A US sun-monitoring satellite, which was taken out of its assigned orbit in September, now is on its way to meet a comet.
It should reach Comet Giacobini-Zinner in September 1985, about half a year before European, Japanese, and Soviet probes close in on Halley's Comet. If the rendezvous is successful, it will be the first time that Earth-bound scientists have been able to inspect a comet at close range.
This is a welcome prospect for US planetary scientists disappointed at being denied a Halley's mission of their own. But it is a loss for some solar scientists who relied on the satellite as a valuable tool for monitoring the flux of radiation, particles, and magnetic fields flowing from the sun toward Earth.
Contrary to some news accounts when the satellite's diversion was first reported, a successful mission will not make the United States the first nation to establish a cometary contact. The satellite, called ISEE-3 (International Sun-Earth Explorer), carries an international payload with instruments for which European as well as US scientists are responsible.
The main import of the diversion lies in the determination of US solar-system scientists to continue exploration even though their once magnificent planetary program is at a nadir.
ISEE-3 is one of a trio of satellites launched in 1977 and 1978 to carry out an international program of sun-Earth exploration. ISEE-3 was put in a solar orbit in which it remained between Earth and the sun. Thus it could monitor the solar wind of particles and magnetic fields coming from the sun.
Taken together, the three satellites were a powerful system for studying the interaction of Earth's magnetosphere with the solar wind. However, at the US National Oceanic and Atmospheric Administration's (NOAA) Space Environment Laboratory at Boulder, Colo., Dr. G. R. Heckman and his associates also found ISEE-3 a unique aid in forecasting such effects as magnetic storms on Earth.
''We would have liked to see it (ISEE-3) remain,'' Dr. Heckman says. ''It is a real il17l,0,16l,5poperational loss. That was one of the best pieces of data-gathering equipment we've had come along. . . .
ISEE-3 experimenter W. Jeffrey Hughes of Boston University agrees that the NOAA scientists have lost a useful tool. However, he explains that, from a research point of view, the ISEE system has already done much valuable work. Now there is far more important new knowledge to be gained by sending ISEE-3 to Comet Giacobini-Zinner. This includes knowledge of the magnetosphere and interplanetary space, as well as of the comet.
Although ISEE-3 was never intended for such a voyage, it has enough propellant in its maneuvering rockets to put it on course for Giacobini-Zinner.
ISEE-3 will travel far down the tail of the magnetosphere that is stretched out behind Earth by the solar wind. ''It will travel four times farther down the tail than any probe has gone before,'' Dr. Hughes notes. Then, as it cruises to meet the comet, ISEE-3 will sample the solar wind in interplanetary space. NASA is making 50 percent of the time of its Deep Space Tracking Network available for this research - an assignment Hughes says is a big bonus for solar system research.
ISEE-3 will sample particles, gases, and electric and magnetic forces in the comet's tail. It may come within a few thousand miles of its nucleus. This will give scientists their first direct measurements of material boiling off a comet nucleus as it heads toward the sun.
Later, ISEE-3 will make similar measurements with Halley's Comet. Hughes explains that, unlike the European, Japanese, and Soviet probes, ISEE-3 cannot take pictures. These other probes will pass on the sunward side of the comet to get the best views. But ISEE-3 will pass behind it and sample the comet's atmosphere - the only probe to do so.
Diversion of ISEE-3 for this mission makes little demand on NASA's strained budget. The direct cost should be under $2 million. The real cost-benefit decision was that of priorities. The fact that most of the scientists involved, including the European partners, favored using ISEE-3 for this purpose shows the value they place on learning more about the comets.