Hubble as seen during its 2002 servicing. Credit: NASA

From an operational standpoint, the two most serious issues facing Hubble are the observatory's batteries and gyros. The gyroscopes, which help Hubble slew and lock onto targets, are the limiting factor on science. But the batteries, which have never been replaced, are the limiting factor when it comes to simply keeping the telescope alive.

When Hubble was launched in 1990, its six state-of-the-art batteries, charged during the daylight portion of each orbit, provided about 540 amp hours of capacity to keep the telescope warm and to run its instruments, computers and communications systems during orbital darkness.

At the end of 2005, the batteries had about 300 amp hours of capacity. A 2004 battery test showed they were declining at an average of about 6.3 amp hours per battery per year.

"In order to get through an orbital night period, we need 40 amp hours total for the whole system," HST Program Manager Preston Burch said in a 2004 interview. "But that means you would come out of the orbital night period with nothing, so you need some reserve. It's sort of like flying an airplane. You wouldn't fill the tanks with just enough gas to get there. You'd want extra.

"So our benchmark that we've set for ourselves is we would like a minimum of 110 amp hours. (That) would give us one orbit to cope with any kind of a major failure on the system and entry into a safe mode or something like that."

Hubble cannot survive without power. Within days of a total power loss, low temperatures would cause titanium fittings to unbond and the optical system would lose its critical alignment. In 2004, engineers believed Hubble would reach that 110-amp-hour point in late 2008 or 2009.

"We've now extended that based on the latest data that we've taken," Burch said in a November 2005 interview. "Our best estimate at the moment is we think we're good out to the middle of 2010, so we've got about a four-and-a-half-year window to get up there."

Engineers bought the extra time by changing the way the batteries are recharged.

"We're treating them differently," Burch said. "When we come out of the orbit night period, we're not charging them as hard as we were earlier. So we've modified our recharge technique, if you will, and we believe that's having some positive effects on the rate of loss of battery capacity. We think the middle of 2010 is reasonable. They could go well beyond 2010. If you take into account the change in the trends, you could argue it might go well beyond that, but I would say we can make a good case that the middle of 2010 is a reasonable expectation."

Hubble's gyroscopes are another pressing concern. The telescope was designed with redundancy in mind and while it was equipped with six gyros, only three were required for science operations. But gyros 3 and 5 are failed and gyro 6 exhibits symptoms of a problem that eventually could knock it out of action.

As a result, engineers developed complex computer techniques to continue science operations using just two operational gyroscopes in concert with Hubble's magnetic sensing system, fixed-head start trackers and a fine guidance sensor. The new control technique went into operation Aug. 29, 2005.

"We successfully implemented the two gyro science mode, that works very well," Burch said. "We're flying on gyros 1 and 2 and we turned gyro 4 off. So 3 and 5 are failed and 4 and 6 are off, they're in storage if you will. We could use them if we had to."

The Goddard team initially thought the two-gyro control mode would limit Hubble's science to about 70 percent of what it could do with three gyroscopes.

"I think we're doing actually a little better than that," Burch said. "In terms of our observing efficiency, we've only had a loss of a few percent. So it's going really well. The other thing that's really exciting is if you look at the point spread function, the pointing accuracy of the telescope is totally indistinguishable from being on three gyros.

"We currently are projecting that we'll be good out to the middle of 2008 doing science on two gyros. That means there's a 50-50 probability that when we get to the middle of 2008 that we will lose a fifth gyroscope, which will put an end to our two-gyro science. And so, to guard against that we've embarked on a one-gyro science mode. That's in the early assessment and development stage. It would be quite similar to the two-gyro science mode, only we would use two FGS's and a gyroscope instead of two gyros and one FGS."

But a one-gyro control mode would cause "significantly greater impacts in terms of the scheduleability of observations," Burch said. "So that's one of the things we're currently assessing, to see how friendly that mode is to doing science."

Otherwise, Hubble's upgraded computers and new solar arrays, installed during a servicing mission in 2002, are performing flawlessly. The solar panels, in fact, generate more power than Hubble needs given the new battery recharging procedure. Fine guidance sensor 3, in operation since Hubble's launch in 1990, has a problem with the mechanical bearings in a servo subsystem. While it's not causing any problems at present, the control team is "babying it," Burch said.

If SM-4 is launched and if the new batteries and gyros are successfully installed, Burch believes Hubble will be able to continue its scientific observations for at least five more years.

"That's what we're gearing ourselves for," he said. "I think there's a good chance we could go beyond five, but our nominal end of mission would be five years from the date of the servicing mission."

Depending on when SM-4 is launched, "you'd be looking at the end of 2012 or sometime in 2013 for Hubble's end of life."

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