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Hubble's life expectancy predicted When Rodger Doxsey presented the Hubble's health report last week, he called the analysis of Hubble's life expectancy educated guessing. Amid the large amount of uncertainty, by far the biggest worries are the gyros and batteries.
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Steven Beckwith, who oversees operation of the Hubble Space Telescope, said the other day that the observatory could be knocked out of commission the next day by a meteoroid. It wasn't. But the sentiment reflects the precarious nature of any satellite.

Hubble's status right now is now more precarious than it has ever been.

While astronomers say the 14-year-old telescope is doing its best science ever -- please don't call it an "aging observatory," they ask -- its batteries are waning much in the same way the power of a cell phone that's been charged too many times dissipates. And Hubble has been through more than a dozen gyroscopes and is down to four.

NASA nixed a planned servicing mission by astronauts that would have replaced the batteries and nonworking gryos, along with adding two new instruments to essentially put Hubble on steroids for its final decade.

Prodded by astronomers, politicians and the public to reconsider, the agency is now thinking about sending a robot to do the job. The nervousness over the decision, expected in early June, is palpable in the entire astronomy community and downright intense at the Space Telescope Science Institute in Baltimore, where Beckwith leads a team of scientists and engineers who run the telescope for NASA.

With all the uncertainty, one obvious move was to analyze Hubble's life expectancy. Enter more uncertainty.

Educated guesses
Rodger Doxsey, who presented Hubble health report last week to a group of astronomers and cosmologists gathered to discuss Hubble's final years of observing, calls the analysis educated guessing. He compared it to cosmology, a field that explores the origin and fate of the universe and which is notorious for being long on theory and short on data.

"I've certainly done a lot of simplification," Doxsey said. "And there are a lot of uncertainties. If you want certainty, you should be a cosmologist."

That amusing caveat in mind, here's the outlook:

All of Hubble's instruments and major systems are working fine. By far the biggest worries are the gyros and batteries.

"I think it is relatively unlikely that some other major problem will impede science operations," Doxsey told "That doesn't mean it can't happen, just that it is unlikely. The most likely impediment to continued operations is failure of the gyros. The next most likely impediment is the batteries."

The gyros
Hubble carries six gyros. Three are needed to point the telescope, and they fail regularly. About a dozen have been replaced in previous servicing missions. Only four are working now.

Based on the history of wear-outs and random failures, Doxsey figures the odds are 50-50 that three gyros will still be working in March 2006. Engineers have already uploaded a portion of new software that's being written to allow Hubble to point with just two gyroscopes if needed (engineers of NASA's SOHO spacecraft were forced to employ a similar technique). The software will be tested in January with more code to follow.

Running on two gyroscopes would cause some science operations to be about half as efficient.

The probability of still having two gryos operating by mid-2007 is also 50 percent, which is to say there's a 50-percent chance the telescope will be down to one and cease operations then if it is not serviced first. By March of 2009 the chance of having two gyros still going drops below 10 percent.

Doxsey notes that there are reasons to hope the current gyros, which are different than older models, will last longer, but he has no engineering data to support the hope.

"We'll probably be good for three to four years," he said of the gyros, "but we'll have to keep our fingers crossed."

The batteries
Hubble's batteries are expected to fail between 2008 and 2010, according to the analysis. Engineers are working on ways to stretch the battery life. Hubble uses six nickel-hydrogen batteries. It's a type that's never been used this long in orbit, Doxsey said, so it is "new engineering territory" to try and calculate how long they will last.

"The batteries are charged and discharged every single orbit of the telescope," he explained. "We're now up to 75,000 orbits." Hubble circles the Earth every 95 minutes.

Hubble was outfitted with new solar panels and a new power control unit on a previous mission. But the batteries are original. They provided 600 ampere-hours when the mission began in 1990. That's down to 300 now. It must be above about 100 ampere-hours to maintain heat to critical systems.

What if Hubble is serviced within three years, but never again? How many years would that add to its life?

"That is really hard to answer," Doxsey said. "I think a new set of batteries should last as long as the current set, more than 14 years. Starting from a fresh set of six new gyros, I think we should expect them to last six to eight years."

But, he added, there is no way of knowing whether something else might cause failure before then.