Comet data confirm some expectations, unveil surprises. Results support `dirty-snowball' theory of comet composition
Data produced by the International Comet Explorer as it passed through the tail of Comet Giacobini-Zinner a few days ago show some expectations fulfilled and some surprises. Results indicating an abundance of water are just what comet experts expected. NASA astronomer John C. Brandt called the detection of water molecules ``a pleasant confirmation . . . which fits in nicely with our picture of a comet as a dirty snowball.''
Other data, however, are unexpected and puzzling, as in the case of the high-energy particles that the comet seems to be emitting. Robert Hynds of Imperial College in London, England, explained that solar system physicists had not suspected comets to be a source of such cosmic rays. He called the discovery ``very exciting, scientifically.'' But, he added, ``It's going to take us some months before we get a hard grasp on this.''
At a conference summarizing the International Comet Explorer's (ICE) findings for the press, Dr. Brandt said: ``The results we've seen have exceeded my expectations!'' At this stage, however, ``We're all trying to understand what we saw,'' added Samuel Bame of the Los Angeles National Laboratory.
What the ICE scientists actually ``saw'' during the comet encounter last Wednesday morning was a stream of data from a battery of instruments. These measured magnetic fields, electric currents, the quantities, types, and velocities of electrically-charged molecules called ions, and waves traveling through interplanetary medium and through the substance of the comet itself.
The European-American scientific team has been working hard with these measurements to sketch a picture of the comet as it appeared when the spacecraft zipped through its tail, coming within 8,000 kilometers (about 4,970 miles) of its nucleus. It was this ``quick look'' interpretation that some of the ICE scientists presented in their press briefing Friday. Edward Smith of the NASA Jet Propulsion Laboratory warned that the team had yet to get down to the hard work of ``trying to understand the physics''
of what was going on 70 million kilometers from Earth.
One of the most puzzling scientific questions, at this point, asks whether or not Giacobini-Zinner is generating a bow shock. Just as water piles up before a speeding boat or air piles up before a supersonic aircraft in a sonic-boom-generating shock wave, so did some scientists expect the solar wind of particles to pile up in a bow shock in front of Giacobini-Zinner. ICE went through the region where the shock is theorized to exist. But though it was expected to settle the question, data, thus far, have
only baffled the experts.
Some instruments that should have detected a shock wave didn't ``see'' any. Other instruments reported what may, indeed, be a bow shock. But, if it exists, it is quite unlike any interplanetary shock wave yet encountered.
Fred Scarf of TRW, Inc., whose instruments recorded wave phenomena within the range of sound, played a recording of what ICE ``heard.'' It is a weird, drawn out sound of rising intensity which could well serve as background for a Steven Spielberg space fantasy scene. ``This is not a normal bow shock where you just have a big bump and your're in,'' Dr. Scarf observed.
Edward Smith's magnetic sensors were among the instruments that did not ``see'' a bow shock. But they did give a good picture of the magnetic features associated with the comet.
Dr. Smith explained that a magnetic field lends structure to a comet. The electrically-charged ions in a comet's head and tail are grasped by magnetic force and organized into distinctive structures, such as the tube-like tail itself. A comet has no magnetic field of its own. It gathers its field from the magnetic force lines that lace the solar wind. Most of these originate in the Sun.
Dr. Smith said his data show the comet to be fairly efficient in gathering in such magnetic field lines. They drape around it like limp spaghetti hanging over a fork, he said. Noting that a comet can't accumulate magnetic field lines without limit, he reported evidence that excess field lines are sloughed off and move away with the solar wind downstream of the comet.
Commenting on ICE's findings, especially the surprises, John Brandt observed that ``cometary physics has fundamentally changed.'' From now on, he said, theories of what a comet is like, ``will have to confront reality.''
Meanwhile, ICE, undamaged by its encounter is now heading for its next station upstream of Comet Halley where it will monitor the solar wind as a squadron of five Japanese, European, and Soviet probes inspect Halley next march.