NASA trying to fix power glitch in Hubble spectrograph Ultraviolet-wave studies affected

September 04, 1991|By Doug Birch

NASA engineers are trying to outwit a new glitch in the star-crossed Hubble Space Telescope, an intermittent power failure that afflicts one of the most productive of the telescope's five scientific instruments.

Because of the problem, the National Aeronautics and Space Administration has stopped using one of two ultraviolet detectors housed in the telescope's Goddard High Resolution Spectrograph. This postpones indefinitely about 30 percent of the research planned for the spectrograph.

It's behaving exactly the way my dryer at home has been working recently," said Doug Duncan, an astronomer at Baltimore's Space Telescope Science Institute, the center for Hubble research. "You throw your clothes in and turn it on and you never know whether you're going to have perfect results or soggy results."

The power problem has not affected the Hubble's four other instruments, including its two cameras, its Faint Object Spectrograph and its sensitive light meter, called the High Speed Photometer.

Joe Rothenberg, NASA's associate director of flight projects for Hubble, said there was still a chance that experts at NASA's Goddard Space Flight Center in Greenbelt would be able to fix the telephone booth-size Goddard spectrograph, located behind the Hubble's main mirror, with commands radioed from the ground.

"Engineers are still examining all options," he said. "They've been known to find ways of operating things in the past that were originally not envisioned."

But if they can't, he said, repairs will probably have to wait until a scheduled 1997 shuttle mission. A replacement instrument, he said, couldn't be built in time for the shuttle's first Hubble repair flight, due in November or December 1993.

The instrument's troubles are only the latest for the Hubble, which has not been able to do some of the groundbreaking research it was designed for because of an error in the grinding its 94.5-inch primary mirror. The telescope, launched in April 1990, is also plagued by vision-jarring "jitters" that occur as its solar power panels heat up and cool down.

Two of the six gyroscopes that keep Hubble pointed at celestial objects have malfunctioned, and a third threatened to do so -- although so far, the telescope is still able to lock onto its targets.

The Goddard spectrograph was designed to study faint ultraviolet emissions given off by very bright objects such as active galaxies and supernovae.

It recently gave scientists their first look at huge clouds of intergalactic matter that may be the flotsam of the early universe.

Ultraviolet waves, like visible light, are part of the electromagnetic spectrum, and astronomers use them to establish the presence and proportion of certain elements in stellar objects.

This type of research cannot be done on Earth because the atmosphere filters out all ultraviolet light except a fraction of that produced by the sun.

Mr. Duncan and David Lambert of the University of Texas are using the Goddard spectrograph to study invisible ultraviolet emissions from some of the oldest stars in the Milky Way.

Mr. Duncan tried to use the Goddard spectrograph over about a 12-hour period Monday. But he said engineers could keep it working for only about six hours.

Because of the telescope's tight schedule, he said, "There's no possibility of saying, 'Oh, well, that didn't work. Let's try something else.' " The research time is lost.

The spectrograph's problems began July 24 when a test lamp in the instrument blinked erratically and all data switched off.

NASA re-established contact with the instrument. But on Aug. 5, similar problems developed and engineers began to suspect that the spectrograph's main power source was flickering on and off.

NASA engineers concluded they could control the problem as long as they didn't use too much power.

So they decided not to employ the Goddard spectrograph's "Side 1" detector, which can register ultraviolet waves between 100 nanometers and 170 nanometers in length. (A nanometer is one-billionth of a meter.)

Instead, all research has been shifted to the "Side 2" ultraviolet detector, which is sensitive to a slightly higher part of the ultraviolet spectrum, waves of from 120 nanometers to 320 nanometers in length.

Mr. Rothenberg said yesterday that power to Side 2 was still blinking off sporadically for periods ranging from 15 minutes to several hours.

"It's as if something that you owned, like your computer or your TV, sometimes worked and sometimes didn't," Mr. Duncan said. "What you'd suspect is that you had a bad connection in there." A NASA document suggests that electronic switches in the spectrograph's power supply may have worn down with use, like points in a car's distributor.

Engineers are trying to decide whether to switch to a backup power supply for Side 2, Mr. Rothenberg said. But there is a risk that the backup may not work. "It's something that you just don't do lightly," he said.

If the backup supply is switched on, Mr. Rothenberg said, Side 1 could no longer be used.

Some astronomers say the Hubble's most interesting work so far has been performed by the Goddard High Resolution Spectrograph and its Faint Object Spectrograph. The August edition of the Astrophysical Journal focused exclusively on research conducted by these two Hubble instruments.

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