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Last Call

The final mission to the Hubble Space Telescope could radically transform the observatory, but the crew faces some special challenges

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11:47am, September 12, 2008
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Loosening a half-inch screw may not sound like a job for a rocket scientist.

But now imagine performing that task 32 times with your hand inside a pressurized glove so stiff it’s hard to bend your fingers, let alone grab a screwdriver. Now try holding on to the screwdriver in your gloved hand but while you’re floating in space. One additional obstacle: A strut blocks direct access to the screws.


That’s just one of the challenges facing the crew of the space shuttle Atlantis, set to pay a final servicing call to the most celebrated observatory in history—the Hubble Space Telescope.

From its breathtaking images of star-forming regions in the Milky Way to its measurements of distant supernovas confirming that the universe is revving up its rate of expansion, Hubble has both revolutionized astronomy and captured the imagination of the public.


If all goes according to plan, this last mission will transform Hubble, in orbit since 1990, into the most powerful imaging tool and spectral analyzer ever to operate in space.

A new instrument, the Wide Field Camera 3 or WFC3, will capture the portraits of stars and galaxies at wavelengths ranging from infrared to ultraviolet and examine the evolution of galaxies over 13 billion years of cosmic history. Because it can capture such a range of colors, the camera will be able to record, for example, the entire population of stars in a nearby galaxy—both elderly, reddish stars and newborn, bluish stars.


“The Wide Field Camera is going to blow people’s socks off,” says astronaut and Atlantis crew member John Grunsfeld. About 20 times more sensitive than Hubble’s current wide-field camera, WFC3 can record across many wavelengths, simultaneously. With it, astronomers can take a multiwavelength portrait of a celestial object in a single snapshot. “This is the first hyperspectral camera to go up on Hubble,” Grunsfeld says.

WFC3, in tandem with a revived Advanced Camera for Surveys, or ACS, “is just a gangbusters combination for imagery,” says senior Hubble scientist David Leckrone of NASA’s Goddard Space Flight Center in Greenbelt, Md.


The mission’s success would also give Hubble a full toolbox of instruments for the first time since 1993, when the observatory’s photometer was removed to make room for COSTAR, the corrective optics that compensated for Hubble’s flawed primary mirror.

And the upgrade, expected to last five years, could make Hubble a leading observatory for studying the atmospheres of planets beyond the solar system. With a repaired and refurbished Hubble, “this could be the decade when we find an Earthlike planet” beyond the solar system, Grunsfeld says.

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Not bad for a mission that almost never happened.

Temporarily grounded


It was in January 2004, nearly a year after the demise of the Columbia space shuttle and its crew, that then–NASA-administrator Sean O’Keefe announced there would be no more shuttle missions to service Hubble. It was just too risky, he said. Despite the results of a National Academy of Sciences study indicating that the risk to service Hubble was not significantly greater than a shuttle trip to the space station, O’Keefe refused to reconsider.

“For the first time in my life, I was truly stunned,” recalls Grunsfeld. “It was like someone had hit me with a two-by-four.… The fact that I was the chief scientist at NASA and wasn’t in the decision matrix is another whole story.”

O’Keefe did tell scientists at NASA-Goddard that they could work on other ways, short of a shuttle repair mission, to prolong Hubble’s life. And that they did. “We managed to keep the team intact, and managed to do engineering work that was useful not just for Hubble” but also for other missions, Leckrone says.

“We knew fools would come and go, but if we could stick together and keep doing work toward the mission, that some day it would be made to happen,” he adds. “Of course the wonderful thing is that we got an administrator [Michael Griffin] who was a very technically astute person. And he didn’t just have to rely on what people told him.”

Moreover, notes Leckrone, the new spectrograph and camera for Hubble were already in the works, first designed in the mid-1990s. “We’re just trying to finish the work that we started,” he says.


A new task not in the original plan is to fix two dead instruments on-orbit. “We’ve learned how to take instruments out and put them in, how to open the telescope, how to close the telescope,” says Hubble deputy program manager Mike Weiss of NASA-Goddard. “But now we’re adding something to it that we do not have any experience for—in situ instrument repair.”

Maneuvers in space


“I describe [the mission] as running a marathon at a sprint pace,” says Grunsfeld, who has flown on three other missions to repair Hubble. After inspecting the shuttle for damage and reconfiguring the shuttle’s orbit, the astronauts will grapple with the telescope, installing it in the shuttle’s payload bay. Each space walk, notes Grunsfeld, requires a different set of some 40 tools.

“One trait that makes this mission unique compared with the others is the instrument repair work,” says Hubble mission manager Preston Burch of NASA-Goddard. “We’ve raised the bar on this mission, in terms of complexity and difficulty for the astronauts. We’ve had to build a lot more tools for this mission, and we’ve had to do more training than was anticipated.”


Another new device the crew will install is the Cosmic Origins Spectrograph, or COS, which will separate ultraviolet light into its component wavelengths. The spectra will provide new information on supernova remnants, the expanding rings of debris encircling exploding stars. COS will also examine young stellar objects, trace the largest visible structures in the universe—galaxies that have clumped into mammoth superclusters—and search the gas between galaxies for missing ordinary matter. The Big Bang model predicts that more of this matter exists in the universe than has been detected.


Along with installing new instruments, astronauts will revive two existing workhorses, the ACS and the Space Telescope Imaging Spectrograph, or STIS. ACS has taken the sharpest visible-light images in orbit and has been the main camera capturing Hubble’s historic images. STIS is equipped to study planets outside the solar system.

Neither instrument was designed to be taken apart and repaired in space, but that’s just what Grunsfeld, along with a crew of six other astronauts, will attempt.

Sprucing up a planet hunter


Installed in 1997, STIS separates ultraviolet light into its component wavelengths to reveal the chemical composition and temperature of gas and stars. It adeptly measures the composition of the atmospheres of extrasolar planets.

In combination with STIS, WFC3 would also hold particular promise for finding planets beyond the solar system and studying their atmospheres, notes Grunsfeld. The new camera can find small transiting planets—orbs as small as twice Earth’s diameter that periodically block a tiny amount of light from their parent stars as seen from Earth. Spectra taken by STIS will help scientists study the starlight that filters though the transiting planet’s atmosphere, revealing its chemical composition.


In 2004, the STIS power supply failed and engineers determined that the problem resided within a printed circuit board. NASA scientists were already planning to repair STIS when ACS stopped operating in 2007 after a backup power supply of a unit that had failed a year earlier also experienced a short circuit.

Leckrone says that it had seemed at the time that fixing both STIS and ACS on one, already oversubscribed repair mission would overtax astronauts. But as engineers explored the possibility, they realized some of the maneuvers required to fix ACS were similar to those already planned for STIS.


STIS and ACS require access to failed or no-longer-needed circuit boards, which astronauts will have to pull out of the instruments while wearing pressurized gloves. To open up STIS, astronauts will need to remove 111 screws—all in the few hours allowed for a typical space walk.


Engineers at NASA-Goddard have developed a high-speed power screwdriver with low twisting action that can remove the screws quickly without breaking them. Metal shavings, loose screws and other floating debris from the repairs could damage the delicate, precisely aligned instruments on Hubble.

To contain the screws, engineers have developed capture fastener plates that fit over the access panels of the two instruments. Tiny holes in each plate, which match up with the positions of the screws, are big enough to allow astronauts to insert a drill bit but small enough to contain the loosened screws. When all the screws are loosened, the entire capture plate is removed as a single unit, safely taking all the debris with it.


Unwanted or damaged circuit boards will be removed and a new cover panel will be snapped in place with leverlike latches instead of screws.

Although STIS has many more screws than ACS, the instrument is directly accessible. “When you open the doors to Hubble for the STIS repair, you look right at the panel,” Weiss notes. “If you keep going you’d run into it.”

In contrast, the ACS repair involves only 32 screws. Because the camera is partially hidden behind a strut, Grunsfeld, the designated astronaut for this repair, will have to operate the power screwdriver at an angle and perform some of the maneuvers solely by feel rather than by sight. He will also hang a new low-voltage power supply outside the instrument, overriding the damaged components.


“I’m actually looking forward to it in a sick kind of way,” Grunsfeld says.

During five planned space walks, the crew will replace all six of Hubble’s gyroscopes, devices that keep the telescope stable and pointing precisely at celestial targets. Only three gyros are still working, and Hubble has been relying on only two since 2006, reserving one in case of failure. One of the two now in use recently showed a spike in its motor current, a sign that it could be failing, Burch says.


Astronauts plan to replace all six nickel-hydrogen batteries, which unexpectedly have survived 18 years. The batteries weigh a collective 960 pounds, and each stores 2,000 times as much energy as that in a typical digital camera battery.

The crew will also cover Hubble’s thermal blankets with new stainless steel panels, ensuring that the craft is protected from the extremes in temperature it experiences 15 times a day, each time it passes out of Earth’s shadows and into sunlight. Over time, the exterior of some of the old blankets, made of 16 layers of aluminum with an outer layer of Teflon, have cracked. The astronauts will be on a particularly tight schedule, notes Weiss. The crew will spend the first day and a half photographing the heat-shield tiles on the shuttle and making sure the tiles haven’t fallen off or severely degraded as they did in February 2003 on the doomed Columbia flight. Adding an extra space walk to the five already scheduled isn’t an option, says Grunsfeld, since exhaustion usually sets in after four or five space walks, even with crew members alternating tasks.

“The new instruments, the batteries and the gyros that will be changed out, we’ve tested the bejabbers out of them,” Burch says. “Those jobs have been done routinely on other missions. The in situ repairs, we’ve never done that before, and all we can say is stay tuned.”

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