NASA's Hubble Space Telescope is now on duty minus one operational gyroscope.

The space agency announced August 31 that ground controllers have shut down one of the three operational gyros onboard the orbiting eye on the universe. Doing so is expected to preserve the overall health of that third gyro--thus extend the space observatory's science gathering through mid-2008, an eight-month extension.

Hubble Space Telescope (HST) gyroscopes are critical to running the Earth orbiting facility's complex pointing control system. That system maintains precise pointing of the telescope during science observations.

NASA has noted that the system was originally designed to operate on three gyros, with another three in reserve. Two of the six are no longer functional.

"Hubble science on two gyros will be indistinguishable from the superb science we have become accustomed to over the years," said David Leckrone, a senior Hubble scientist at the space agency's Goddard Space Flight Center in Greenbelt, Maryland, as highlighted in a NASA press statement.

Science impact

But while HST's observational campaigns can still be accomplished, working on two gyros does come at a price.

"Saying there is little or no impact on science data quality is not quite the same thing as saying there is no impact on the overall Hubble science program," said Bruce Margon, Associate Director for Science the Space Telescope Science Institute in Baltimore, Maryland.

The two gyro mode "does make overall scheduling considerably more complex, and, perhaps worse," Margon told SPACE.com, and excludes observations of certain parts of the sky for a fair fraction of any given year. 

"So we eventually can still get to any target we could get to with three gyros, but maybe not when we would like to," Margon said. "This makes following up unexpected
transient events, for example, far more awkward and sometime not possible."

Margon said that initial tests of the two gyro mode, and the requisite software needed, were fleshed out many months ago. It was found that the image degradation was surprisingly small, so that science impacts on a given observation should be almost negligible, except for a very small number of rather arcane modes. 

"Based on those tests, NASA approved permanent entry into two gyro mode starting this week," Margon said. "Our initial scientific results from observations this week appear to show no surprises, although those very recent data are still being studied."

Reboost, deorbit decisions

Meanwhile, there are indications that a robotically-attached deorbit module for the Hubble Space Telescope has been cancelled. That no-go decision appears predicated on the ability for a human servicing mission to Hubble, sometime in the future.

As reported by SPACE.com August 22, the idea of hooking a special deorbit module to the Hubble Space Telescope has apparently been scrapped by NASA.

"It does not look like a propulsion module will be necessary for a shuttle servicing mission," said Chris Shank, special assistant to NASA chief, Michael Griffin, at the 8th International Mars Society Convention, held August 11-14 at the University of Colorado at Boulder.

Meanwhile, the HST is not likely to fall back to Earth prior to 2020. Although if the Sun is much more active than expected next cycle, reentry might occur a little earlier...perhaps by a few years, said Nicholas Johnson, NASA Orbital Debris Program Manager and Chief Scientist for Orbital Debris at the NASA Johnson Space Center in Houston, Texas. 

Johnson emphasized that this is considered very unlikely. "If another servicing mission is undertaken, HST would probably be given another small boost in altitude at its conclusion. This would further delay a natural reentry of HST," he told SPACE.com via email.

In a related development, Shank noted at the Mars Society meeting that HST's follow-on space scope--the James Webb Space Telescope--is skyrocketing in cost. "There's a $1 billion cost overrun that we're looking at," he said.