Giacobini-Zinner Targeted : U.S. Satellite Will Be First to Reach a Comet

Six months before the first foreign space probes reach Halley’s comet, an American spacecraft will pass through the tail of another comet in a snappy demonstration of scientific and bureaucratic derring-do.

The craft is an aging but dependable satellite launched in 1978 and recycled for the new mission over some strong objections within the National Aeronautics and Space Administration.

The mission to comet Giacobini-Zinner falls far short of the dreams of some scientists to send a probe to Halley, the most famous comet of all, but it does mean that the first spacecraft to reach a comet will be from the United States, not the Soviet Union, Japan or Western Europe.

Stationary Orbit

“We wanted to be first,” said Robert Farquhar of NASA’s Goddard Space Flight Center. He is one of three scientists credited with pulling off the adventure and the one who figured out how to get the craft out of its stationary orbit, rocket it around the moon and on its way to a rendezvous this September with the comet.

Some within NASA are furious over the fact that the International Sun and Earth Explorer was diverted from other duties and sent off to chase a comet, but Farquhar and his cohorts show no shame.

“We stole it,” said Fred Scarf, a top scientist at TRW and one the leaders of the rebellion.

The road to Giacobini-Zinner proved a little bumpier than Scarf had anticipated when he first tried to persuade NASA that Farquhar could make the moon do what the satellite’s small rockets couldn’t do on their own--sling the satellite out to the comet. What Scarf didn’t know until after NASA began to show real interest was that Farquhar wasn’t sure how to do it.

“Fred set me up,” Farquhar said.

Move Proves Fortunate

As it turned out, he set up the right man. Farquhar’s calculations have put the craft, renamed the International Cometary Explorer, right on course. About the only thing that remains to be seen at this point is whether the spacecraft will survive a treacherous high-speed flight Sept. 11 through the dust storm that follows the comet.

If it does, and Scarf thinks it might, it will continue on around the sun and return to the vicinity of Earth in 2012. And if that comes to pass, Farquhar wants to go out and get it.

The spacecraft was sent up seven years ago, loaded with instruments to study the sun. It came to rest in an area where the gravitational pull from Earth and the sun is equal. It remained suspended in that area, providing warning of solar disturbances that could affect Earth’s weather and studying a wide range of solar phenomena.

Meanwhile, in 1981, NASA scientists were drafting a plan to send a sophisticated spacecraft to Halley’s comet, furthering U.S. preeminence in space exploration.

“The scientists involved with planning for Halley were a little too ambitious,” Farquhar said. “They were more interested in the advancement of their technological capabilities than in getting to Halley’s comet. And they were coming in with projects that would cost almost $1 billion.”

To win support for their proposals, the Halley planners argued against a simple comet fly-by. But as their plans proved too expensive, NASA abandoned the idea of going to the famous comet.

Japan, a consortium of Western European nations and the Soviet Union all announced missions to Halley, meaning that the United States would be the only nation with a space program that would not be sending a probe to the comet.

Heavy Disappointment

That disappointment weighed heavily on many members of the scientific community, and in August, 1982, Scarf, Farquhar and Ed Smith of Pasadena’s Jet Propulsion Laboratory gathered with other scientists at Goddard to discuss the future of the International Sun and Earth Explorer, as it was known then. The scientists, who had experiments aboard the craft, were members of an advisory committee set up to guide NASA on the use of the spacecraft.

The three scientists were concerned over the wisdom of leaving the craft where it was, doing the same thing it had already done for several years, if there was something new that it could do--like intercept a comet.

Scarf told the dozen or so scientists that the craft could be sent around behind the Earth, where it could study materials that trail the planet as it revolves around the sun, giving the craft somewhat of a new mission.

Everybody seemed to like that idea.

Then he dropped a bombshell.

Figured Out Interception

Scarf said Farquhar had figured out how to get the spacecraft out to intercept Giacobini-Zinner, “an interesting comet” that passes the Earth every 6 1/2 years.

“What I didn’t know at that time was that Bob, at that point, didn’t know how to get us there,” Scarf said.

Farquhar agrees.

“I had talked to Fred on the side about this comet thing, but I didn’t have all the details worked out yet, and I really wasn’t very sure it could be done,” he said.

Farquhar said he was kind of “on the spot,” and he did not tell the advisory committee that he did not know how to do it.

The committee voted overwhelmingly against it, however, because most of the instruments aboard the craft were designed to study the sun, not comets, and would not be very useful in an encounter with Giacobini-Zinner. Smith, who said he found the idea irresistible, was put in charge of a committee to take the matter before the National Academy of Sciences for an independent assessment. The academy bought the Scarf-Farquhar-Smith concept, and the recommendation of the advisory committee was rejected, something NASA rarely does.

The cometary race was on.

‘Caught Up in Spiral’

“I kind of got caught up in the spiral,” said Farquhar, who still was not sure it could be done. “I knew I was going to look kind of stupid if it didn’t work. We tried several things and none of them worked” because, he said, the moon and the Earth were not in the right position relative to each other.

“The phasing is very difficult,” he added. “In order to get out to the comet, I had to pass the moon at just the right angle at just the right time at just the right speed.”

The idea was to pass the moon close enough to use the lunar gravitational pull as a slingshot, whipping the craft out toward the comet. Using sophisticated computers and models of the solar system, Farquhar finally determined that it would work if he fired the spacecraft’s rockets several times on “five lunar swing-bys.”

The spacecraft’s rocket motors, built by TRW, were not designed as primary propulsion engines. They were designed to keep the craft on station as it studied the sun, and they are much smaller than the engines used to loft satellites. Still, they proved sufficient for the task. In January of last year, the spacecraft rounded the moon and headed out for its encounter with Giacobini-Zinner, and it was given its new name.

Several instruments aboard the spacecraft are ideally suited to study the comet. Among them are Scarf’s space plasma equipment, which should tell scientists more about the nature of a comet’s tail, and Smith’s magnetometer, which will study the comet’s magnetic field.

But the craft does not carry the full range of equipment found aboard foreign vehicles that were designed specifically for cometary exploration. There are no cameras aboard, for example, so there will be no pictures from the spacecraft of Giacobini-Zinner.

Can Fly Through Tail

That does not worry people like Scarf, because it means the craft does not have to remain on the sunny side of the comet and is free to fly through its tail instead, gathering information that will be denied to the Halley probes.

What does worry just about everybody is the lack of protection for the spacecraft. By contrast, the European Halley probe, which was launched last month, is protected with a double shield of the same material used to make bulletproof vests.

When the American craft reaches Giacobini-Zinner, it should be able to collect much data about the comet’s tail, but the craft uses wirelike antennas that stretch out in several directions nearly the length of a football field. If those are sheared off, most of the data will be lost.

Giacobini-Zinner’s tail apparently includes heavy dust particles. Twice in this century the Earth passed through the comet’s tail, and each time the comet “gave us some of the most intense meteor showers in history,” Farquhar said.

Dust particles could damage the craft’s solar cells, or larger particles could simply bump it so that its antenna no longer points toward Earth.

“I think there is a strong possibility that we may not make it through the encounter,” Farquhar said. “It doesn’t take very much to wipe out a spacecraft that doesn’t have a shield on it. This is very risky.”

Farquhar said he hopes “we will get data at least halfway through” the tail.

The craft should eventually meet Halley, but it will be nearly 19 million miles away at its closest approach.

By then, it should be a veteran, the first man-made instrument to have visited a comet.