If NASA had kept to its original schedule, astronauts would have made their fifth and final mission to service the Hubble Space Telescope back in August.It's a good thing they didn't.
The space agency announced today that an onboard communication problem has temporarily shut down the World's Greatest Telescope — and postponed the planned house call in orbit, scheduled to begin October 14th, until no earlier than next February or perhaps April.
The failure occurred in the Command Unit Science Data Formatter, an electronics package that moves digitized streams of data from the science instruments to spacecraft's digital tape recorder for later playback to Earth. The CU/SDF has worked great for 18 years, so faulty craftsmanship isn't the issue. Nor is the venerable space observatory in any real trouble.
The good news is the that Hubble's designers included a second unit for redundancy. As far as engineers know, it still works — but it hasn't been checked since before HST's launch more than 18 years ago. Changing from one to the other is entirely doable but a lot more involved than just throwing a switch from "A" to "B". So Hubble's handlers are dusting off the owner's manual to begin the process; that might be completed by early next week, at which point the observatory will come back online.
More weighty, for the moment, is whether replacing the defective unit should be added to the already jam-packed "to do" list for astronauts on the forthcoming servicing mission, STS 125. A spare CU/SDF does exist at NASA's Goddard Space Flight Center (which manages Hubble's science payload) and could be made flight-ready soon.
During a hastily convened briefing for reporters today, Preston Burch, Hubble manager at NASA-Goddard, noted that replacing the 136-pound unit, which is roughly the size of a two-drawer filing cabinet, should be "a relatively straightforward activity" that would add only about 2 hours to one of the mission's five planned spacewalks.
It'll take a few months to certify that the spare is flightworthy, but NASA officials seem willing to accept that delay. This means the Space Shuttle Atlantis, already on the launch pad at the Kennedy Space Center in Florida, will be rolled back into its shelter. So will a second shuttle, Endeavour, that's standing by just in case a dramatic rescue of the Hubble repair team had been necessary.
As detailed in Sky & Telescope's October issue, the STS-125 crew hopes to install two scientific instruments (the Cosmic Origins Spectrograph and a replacement Advanced Camera for Surveys), repair a third, and swap in new batteries, gyroscopes, and other components. The spacewalkers will also attach a mechanism to allow the docking of a rocket stage at some future date for Hubble's safe disposal.
Should the replacement data formatter prove unfit to fly (considered unlikely), the repair mission will be hustled back to the launch pad as soon as it can — probably in November.
"Hubble has a habit of coming back from adversity," notes Edward Weiler, who heads NASA's Science Mission Directorate. "This particular failure was anticipated 20 years ago, and we have spare hardware ready to go."