Astronomers Scramble To Salvage Data From Hipparcos

CAMBRIDGE, ENGLAND—Four months after their $500 million satellite failed to settle into its target orbit, European space scientists are hoping to salvage as much as half of the data that they expected to receive from the Hipparcos mission. Their optimism is based on weeks of hard work modifying the hardware and software that control the astronomical observatory to mitigate the scientific impact of the satellite’s failure to reach its proper orbit. Ironically, the reduced flow of information from Hipparcos has made theirjob somewhat easier. “We have less data, so what we do have we can examine more carefully,” says astronomer Floor van Leeuwen of the Royal Greenwich Observatory. Van Leeuwen is one of more than 100 scientists from 13 nations in the European Space Agency (ESA) who have spent most of the decade preparing for the launch of Hipparcos. Its mission is to measure the position of nearly a half-million stars during a three-year sweep through the heavens. Van Leeuwen has spent eight years writing software for the mission. His payoff was to be the gathering of enough data to fill 10 high-density magnetic tapes each week, all beamed down to a ground station in Darmstadt, West Germany. But that plan had to be scrapped after the satellite’s booster rockets failed to lift it into an orbit 22,000 miles above the earth, where it would remain stationary above a single point. In fact, the malfunctioning booster rocket was thought to be so reliable that engineers did not include features that might have allowed them to diagnose the problem. Instead, Hipparcos is moving through an elliptical orbit that takes it anywhere from 130 to 300 miles above the earth. That places it out of sight of Darmstadt for long periods, which means less data flowing earthward. Swift Reaction But the scientists who created Hipparcos were determined not to be denied the fruits of their labor. Once the booster motor failed to lift Hipparcos into its geosynchronous orbit, a half-dozen technicians were sent to Perth, Australia, to prepare antennas at a ground station there. Within a few weeks, they had the site up and running. Their next stop was Kourou, French Guiana, where a third station stood ready to be activated. On November 6 that station went into operation, and scientists are now able to track the satellite during 80% of its orbit. If the team is successful in its attempt to add a station in Goldstone, Calif., the figure would increase to 94%. The next problem, according to ESA project scientist Mike Perryman, “was trying to work out the impact on the science program of having the satellite in completely unexpected orbit.” In the original plan, Hipparcos would have spun on an axis inclined at 42 degrees to the sun, working 24 hours a day to observe the sky in a crisscross pattern. From these observations, astronomers hoped to learn more about both the distances of individual stars and the scale of the universe itself. Scientists figured that Hipparcos could compile a basic star catalog in 18 months, and spend the rest of its mission adding important details. Its revised orbit forced them to change their thinking, however. Van Leeuwen and his team estimate that there’s an even chance that Hipparcos will last for at least 18 months. The big question mark is the ability of its solar panels to survive the radiation from the Van Allen belts, which it traverses every 10 hours m its new orbit. If the panels are damaged, the satellite could lose its source of electrical power. But initial pessimism about the panel’s durability has faded, and scientists are growing more confident that it can survive the deadly radiation. That revision is typical of the change in attitude among project scientists since the days immediately following the August 10 failure of the booster rocket. The anguished comments of scientists struggling to understand the signals from the crippled satellite generated media coverage that was uniformly pessimistic. Now that many of the problems have been overcome, some scientists are cautiously optimistic. One reason is the possibility of another mission. ‘The idea is that the combined results of the two missions will be so good that it would warrant a second launch,” says van Leeuwen. “With a spread of maybe six years, we could get the star motions and positions to a precision that would make our star catalog valid through the 21st century. But we still have to persuade ESA to launch a second Hipparcos.” Scientists say the next crucial test of the satellite’s staying power is a peak in solar activity, expected to occur in March. The increased activity will force them to temporarily break off communications, raising the possibility that they will not be able to resume the link. But most team members appear to be confident that the worst is behind them. Says Perryman, “We have moved from extreme pessimism to a reasonably optimistic outlook.” Simon Mitton , formerly a practicing astronymer, is now science director of Cambridge University Press.

Astronomers Scramble To Salvage Data From Hipparcos

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