For the first time in astronomical history, an instrumented probe has passed through a comet's tail. From now on, the study of comets will be made in the light of a growing body of knowledge derived from on-site investigation. With its data-transmission tone sounding like an owl's hoot, the International Comet Explorer (ICE) swooped down on comet Giacobini-Zinner at 6:53 a.m. Eastern daylight time Wednesday. Steven P. Maran, a senior scientist here at the Goddard Space Flight Center, pointed out that ICE actually went through the comet's head when it passed within 8,000 kilometers of its nucleus on the side away from the Sun.
NASA Administrator James M. Beggs promptly dubbed it ``one of NASA's prouder moments.''
He explained that, as the ICE data are digested ``we expect the mission . . . to enable us to get better use'' out of the Japanese, European, and Soviet probes that are to intercept Halley's Comet next March.
The part of the comet through which ICE passed turned out to be several times wider than expected. Instead of a four- or five-minute passage, ICE spent 18 minutes in this region, emerging at 7:11 a.m. EDT, giving scientists much more data than they originally hoped to get.
Surprisingly, ICE encountered little or no dust. Flight Director Robert W. Farquhar had warned that cometary dust might severely damage the spacecraft. He was especially concerned that dust would coat ICE's solar cells and rob it of electric power. However, the wavey white line that indicated the solar-cell current on the flight-control monitor screen remained rock-steady.
Obviously pleased that the spacecraft has survived, Dr. Farquahar observed, ``I guess this means we can plan on observing Comet Halley.'' ICE is in an orbit that will position it between the sun and that comet when the Halley's probes arrive next March. It then can provide important information on the solar-wind particles flowing outward from the sun past ICE and Halley's.
Farquhar added that he doesn't know what ICE's survival ``means for my recovery plan.'' He was referring to one of his dream projects to have a space shuttle retrieve ICE when its orbit brings it close to Earth in 2012.
Meanwhile, ICE scientists are ecstatic over the encounter's results. Dr. Edward J. Smith of NASA's Jet Propulsion Laboratory exclaimed, ``Mankind's first encounter with a comet has to be ranked as an unqualified success. The data are full of surprises.''
This is the kind of result scientists like. Data that do not raise new questions and open new avenues of exploration are much less interesting, Dr. Smith explained.
Fred L. Scarf of TRW Inc., one of the ICE's experimenters, reported that one of the surprises was finding a much higher level of activity than comet scientists expected. He said scientists generally picture a comet as relatively benign, cruising along and evaporating gases as it approaches the sun. However, his team's instruments found Giacobini-Zinner surrounded by a zone that was generating highly-energetic particles and a great deal of turbulence. From now on, Dr. Scarf said, scientists will have to picture comets as much more active bodies than they had imagined.
These and other findings that will emerge as the data are studied will keep scientists busy for many months if not years. Asked why such activity was not foreseen, Dr. Smith replied, ``Scientists aren't all that good at predicting. . . . We're a lot better at explaining things after they happen.''
He added that the spacecraft apparently made a very clean passage right through the area of the comet at which it was aimed. He said ``we certainly made the right decision'' in sending ICE on this mission.
Built and launched by the United States, ICE is a joint project of the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). Representatives of ESA are here, as are representives of Halley's probe teams. The latter want to learn all they can about comet probing before their own spacecraft reaches its target.
Referring to this, Beggs noted that Raould Sagdeyev, who heads the Soviet civilian unmanned space-research program is here. He is ``very welcome,'' Beggs added.
Academician Sagdeyev is unofficially discussing possible joint projects with US space scientists. Asked about the prospects for further US-Soviet space cooperation, Beggs said that for unmanned space research, they seem fairly bright. In particular, he said, ``prospects for an unmanned mission to Mars are probably pretty good.''
Such cooperation will be strengthened when ICE's solar-wind data are used to support the two Soviet Vega Halley's probes.
Speaking of Halley's, Beggs said that, personally, ``I regret that the United States didn't [itself] go to Halley's. . . . I think the money that would have been spent in doing it would not have been missed.