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Technology: Salvaged satellite will work better than ever

作者:尉迟乱瘩    发布时间:2019-03-03 07:08:04    

By BEATRICE LACOSTE in PARIS A European satellite launched in 1989 to map the positions of hundreds of thousands of stars was thought to be a dead loss after a faulty booster motor failed to lift it into its proper orbit. But last week, scientists from the European Space Agency described how a mixture of luck and clever improvisation will allow the Hipparcos satellite to complete its original mission and probably do even better. The original mission was to establish two catalogues. The Hipparcos catalogue will be a ‘Who’s Who’ of 120 000 stars with an accuracy of two thousandths of a second of arc, 30 000 times as accurate as the naked eye. The less accurate Tycho catalogue was planned to include 400 000 stars. ESA believes Hipparcos will improve its accuracy for the Hipparcos catalogue and increase the size of the Tycho catalogue by lengthening the mission. The satellite fixes positions by masking repeated observations of each star, improving accuracy each time. So a longer mission means better positions. ESA has funds to keep Hipparcos operating until the end of this year. But it believes there is enough fuel in the satellite’s thrusters to last till the end of 1993, and hopes to be able to continue operations until then. Our galaxy has more than two hundred billion stars, and the Hipparcos mission will help astronomers unravel their dynamics. Michael Perryman, the project’s chief scientist, says, ‘Hipparcos will improve our knowledge of scale within the galaxy, and for this we can infer the distance of other galaxies.’ Hipparcos was meant to sit in geosynchronous orbit, following a fixed point above the equator. The booster failure left Hipparcos in a highly elliptical orbit, swooping to within 500 kilometres of the Earth and then soaring 36 000 kilometres into space every 10 hours. Following unsuccessful attempts to fire the booster motor to send the satellite into its proper orbit, there was talk of trying to shake the satellite to start the motor. But this might have damaged the instruments and it was decided instead to revise the mission. In its geosynchronous orbit, Hipparcos would have beamed all its data directly down to a tracking station near ESA’s control centre at Darmstadt in Germany. But the satellite now had a rapidly moving orbit that circled the Earth in 10 hours. Experts worked day and night to set up equipment to receive the data at three other tracking stations: Perth in Australia, Kourou in French Guiana and Goldstone, a NASA tracking station in California. With these stations they could keep in contact with the satellite 70 per cent of the time. The onboard software had to be reprogrammed by a team whose members came from Matra, which was the main contractor for the satellite, as well as from other companies and ESA. The engineers were also concerned that as the satellite swoops close to the Earth twice a day, protons and electrons trapped by magnetic forces in the Van Allen belts above the equator would wear out the solar arrays and shorten the life of the satellite. They found, however, that the arrays were degraded much more slowly than expected and they believe that Hipparcos will be able to operate for four years instead of its planned mission of two-and-a-half years. More hazards befell the satellite in March 1990 when the relative positions of the Sun and Earth conspired to leave it in shadow for long periods, depriving it of power. During the longest such eclipse, of 105 minutes on 16 March, Hipparcos scraped through with just five minutes’ power in reserve. Hipparcos collects 24 000 bits of data per second as it spins slowly around its own central axis in space. To date,

 

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