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Hubble’s life gets longer and more complicated The Hubble Space Telescope has switched to a two-gyro mode that should extend its operational life but will make scientific planning more complicated.
This picture of the Hubble Space Telescope was snapped from the shuttle Discovery during a 1997 servicing mission.
This picture of the Hubble Space Telescope was snapped from the shuttle Discovery during a 1997 servicing mission. NASA via AP
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NASA’s Hubble Space Telescope is now on duty minus one operational gyroscope.

The space agency announced Wednesday 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 extending the space observatory’s ability to gather scientific data through mid-2008, an eight-month extension.

Hubble's 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, Md., as highlighted in a NASA press statement.

Science impact
But while Hubble’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 at the Space Telescope Science Institute in Baltimore.

The two-gyro mode “does make overall scheduling considerably more complex, and perhaps worse,” Margon told, 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 on 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 canceled. That no-go decision appears predicated on the ability for a human servicing mission to Hubble sometime in the future.

As reported by on Aug. 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,” Chris Shank, special assistant to NASA Administrator Michael Griffin, said at the Eighth International Mars Society Convention, held August 11-14 at the University of Colorado at Boulder.

Hubble is not likely to fall back to Earth prior to 2020 — although if the sun is much more active than expected next cycle, re-entry might occur a few years earlier, said Nicholas Johnson, NASA orbital debris program manager and chief scientist for orbital debris at Johnson Space Center in Houston. 

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 re-entry of HST,” he told via email.

In a related development, Shank noted at the Mars Society meeting that Hubble’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.