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First to a comet: 'a long, wonderful night'.

The first visit to a comet by a man-made object resulted last week from a cleverly designed and remarkably complex series of maneuvers carried out by a spacecraft originally created--and used--for a totally different purpose, studying the sun. The comet mission's intricacies had been widely publicized, including the fact that the U.S.-built probe would reach a comet called Giacobini-Zinner six months before a group of Soviet, European and Japanese craft was due at Comet Halley. Less mentioned, however, was the worth of the science that the little U.S. probe, called the International Cometary Explorer (ICE), would be able to perform when it arrived, carrying no cameras, nothing designed to study a comet's dust. But when the counter was finished, Edward Smith of Jet Propulsion Laboratory in Pasadena, Calif., was not atypical of the mission's scientists in dubbing it "an unqualified success."

Early on the morning of Sept. 11, at about 46,000 miles an hour, ICE hurtled toward the comet, aimed to pass through the comet's tail about 5,000 miles "downstream" from the icy nucleus. A major question was whether the craft, or at least its power-generating solar panels, would be damaged by dust streaming back from the nucleus as it was released from the vaporizing ice. But ICE survived virtually unscathed. The result was valuable information for officials of the various Halley-bound missions, whose craft will not arrive until next March but who followed the encounter from the ICE control center at NASA's Goddard Space Flight Center in Greenbelt, Md.

Lacking formal dust detectors, ICE nonetheless offered at least five possible indicators--only one of which appeared initially to show any dust at all. A plasma-wave detector operated by Frederick L. Scarf of TRW Systems in Redondo Beach, Calif., recorded pulses believed to represent little ionized clouds formed by dust particles vaporizing as they hit the spacecraft. The pulses seemed after very preliminary analysis to be occurring only about once a second, and other symptoms such as wobbling of the spacecraft or degradation of its solar panels were completely absent.

Some scientists had thought that with no cameras, the blind spacecraft's first awareness of Giacobini-Zinner might be its detection of radio waves possibly generated in the comet's tail. But Jean-Louis Steinberg of the Paris Observatory reported "no evidence of any radio emission before we reached the comet."

Instead, the honor went to an instrument that project scientist Tycho von Rosenvinge of Goddard says was once rated least likely to make a contribution among the seven sensors to be used during the encounter. A particle detector in the charge of Robert Hynds of Imperial College, London, it first spotted the signs when ICE was still about a million kilometers from its goal.

"What we really expected to see," says Hynds, "was some very low-energy particles, possibly very close to the tail of the comet--I suppose you might say we expected to see a very pale imitation of the earth's magnetosphere. And what actually happened was a considerably surprise." Beginning hours before the encounter, the instrument detected the presence of highly energetic ions, and the readings continued afterward for an even longer time. "when it was proposed to send ICE to a comet," Hynds admits, "I must say that, like Dr. Steinberg, I had some doubts as to the merit [of undertaking] the operation. I'm very glad they were overcome."

Hynds's findings may also be of particular interest to the mentors of the various Halley missions. His instrument was picking up signs of Giacobini-Zinner's presence over a span of more than 42 hours--representing a domain "far bigger than any of us expected," says Scarf--and one or more of the Halley craft may be limited in the time during which all their instruments can operate at once. The reason, on the other hand, has been the anticipated need to depend on battery power in case the solar panels are damaged, and ICE's solar panels showed no power reduction at all.

One surprise lay in ICE's answer to whether a comet would have a magnetic "bow shock," formed where the comet blocks the incoming, supersonic flow of ionized gases called the solar wind. Advance speculation ranged from "yes" to "no," but ICE initially appeared to show evidence of both possibilities. During the approach to the comet, Scarf's instrument detected high-frequency electrostatic waves of the sort familiar from spacecraft studies of planets such as Jupiter and Saturn, where bow shocks are clearly present.

On the other hand, Smith's magnetometer failed to show what in the case of such planets would be a clear-cut jump in the strength of the magnetic field. "We don't see evidence for a bow shock," he said after the encounter. The emissions recorded by Scarf, some of which were audible over loudspeakers as a sort of gentle gurgle, at first intensified as expected, but then failed to climax in a pronounced "whomp!" as ICE penetrated the region where the shock wave may have been.

Samuel Bame of Los Alamos (N.M.) National Laboratory (whose electrostatic analyzer had also detected the comet hours in advance) also got readings near the presumed shock region--but instead of a clear "signature," he says, "we saw something very different from what we see when we go through a shock." Instead, he describes a region of great turbulence in the electrons his sensor was measuring. Says Bame, "I donht know whether it should be called a shock at all."

As for what the comet is actually made of, Keith Ogilve of Goddard was not particularly surprised at his early results. "We confirmed the conventional wisdom that the 'water group' [ions such as H.sub.2.O.sup.+ and H.sub.3.O.sup.+, with mass numbers of 18 and 19] would be the most common ionic group that we would see. We believe that there is probably CO.sup.+ there [carbon monoxide, a probable part of an observed concentration of ions with mass numbers of 28 to 32]."

Another concentration was observed in ions of mass numbers 23 and 24, yet to be interpreted, but Ogilve was pleased after a mere glance at his data by how well his instrument performed. Prior to the Giacobini-Zinner encounter, Ogilve later told SCIENCE NEWS, he was worried that if the solar wind started moving too rapidly, it might accelerate the ions until they were beyond the device's measuring capability. Instead, he says, "we couldn't have done better if we'd known about it [the solar wind velocity] in advance." During ICE's three-plus-hour trip across the comet's tail, the instrument completed seven sweeps through its full mass-measurement range, the first direct samplings of a comet's composition.

Before and during ICE's encounter, attempts were also made to observe Giacobini-Zinner from at least three other spacecraft at widely differing locations: the International Untraviolet Explorer circling the earth, the Pioneer Venus Orbiter (PVO) around Venus, and the latest member of the fleet bound for Comet Halley, Japan's Planet A, launched on Aug. 18. All (except ICE itself) were equipped to record Giacobini-Zinner's ultraviolet hydrogen emissions, promosing measurements of how much water was actually coming off the comet due to the sun's heat.

The emissions appeared about twice as bright as expected, says Ian Stewart of the University of Colorado in Boulder, in charge of PVO's ultraviolet spectrometer. Combined with the data from ICE, he says, the result will be the first set of quantitative measurements linking a comet's sublimation (ice vaporization) rate with the ion densities and structure in its tail. The PVO instrument was able to monitor the comet for 10 hours, resulting in especially noise-free measurements and encompassing the whole span of ICE's visit.

There are no comet "experts"--far too little is known for anyone to merit such a term, even after ICE's pioneering visit. But according to John C. Brandt, chief of Goddard's Laboratory for Astronomy and Solar Physics, "our concept of cometary physics has fundamentally changed as of Sept. 11, 1985." Added Scarf after the encounter, "It's been a long, wonderful night."
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Title Annotation:International Cometary Explorer
Author:Eberhart, Jonathan
Publication:Science News
Date:Sep 21, 1985
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