NASA's new chief, Michael D. Griffin, has told the Hubble servicing team at the Goddard Space Flight Center in Maryland to resume preparations for a possible shuttle flight to upgrade the orbiting observatory. Although a decision on returning astronauts to Hubble won't be made until after at least two successful shuttle missions to the International Space Station, the telescope's prospects look better now than they have at any time since Griffin's predecessor, Sean O'Keefe, abruptly canceled Hubble servicing in January 2004 a decision that outraged astronomers, key members of Congress, and the public.
At his Senate confirmation hearing in early April, Griffin said he would consider reauthorizing a Hubble house call once the shuttle was flying again. But after just two weeks on the job he called a press conference to announce a two-month slip in the first post-Columbia launch, from mid-May to mid-July. This means that the second of two "return-to-flight" missions won't occur before September. By then there may not be enough time to mount a servicing mission that would reach Hubble before its electronics give out. So, said Griffin, "what we're going to be doing is getting the...folks at Goddard started on the work that they would have to do if a servicing flight can yet be done."
Griffin reiterated that a robotic servicing mission, once offered as a less risky alternative to a shuttle flight, is no longer under consideration. Robotic servicing "is just not feasible within the time and the money that we have to allow for it," said Griffin. "So that's off the table."
Astronauts have serviced Hubble four times between 1993 and 2002, replacing failed components and installing new instruments. If they do it again, they'll have three main goals. One is to extend the spacecraft's operational lifetime by replacing its onboard gyroscopes and batteries. Another is to upgrade the telescope's scientific capabilities by installing a new camera and spectrograph. The third and most important task is to attach a retrorocket module that will steer Hubble to a harmless splashdown in the Pacific Ocean at the end of its mission early in the next decade. If the shuttle's return to flight doesn't go as planned, or if something else happens to preclude servicing Hubble, NASA probably will instead fly a limited robotic mission just to attach the deorbit module.
Meanwhile NASA has figured out how to buy Hubble a little extra time, with or without servicing. Normally celestial targets are acquired and tracked using data from three gyroscopes that sense the telescope's orientation in pitch (up-down), yaw (side-to-side), and roll. According to program manager Preston Burch (NASA/Goddard), recent tests showed that Hubble can be pointed and guided with just two gyros and information from the onboard star trackers and fine-guidance sensors. "The images remain within specifications," says Burch, "though we take some hit in observational efficiency."
Two of Hubble's six gyros have failed, a third was shut down after behaving erratically but remains available as a spare, and a fourth is likely to wear out by mid-2006. At that point Hubble's controllers would be forced to switch to two-gyro mode, which would last until another unit failed, probably sometime around mid-2007 about the earliest that a Space Shuttle could be dispatched to the telescope. But by turning off one working gyro now and holding it in reserve, NASA might extend two-gyro science by 9 months or so, keeping the telescope going through early 2008 and easing pressure on the shuttle program.
Whether it goes by shuttle or by expendable rocket, the deorbit module has to reach Hubble before the telescope's rechargeable batteries lose their potency, once projected to occur around mid-2009. Without sufficient battery power, the observatory can't be held steady so that the retrorocket package can be attached by astronauts or docked robotically. But as with the gyros, preventive measures may extend the batteries' lifetime. "By slowing the rate at which we recharge the batteries in sunlight," says Burch, "it looks like we can maintain enough power for Hubble to survive until mid-2010."
For astronomers who despaired over NASA's apparent willingness to abandon what one panel of experts called "the most important telescope in history," these latest developments are a hopeful sign. "Every public statement Griffin has made has been in favor of Hubble servicing," says Steven Beckwith, director of the Space Telescope Science Institute. "People are optimistic."