It offered them a chance to examine a pristine remnant from the birth of the solar system.

But each of the five spacecraft saw a different Halley.

The Japanese probes were so far away that the nucleus was hidden beneath a cloud of dust and gas.

The Soviets’ two probes got much closer, but the comet had changed between the first and second probes. The atmosphere around it was less dusty during the second Soviet encounter, and a different side of the comet was exposed to the sun, whose radiating energy gives it the life that has made it such a marvel.

By the time Giotto got there, the atmosphere was even less dusty, although Giotto penetrated the cloud so much farther that it encountered much more dust than both the Soviet probes combined. Giotto also saw a different area on Halley when the spacecraft and the comet passed at 150,000 m.p.h.

Comparing Results

Scientists from all the participating countries will study their data during the coming months, comparing their results with other experiments.

The Soviets and the Europeans will put their photographs together and will try to construct a three-dimensional model of the comet.

In late October, they will meet again in Germany to compare notes and to try to determine just what all this time and money and energy has brought forth.

By then, Halley will be well on its way back out to the cold countryside of the solar system, where it will remain until the year 2061, when it will once again command center stage.