Hubble Now on Two Gyroscope Mode: Some Science Impacted

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

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

HubbleSpace Telescope (HST) gyroscopes are critical to running the Earth orbitingfacility's complex pointing control system. That system maintains precisepointing of the telescope during science observations.

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

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

Science impact

Butwhile HST's observational campaigns can still be accomplished, working on twogyros does come at a price.

"Sayingthere is little or no impact on science data quality is not quite the samething as saying there is no impact on the overall Hubble science program," saidBruce Margon, Associate Director for Science the Space Telescope ScienceInstitute in Baltimore, Maryland.

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

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

Margonsaid that initial tests of the two gyro mode, and the requisite softwareneeded, were fleshed out many months ago. It was found that the imagedegradation was surprisingly small, so that science impacts on a givenobservation should be almost negligible, except for a very small number ofrather arcane modes. 

"Basedon those tests, NASA approved permanent entry into two gyro mode starting thisweek," Margon said. "Our initial scientific results from observations this weekappear to show no surprises, although those very recent data are still beingstudied."

Reboost, deorbit decisions

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

Asreported by SPACE.com August 22, the idea of hooking a special deorbitmodule to the Hubble Space Telescope has apparently been scrapped by NASA.

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

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

Johnsonemphasized that this is considered very unlikely. "If anotherservicing mission is undertaken, HST would probably be given another smallboost in altitude at its conclusion. This would further delay anatural reentry of HST," he told SPACE.com via email.

Ina related development, Shank noted at the Mars Society meeting that HST'sfollow-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.