NASA craft to rendezvous with a comet

This Thursday controllers at the NASA Goddard Space Flight Center will tweak the orbit of ICE (pronounced ``icy'') to help it keep its late summer rendezvous with a comet. The International Comet Explorer, as the National Aeronautics and Space Administration (NASA) now calls the former Earth-oriented spacecraft, is due to pass through the tail of Comet Giacobini-Zinner at 7 a.m. Eastern daylight time on Sept. 11, coming within 10,000 kilometers (6,000 miles) of the comet's nucleus. If it hits this extremely small target and survives the dusty encounter, ICE will return the first on-site comet data ever gathered.

``Everything has to be right on the money'' for ICE to keep that epochal rendezvous, says project scientist Tycho T. von Rosenvinge. But he adds that he expects success. This kind of midcourse orbital maneuvering is something that Goddard flight controllers know very well how to do. In fact, Goddard calculations put the probability of hitting Giacobini-Zinner's tail at somewhat better than 90 percent.

Thus NASA and its project partners in the European Space Agency (ESA) are preparing for what will likely be a valuable preview of a comet flyby half a year before the more publicized encounter of European, Japanese, and Soviet probes with Halley's comet.

Von Rosenvinge calls the comet-probing attempt ``an interesting gamble.'' ICE was not intended to be in a dusty and possibly high-radiation environment and is not properly shielded. Yet, of all the comet probes now on their way, ICE is the only one equipped to probe a comet tail. The Halley probes have imaging equipment to send back pictures. They can't look at the comet against the sun. They have to pass on the sunward side. ICE, which is instrumented to measure magnetic fields and particles, is well equipped for comet tail study. It just isn't specifically toughened for that perilous task.

Nevertheless, the seven-year-old spacecraft still is in good shape with plenty of maneuvering fuel on board. Von Rosenvinge says that ICE is ``following in the same tradition of some of the early IMPs.'' These Interplanetary Monitoring Platforms have also been unusually long-lived. IMP-8, launched in 1973, is ``still torquing away,'' returning good data as it orbits Earth at a distance of some 40 Earth radii, von Rosenvinge says.

If ICE does survive the Giacobini-Zinner encounter, it should go on to help the other probes when they intercept Halley's comet. ICE will be between the comet and the sun. It can monitor particles and magnetic fields sweeping out from the sun past the comet and help Halley probe scientists interpret what their instruments detect. All told, the comet-related mission could give ICE a more significant place in scientific history than its designers, generally, had imagined. Von Rosenvinge says he suspects that chief mission planner Robert Farquhar secretly cherished a dream of doing this all along. But that would have been considered overly ambitious a decade ago.

ICE is a spacecraft that has had three missions -- two of which were scarcely envisioned when it was launched in August 1978. Then it was known as the Third International Sun-Earth Explorer (ISEE-3). It was the upstream solar wind monitor for two other ISEE craft in a program to study the interaction of Earth's magnetic field with the ``wind'' of particles boiled off the sun's surface that flows throughout the solar system.

ISEE-3 kept station between Earth and Sun at one of the so-called libration points. Here -- at a distance of some 240 Earth radii from the planet in the direction of the sun -- the gravitational pulls of sun and Earth balance out. ISEE-3 could orbit the Sun with the same one-year period with which Earth also orbits. Planet and spacecraft kept pace with each other, maintaining their relative positions.

It is an unstable arrangement. So to correct for any launch errors and to keep on station, ISEE-3 carried a generous amount of hydrazine fuel for its maneuvering rockets. Launch errors were minimal and station-keeping turned out to be less demanding than expected. Thus, after the ISEE project had gathered considerable valuable data, project officials realized they had enough maneuvering fuel to undertake more ambitious exploration. In fact, they could seriously consider what had once seemed the impossible dream of sending ISEE-3 to look at a comet.

The spacecraft itself was in good shape in spite of having only a two-year designed lifetime. Moreover, the moon could help change the craft's orbit. ISEE-3 was given two new missions. First it would explore the long tail of Earth's magnetosphere -- the region where Earth's magnetic field dominates and holds energetic trapped particles. The solar wind drags this region into a long tail. ISEE-3 was sent to probe that tail out to distances of over 200 Earth radii where no data had ever been taken before. Then it would head for the Giacobini-Zinner comet.

In all, five swingbys of the moon were needed. The moon pulled the spacecraft onto a different trajectory each time, giving it a boost on its way. During the last swingby on Dec. 22, 1983, which sent the craft toward the comet, ISEE-3 came within 120 kilometers (75 miles) of the lunar surface. To skim that closely without crashing or entering a wrong trajectory required a precision which von Rosenvinge likens to ``jumping off a high platform into six feet of water.''

But ISEE-3 made it and became ICE, the comet explorer. Whether the 50-50 gamble of successfully probing Giacobini-Zinner's tail pays off or not, the spacecraft has already returned a handsome extra dividend in exploring Earth's magnetotail. And if the comet probing does succeed, the hard-working spacecraft will turn out to be one of the most unexpectedly productive investments in space exploration that NASA and ESA have made.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.