By Richard Tresch Fienberg
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.
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."
For further reading-