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Hubble still shaking despite software 'fix.' (space telescope)

Hubble still shaking despite software 'fix'

During each 96-minute trip around Earth, the Hubble Space Telescope suffers two bouts of uncontrolled jiggling. To compensate for these unsettling episode--which last for about 6 minutes each and thus encompass one-eight of the telescope's potential viewing time -- engineers radioed up a package of new computer instructions on Oct. 15. But the software failed to steady the craft, and engineers deactivated the package two days later.

The jitters result mainly from oscillations of Hubble's electricity-providing solar panels. Each time the panels move between sunlight and darkness, their thermal expansion or contraction initiates a "solar twang," explains astronomer H. John Wood of NASA's Goddard Space Flight Center in Greenbelt, Md.

The telescope's other technical setback--a misshapen primary mirror -- creates focusing problems that lead to fuzzy images (SN: 7/7/90,p.4). The jitters, however, interfere primarily with Hubble's ability to track guide stars -- extraterrestrial "signposts" that help orient the telescope for observations. At times, the guide-star images jitter about so rapidly that the telescope loses sight of them.

Engineers at Lockheed Missiles and Space Co. in Sunnyvale, Calif., the telescope's principal builder, spent months working on a remedial computer program with colleagues at NASA's Marshall Space Flight Center in Huntsville, Ala. (SN: 7/14/90, p.21). When the "software patch" didn't dampen the vibrations, they headed back to the drawing board.

The problem is that Hubble oscillates on several different frequencies -- primarily at 0.1 hertz but also at others between about 0.4 and 0.8 hertz. Ironically, when the remedial software performed its designated task of quelling the 0.1-hertz jitter, it also made the guidance system overly sensitive to the higher-frequency oscillations, says David J. Pine of NASA headquarters in Washington, D.C. And so, Pine says, "we have returned to the old software."

Hubble jiggles not only when its solar panels change temperature, but also on some occasions when its two tape recorders go on or off, or when researchers rotate the carousels that place different filters or prism-like diffraction gratings in front of various instruments. Moreover, the craft twitches slightly when a telescoping strut on a solar-panel mount sticks briefly and then releases, says Gerald S. Nurre, chief of the pointing-control systems branch at the Marshall center. The European Space Agency, which developed the solar panels, is considering using stick-resistant materials or lubricants in the new mounts for the replacement panels scheduled for installation in June 1993.

NASA officials now plan a "dynamics test" in which engineers will give the telescope's position-holding "reaction wheels" a precisely specified nudge and then measure the jitter response, Nurre says. The results should help researchers design a new software correction (or an adjustment to the previous one) to radio up to Hubble around Jan. 1.
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Author:Eberhart, Jonathan
Publication:Science News
Date:Nov 10, 1990
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