STS-125 Mission Preview:
High Hopes for Final Hubble House Call

Editor's Note...
Portions of this historical review were written for Astronomy Now magazine.

CBS News Space Consultant

NASA's fifth and final mission to service and upgrade the Hubble Space Telescope will add one of the most dramatic chapters yet to an ongoing saga that reads like "The Perils of Pauline." Or an over-the-top Hollywood screenplay about a scientific superstar repeatedly rescued from the brink of disaster.

"I don't think anybody except Arthur C. Clarke could have crafted such a great story," said astronomer-astronaut John Grunsfeld, the mission's lead spacewalker. "If it were just about Hubble, it would be a great story. But when you look about the science and the discoveries scientists have made using Hubble, then it just becomes an unbelievable story.

"I'm relatively glib in saying Hubble is perhaps the most important and productive scientific instrument ever created by humans. Only history will tell, but it's a truly remarkable story."

Grunsfeld, commander Scott Altman, pilot Gregory C. Johnson, robot arm operator Megan McArthur and fellow spacewalkers Michael Massimino, Andrew Feustel and Michael Good are scheduled for launch aboard shuttle Atlantis on May 11 at 2:01:49 p.m. It will be the second Hubble visit in a row for Altman and Massimino and the third for Grunsfeld. The rest are shuttle rookies.

Launch originally was scheduled for last Oct. 14, but just three weeks before takeoff a critical circuit in the telescope’s science instrument data system malfunctioned. To restore full redundancy, NASA managers decided to delay the servicing mission to give engineers time to check out and certify a flight spare that had been used for ground testing. The replacement computer was delivered to the Kennedy Space Center on March 30, setting the stage for launch.

Hoping to extend Hubble's life well into the next decade, the four spacewalkers, working in two-man teams, plan five back-to-back excursions to install six new stabilizing gyroscopes, six new nickel-hydrogen battery packs, the new data computer and two new instruments, the $126 million Wide Field Camera 3 and the $81 million Cosmic Origins Spectrograph. Like all modern Hubble instruments, both are equipped with corrective optics to counteract the spherical aberration that prevents Hubble's 94.5-inch mirror from achieving a sharp focus.

The Atlantis astronauts also will attempt to repair two other instruments: the Space Telescope Imaging Spectrograph, which suffered a power supply failure in 2004, and the Advanced Camera for Surveys, which broke down in 2007. Neither instrument was designed to be serviced in orbit, but determined engineers devised custom tools and an ingenious plan for the spacewalkers to bypass the failed electronics.

The repair crew also plans to install an upgraded fine guidance sensor, new insulation and a grapple fixture that will permit attachment of a rocket motor or even NASA's new Orion manned spacecraft in the future to drive Hubble out of orbit when it is no longer able to do science.

"On Servicing Mission 4, we're going to give Hubble another extreme makeover," said Program Manager Preston Burch. "This makeover will be the best one yet because we will outfit Hubble with the most powerful and advanced imaging and spectrographic instruments available and we will extend Hubble's operating lifetime for five additional years."

Without Servicing Mission 4, engineers believe Hubble would be hard pressed to survive past 2010. But if the Atlantis astronauts are successful, they will leave behind an essentially new telescope, one that is equipped with a full suite of five operational scientific instruments for the first time since launch in 1990. And with new gyros and batteries, Hubble has a good chance of remaining fully operational long enough to work in concert with its eventual replacement, the James Webb Space Telescope.

"It's been seven years since we've serviced the Hubble space telescope," said Project Scientist David Lekrone. "And that interval of time, seven years, is twice as long as we should go in terms of servicing intervals. As a consequence of that, over the last few years we've seen significant deterioration within the set of scientific instruments that we provide to the astronomical community. The toolkit that the community uses to do all kinds of science has really diminished in its capabilities.

"I liken this to the situation of a champion athlete who is playing hurt, who has an injury and who is playing through the pain, still doing very well. But now, by golly, it's time to go off and get our surgery and get back to a hundred percent."

NASA has spent about $10 billion on the Hubble Space Telescope to date, making it one of the most expensive science projects in history. Asked whether it made sense to spend more money on a 20-year-old space telescope, former NASA Administrator Mike Griffin, the man who approved Hubble Servicing Mission 4 after it was canceled in the wake of the 2003 Columbia disaster, said it makes all the sense in the world.

"After we get done with it, it's not an old telescope," he told CBS News in a recent interview. "Every subsystem that needs refurbishment is being refurbished and it's getting a new complement of instruments. So the only part of it that's old is the optical metering structure and the glass. And the glass doesn't care. When they're done, it really is not an old telescope, it's a new telescope."

"So the question you want to ask yourself when you look at the value proposition, if for the cost of this shuttle flight - and bear in mind, most of the instrument costs and all that were already paid for - plus the team that we've been carrying, and it's about a $10-million-a-month team, if for whatever all that adds up to you could get yourself a new telescope in space, would you think that would be worthwhile? And I think most people, most astronomers, would say yes.

"Not because it's the biggest telescope, because we can build bigger ones on the ground. And with the new flexible mirror technology and multiple mirror technology, we can get some pretty large apertures," Griffin said. "But, being above the atmosphere still has value, and the best value of all is the coordination of ground-based observations and space-based observations. Between the two, you get a picture that is more than the sum of the parts.

"The question in brief is, if for what we're spending on this mission you could have a new telescope, would you buy one? And I think the answer is yes."

Hubble Space Telescope Facts & Figures (Source: NASA/Lockheed Martin)

Weight.................24,500 lb (11,110 kg) 
Length.................43.5 ft (15.9 m) 
.......................10 ft (3.1 m) Light Shield and Forward Shell 
Diameter...............14 ft (4.2 m) Equipment Section and Aft Shroud 
Optical system.........Ritchey-Chretien design Cassegrain telescope 
Focal length...........189 ft (56.7 m) folded to 21 ft (6.3 m) 
Primary mirror.........94.5 in. (2.4 m) in diameter 
Secondary mirror.......12.2 in. (0.3 m) in diameter 
Pointing accuracy......0.007 arcsec for 24 hours 
Magnitude range........5 mvto 30 mv (visual magnitude) 
Wavelength range.......1100 to 24,000 Å 
Angular resolution.....0.1 arcsec at 6328 Å 
Orbit..................350 statute miles (563 km)
Orbit inclination......28.5 degrees
Orbital period.........96 minutes per orbit
Lifetime...............20+ years

Bruce Margon, former associate director for science at the Space Telescope Science Institute in Baltimore, said shuttle servicing missions and NASA’s ability to upgrade the telescope are the keys to the project’s success.

"The thing to remember about these Hubble servicing missions is they're not just 'let's keep a groaning patient on life support,'" he said. "When you put new focal plane instruments into Hubble, you essentially leave with not only a brand new, but a much better observatory. And when you look at our graph of discoveries as reflected by published scientific papers versus year, it's an amazing thing because it just goes up every single year.

"The reason for that is not that the scientists who are using Hubble are smart. It's servicing. That's the reason, because when you leave Hubble you have not just something with better longevity but something that is an order of magnitude more capable than the previous thing, almost like it's a brand new generation of satellite. And the two new focal plane instruments for SM-4 are predicted to do the same thing. And it's not a whistling in the wind prediction."

For Ed Weiler, NASA’s associate administrator for space science and a former Hubble project scientist, the key point is not the telescope’s serviceability or even its obvious value to the astronomical community. It’s the way Hubble has "brought the universe close up and personal to the average citizen."

"It's images have become part of our culture in our textbooks, magazines, art and even popular movies and TV programs," he said. "Although we probably never will be able to visit these places or objects, Hubble actually allows our human minds and spirits to travel light years and even billions of light years to the farthest reaches of the cosmos. And SM-4 will allow that dream to continue."

As for the telescope’s astronomical price tag, Weiler agreed "you can build a lot of ground-based telescopes for that kind of money. But you can't get rid of the atmosphere."

"We've heard about adaptive optics (for ground-based telescopes) and how that's going to blow Hubble out of the water," he said. "We've heard that for 20 years now. We haven't seen it. What's amazing is, whenever a new telescope comes out on the ground, a press release will always come out that 'oh, this can see a hundred times better than Hubble, or 10 times better.’ Yeah, it can, probably, over a very, very tiny field of view. But you don't see Eagle nebulas [right] on the cover of Time Magazine taken from the ground. It's taken from Hubble."

Launched from Atlantis in April 1990 with a famously flawed mirror, Hubble was equipped with corrective optics during a riveting, make-or-break 1993 shuttle repair mission. Since then, the Lockheed Martin-built observatory has generated a steady stream of discoveries, ranging from a more precise determination of the age of the universe - 13.7 billion years - to confirmation of the existence of super-massive black holes.

It has captured light from infant galaxies in the process of colliding and merging less than a billion years after the big bang birth of the universe. And it has helped refine our understanding of the life cycles of stars, from their birth in vast stellar nurseries to the supernova explosions and more common slow fading that mark old age and death.

In recent years, Hubble's remarkable vision has played a key role in the worldwide effort to understand the nature of dark energy, the enigmatic repulsive force that astronomers believe is accelerating the expansion of the universe.

Throughout it all, Hubble has beamed back a steady stream of spectacular photographs of planets, stars, nebulae and galaxies that have found their way into all facets of modern society, making the telescope an instantly recognized icon of science.

But keeping Hubble healthy in the unforgiving environment of space has not been easy. During a second servicing mission in February 1997, shuttle astronauts installed two new instruments - the Space Telescope Imaging Spectrograph and an infrared camera known as NICMOS - replaced a fine guidance sensor, a gyroscope assembly and installed a solid-state data recorder.

Because of multiple gyro failures in the late 1990s, Servicing Mission 3 was broken up into two shuttle flights, SM-3A in December 1999 and SM-3B in March 2002. During SM-3A, spacewalking astronauts installed a new flight computer, a second solid-state recorder, another fine guidance sensor and a full suite of six gyroscopes.

The objectives of SM-3B included installation of two new solar arrays, the Advanced Camera for Surveys, an experimental cooling system to revive Hubble's infrared camera and a replacement power control unit. The latter operation was analogous to a heart transplant, requiring the telescope to be shut down for the first time since launch.

One year after SM-3B, NASA was well into planning the fifth and final service call when the shuttle Columbia disintegrated during re-entry on February 1, 2003, the victim of heat shield damage caused by a piece of foam insulation falling from the ship's external fuel tank during launch.

A year later, in January 2004, then NASA Administrator Sean O'Keefe sent shock waves through the astronomical community when he abruptly canceled SM-4. The decision was announced two days after President Bush ordered NASA to complete the international space station and retire the shuttle by the end of 2010.

Citing safety concerns in the wake of Columbia and a lack of time and money to properly address them, O'Keefe said it was simply too dangerous to launch astronauts to the space telescope. Heat shield inspection and repair techniques were immature and NASA was still struggling to prevent foam insulation from falling off the shuttle's external tank.

More important, a Hubble crew could not seek "safe haven" aboard the international space station if some post-launch mishap or orbital debris impact prevented a safe re-entry. Hubble and the space station operate in different orbital planes and the shuttle does not carry enough rocket fuel to move from one to the other.

O'Keefe defended his hugely unpopular decision by citing the Columbia Accident Investigation Board, which recommended autonomous heat shield inspection and repair capability for any non-station shuttle flights. Under pressure from Hubble supporters in Congress, he agreed to let engineers explore options for a robotic servicing mission. But the scope of that mission was more limited, the technical risks were high and the projected cost was extreme.

Even so, project managers pressed ahead, fearing subsequent equipment failures in orbit that would knock the observatory out of action once and for all. And they had reason for concern.

In August 2004, the Space Telescope Imaging Spectrograph’s one operational channel failed because of a power supply problem. The observatory's stabilizing gyroscopes were suffering problems and engineers worried the telescope's crucial battery packs, operating continuously since launch in 1990, were slowly degrading. No one knew when one might suddenly fail.

Against this backdrop of concern, NASA pressed ahead with space station assembly flights, implementing a series of upgrades to minimize foam shedding from external tanks. The agency also carried out a series of tests to perfect heat shield inspection and repair techniques.

In one critical test, spacewalking astronauts showed the shuttle's robot arm and a new 50-foot-long tile inspection boom were strong enough to support an astronaut if repairs were needed and the station was not available.

O'Keefe's replacement, Mike Griffin, made no secret of his desire to fly Servicing Mission 4, saying "Hubble servicing represents the highest priority utilization of a single shuttle mission that I can conceive."

Finally, after three successful post-Columbia missions and tests to demonstrate heat-shield repair tools and techniques, Griffin officially reinstated SM-4 in May 2006.

"I don't believe I've talked to anyone in the agency, from flight crew to flight ops managers to, you know, even budget guys, I don't believe I've talked to anyone who thinks we shouldn't do this," he said.

To address the safe haven concern, he ordered the shuttle program to process a second orbiter - Endeavour - in parallel and to have it ready for takeoff within a few days of an emergency being declared to carry out a rescue mission if needed (see "STS-400: Just in Case" for additional details).

"The way we've designed the mission, we've got an answer to each of the risk points that, I think, brings us right into the family of same risk level as going to the station," said Altman. "First, get rid of the debris at the source, fixing the tank. Number two is the ability to detect damage, we've got that. Three is the ability to repair, that's come along pretty well.

"And then the final thing is OK, if you screw all that up and you're stuck there with an unsafe vehicle to come home, what do you do? I think that was a big sticking point before with the administrator and now that we have this launch-on-need plan where another shuttle will come to us and rescue us, we have an answer for that, too."

As if to drive home the need for another servicing mission, the Advanced Camera for Surveys failed in January 2007, the apparent victim of a short circuit in its CCD control electronics. Its high resolution and heavily used wide field channels were knocked out of action, although its more limited solar blind channel continued to operate. That left Hubble with two fully operational instruments: The Wide Field Planetary Camera 2 and the Near Infrared Camera and Multi-Object Spectrometer, or NICMOS.

Then, after a software upgrade prior to the original October launch date for Atlantis, engineers were unable to restart the NICMOS cooling system, presumably because of ice particles that had formed in the coolant lines. Engineers are optimistic about ultimately melting the ice and restarting NICMOS, but as of this writing, the space telescope only has one fully operational instrument - WFPC-2 - and the solar blind channel of the Advanced Camera for Surveys.

"The WFPC-2 has proved to be very durable, but it's been there since December of '93," Burch said. "So it's close to 15 years old and really doesn't owe us anything. So we've got aging science instruments, we've got a weak complement of gyros. I think it's really tough to imagine going much beyond 2010 (without SM-4). And if we lost NICMOS and just became basically the WFPC-2 observatory in space, I think our operation would be cut back substantially. It costs a lot to operate this observatory, the operational cost per year is on the order of a hundred million dollars plus, which includes all the science grants and what not. And to only have the use of WFPC-2 with no prospects of a future servicing mission, I think NASA would feel strongly that they'd want to start putting the money toward the future rather than the past."

The goals of Servicing Mission 4 are roughly the same as those of the mission O’Keefe canceled, with the addition of the science instrument data computer:

"All of the tasks kind of break down into two big categories," said SM-4 mission director Chuck Shaw, an accomplished amateur astronomer. "The life extension tasks and then the mission science extension tasks. And the life extension tasks are clearly the most important, to keep the facility operating, and we'll get those done and then the mission science extension tasks, where we install new capability to do science above what it can do now."


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 550 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," 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.

"What's happened with the batteries is we have become smarter in managing them and we have been able to arrest the rate of decline and their current capacity has stayed about the same for the last couple of years and it's on the order of 300 amp hours," he said in 2008. "We modified our method of handling the recharging of them, we also are avoiding deep discharging them. So-called battery reconditioning turns out to have negative aspects to it. ... Overall, it hurts the batteries in terms of their charge capacities. So we've ceased and desisted on that.

"The batteries are kind of going sideways. You might say, well gosh, that's not too bad. Why not just not change them out? The thing is, they are 20 years old. They were built a couple of years before we launched in '90. We're so far beyond the design lifetime it's anybody's guess as to how long they could continue to go. We know it's not infinite. So our best judgment is we should go ahead and still change them out. However, there are some reduced mission scenarios where if we got into an extreme case of having lost a few EVA days, we, in fact, might only change out one module. That could come into play, but only a very extreme situation."

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 2, 3 and 5 have failed and gyro 6 exhibits symptoms of a problem that eventually could knock it out of action.

"We're flying on 1 and 6. 4 is in reserve," Burch said. "However, 6 you may recall has some flaky characteristics that were detected not too long after it was installed on servicing mission 3A. We suspect it has to do with the suspension system in it. When you slew the observatory, the drift rate on the gyro changes significantly on it. That's the bad news. The good news is, it changes in a very predictable way. We cleverly put some flight software on board that enables us to use gyro 6 and not be confused or whatever by the shift in the gyro drift bias.

"Now gyro 1 recently had a sudden surge in its motor current which is indicative of a temporary rotor restriction event. And this has happened (several) times. The current has gone up, but it's come back down. But it's still running at a value slightly higher than normal. So our best experts and our past experience tell us 1 is living on borrowed time and it could go at any time. Gyro 4, although it's off and held in reserve, was used for a long time and has a lot of run time on it. It's up there, it's up around the 50 percent point in terms of probability of failure. It's not clear how long gyro 4 could last if and when we had to turn it on and use it.

"So the bottom line is, all three of the remaining gyros have got liens against them if you will," Burch said. "Six because of the flaky suspension, 1 because of the flaky motor current and 4 because it's got a lot of run time on it. So you ask, how much longer can you guys keep going on gyros, even with a one-gyro science mode, and that becomes highly speculative. ... Our previous calculations showed we could probably get through 2009 with the gyros that we have. I think getting much past 2010 would be a bit of a stretch."

Protecting against future failures, engineers earlier developed complex computer techniques to continue science operations using just two operational gyroscopes in concert with Hubble's magnetic sensing system, fixed-head star trackers and a fine guidance sensor. The new control technique went into operation Aug. 29, 2005. Engineers then developed a one-gyro control mode for worst-case failures.

"Basically what we're doing is, instead of using gyros we're using the other sensors," Burch said. "The fine guidance sensors were never intended to be used on a continuous pointing basis. You always used the gyros and then you update the gyro reference with the fine guidance sensor information and you just do that from time to time. Now, what we do is, we put the fine guidance sensor into the control loop so it's an active controlling sensor as opposed to being a sort of partial consultant, you might say. It required a major change in the flight software and, of course, a huge change on the ground for how you schedule these things. ... The problem with using the FGS's, of course, is that they get occulted and when they're occulted, you're back to just gyro information and if you've only got one or two gyros then you've really got problems."

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 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."


Repairing the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys represent major challenges for the SM-4 spacewalkers. Neither instrument was designed to be repaired in orbit.

"If we pull it off, it'll be amazing, frankly, especially if we pull off both," Weiler said. "We're not claiming we have to fix both of those for minimum mission success. But if we fix both of those, it goes well beyond full mission success. Right now we have two dead instruments. We got our full scientific value that we had planned. But they represent a quarter-of-a-billion-dollar investment by the U.S. taxpayer. And that was in 1999 dollars, so you can imagine what they would cost today. If we can pull even one of those instruments back into life, we're getting our dividends again, it would be a real home run. We've got one of the best astronaut crews we could hope for. ... I think we've got a good shot at it."

STIS broke down in 2004, the victim of a blown power supply. To fix it, the astronauts must remove a cover held in place by more than 100 screws and then replace a circuit board that is locked in place.

"In order to get at a failed electronics board inside the STIS main electronics box, we need to take the cover off the box," Burch said. "We're very fortunate in that when the astronauts open the doors to the aft shroud and look at this instrument, that cover is sitting right there in front of them. The challenge is the 111 screws that are holding it on. The screws are not captive. So they have to go in there and take all these screws out. You can imagine what went through a lot of people's minds when we first started thinking about this, you know, 111 screws floating around all inside Hubble. That was unacceptable.

"So, we came up with a very clever device called the fastener capture plate, which is basically made out of a Lexan-type material. This plate goes over the top of the MEB (main electronics box) cover, it's aligned and fastened on there. And then this fastener capture plate has a series of little holes in it that line up with all the screws. The holes are small enough to allow the tool bit to go in so you can turn the screw, but they're small enough to keep the screw from falling out. So once you get all 111 screws taken care of, the cover stays attached to the fastener capture plate and you move the whole thing out. So all the debris and all the screws are captured in there."

An astronaut-friendly replacement cover was developed that will be installed in place of the main electronics box cover that was removed.

"Once we're done servicing, we take the new cover and put it on," Burch said. "There are two latches, you just throw the latches and bingo, it's on there. And then there's a third latch they throw that has some fingers that grab the electronics boards and mate them to the cover."

That was one challenge. Another was making sure the astronauts could replace the circuit card with the failed power supply.

"If you've ever fooled around with your desktop computer, those things usually aren't much of a challenge," Burch said. "But the way these instruments are built on Hubble, these boards slide into slots in the box but they're held in place by things called wedge locks. And the wedge locks are designed to keep the boards from rattling around and they also provide a heat path to reject waste heat out to the sides of the box so things stay nice and cool.

"Unfortunately, these wedge locks have a property like these Chinese finger handcuffs you may have played with as a kid. You put them on and the harder you pull, the tighter it gets. Well, the wedge locks have this kind of a property and when you loosen the bolts on them sometimes you can slide the board right out and sometimes you have to wrestle with it for a half hour or an hour to get it out.

"We obviously needed a tool to overcome this problem. So we have a card extraction tool that was developed. We went into a small research program to see even if these wedge locks jammed in their worst possible way could we pull the board out without having the board disintegrate and leave a pile of debris. I'm happy to report we've come up with a tool that enables us to do exactly that. So those were the major challenges."

STIS will be repaired during the crew’s fourth spacewalk.

Repairing the Advanced Camera for Surveys posed another difficult challenge for Hubble engineers and the task originally was spread across two spacewalks. But the addition of the science instrument data computer replacement forced NASA managers to consider a one-shot repair attempt for ACS. Grunsfeld, practicing the procedure in NASA’s spacewalk training pool, said he’s confident the repair can be completed in a single spacewalk.

But engineers initially were "pretty negative" about attempting any sort of ACS repair, Burch said.

"We knew how long it was taking us to get the STIS repair done," he said. "That took us over three years to get that done. And when ACS failed, we didn't have the luxury of three years to get that together. That kind of told us this was going to be a huge challenge. Second of all, we had failures on both sides of the main electronics box, 1 and 2, which we had on STIS also. But the problem was that on ... one of the sides (of the ACS), we couldn't get into the box because it was blocked by the NICMOS cooling system and we'd have had to disassemble that partially to get in there and nobody wanted to do that. The other side that was accessible was difficult to get at and if you got it open, we were concerned about a contamination risk because of the catastrophic nature of the failure. So much current went through there that we we thought there was the potential for a lot of collateral damage and you open that box up and it's like Pandora's box, you don't know what's coming out of there and we didn't think that was a healthy scenario.

"So we had to come up with a whole new approach to repairing ACS," he said. "We can't get into either of the low voltage power supplies on ACS. So our approach is, we're going to provide an additional low voltage power supply and we'll just hang it on the outside of the instrument and we'll tap into the power connector coming into the instrument. So we'll kind of T off the power to that. Ideally, we'd like to restore both the wide-field channel and the high-resolution channel on the ACS. It turns out, even though the high-resolution channel as the name implies provides the best, the deepest pictures, the most sensitive and the highest resolved pictures, it was not the most popular channel by the astronomers because of its very narrow field of view. They found the wide-field channel very useful for the majority of observations that they wanted to make. So the wide-field channel was used a very high percentage of the time, it was on the order of 70 some odd percent whereas the high-res channel was maybe 20 percent or less and the solar blind channel was like 5 percent.

"So we said OK, let's look at how we might do this. And the technique that we came up with, it turns out you can get access to the CCD electronics box that powers each of those channels, you can gain access to those somewhat conveniently going in through the outside of the instrument.

You don't have to take off a bunch of covers and go through a lot of stuff to get at them. but it's not real easy, either. There are two CCD electronics boxes, one for the wide-field channel and one for the high-res channel. In order to get access to them, you have to cut off an EMI grid. There's like this screen, this very coarse screen on the outside. So we came up with a special cutter tool that cuts that screen away and it cuts the individual wires. There's roughly a dozen wires or so that need to be cut. Once you've done that, you're now looking at a plate that needs to be removed and it's got 30 some odd screws in it. So you put a fastener-capture plate on that and remove the screws and once you pull that plate out, you're

now looking at four printed circuit boards in each of those cavities that contain the electronics that power and control the CCD for each of those channels.

"So the idea is, pull those boards out and put in a new set of boards but wire them up in a way that they bypass or ignore the damaged areas coming from the existing main electronic box. This new module that would go in that replaces those four boards, it'll be powered by the external low voltage power supply that you've just attached to the outside of the instrument and it in turn will provide the power and control signals to the CCD using the existing wires that are in there, but it can be done in a way that avoids the damaged areas in the main electronics box.

"The downside here is we just didn't have the time and the money to replace the electronics in both the wide-field channel's CCD electronics box and the high-resolution channel's CCD box. So we came up with a scheme, it turns out there are shared copper paths between the electronics for both of those channels. So what we said was, hey, why don't we get to the high-res channel through the electronics path that are connected to the wide-field channel? We'll just back power the existing printed circuit boards that are in the high-resolution channel CCD. We tested that on the ground and sure enough, it turns out to be feasible to do that. The only question mark is the status of the low voltage power supply on the MED 1 and MED 2 sides. In other words, it's possible that if there's damage on the sides of the interpoint converters, the secondary sides that are powering the high-resolution channel, it's possible there are some short circuits there that will prevent this scheme from working. Particularly on the one side that suffered the major damage. That may not work very well.

"In any event, what we decided to do is, we're providing an additional built-in power supply that will try to rejuvenate, or bring back to life, the high-resolution channel by back powering the high-res channel through these shared copper paths that connect to the wide-field channel," Burch said. "I won't say it's a long shot. It IS somewhat of a long shot, but people need to understand that this doesn't have the same degree of rigor as, let's say, building a brand new science instrument or a new black box that we're hooking up to standard interfaces that already exist on the telescope. This is really a bit of an experiment." February 2008: Testing of an engineering version of the CCD electronics box at the

Even if ACS is revived, engineers would face yet another hurdle: "tuning" the CCD control electronics to get optimum performance.

"They go to great pains on the ground to tune the electronics to get optimum performance out of these things to get the best sensitivity," Burch said. "Unfortunately, the detectors are up there and we're down here and we don't have that opportunity. So the question is, well, how do you make that happen? What we did was, we borrowed some technology from James Webb Space Telescope. We have employed the use of an ASIC chip, an application specific integrated circuit known as a sidecar, which is basically a video processing chip. And this chip is going to be key to enabling us to fine tune the control electronics, the new electronics we're putting in for the wide field channel so that we can get the lowest possible read noise out of the system when it's installed on orbit and operating.

"We're very fortunate that we have an excellent flight spare detector for ACS right here on the ground. Actually, we have several and we've experimented with those and saved our final testing for the best chip. And so we were able to put this into a dewar, get the temperature down to what it's experiencing on orbit and we've been able to fine tune the electronics with the software to demonstrate that this technique works and that we can get the kind of performance that we're looking to achieve. As a matter of fact, I probably shouldn't say this, if it works out up there the way it's worked out on the ground we'll be getting better pictures out of the ACS wide field channel than before the failure occurred."

Developing the ACS repair concept and perfecting the techniques required has required "a super human effort," Burch said.

"The general feeling is that this will be the most challenging mission that we've had to date," he said. "It is jam packed. It's got two instruments that are being repaired and these two instrument repairs are about roughly a day and a half worth of time. This is fine work using new tools, this is stuff that hasn't been done before, getting access to this ACS CCD electronics box area is very, very difficult because it's up near the top of the instrument, there's some structure that's in the way that makes getting direct viewing of this area exceedingly difficult. We've had to build and modify tooling to get in there. It's going to be tough. With the bulky suits and gloves, it's going to be tough work."

But "if these instrument repairs don't go well, they won't do any harm to the observatory so we won't be any worse off for not having tried. But it is a pretty packed timeline and the crew and our engineers have worked very hard to refine the tools and the techniques to get all this stuff to fit within five EVA days. It's a very ambitious mission."

For his part, Grunsfeld said he’s confident the astronauts can successfully repair the broken instruments.

"The extra time we've had with the flight delay has allowed us to practice over and over again the removal of these tiny screws," he told CBS News. "For both the STIS repair, with Mike Massimino at the screw driver, and myself for the Advanced Camera for Surveys repair, we've really honed it to the maximum efficiency. As a result, I have high confidence going into it that we'll be able to finish it in the EVA day, maybe a slightly extended EVA day, but that's in the absence of any surprises. And one thing I've learned from the first two missions and involvement in all the Hubble missions is, Hubble is always full of surprises. So we'll have to see on the day we get there."

Burch said he believes "the odds are better than 50-50 for ACS and I think they're much better than 80 percent for STIS. But I hope I don't have to eat my words after this mission."


The Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), both with built-in corrective optics to compensate for the flaw in Hubble's primary mirror, are expected to boost the observatory's data output 44 times above what it was 10 years ago.

The new camera will capture stunning views of planets in our solar system, distant Kuiper Belt objects and all the other usual deep space targets.

WFC3 is the first panchromatic instrument built for Hubble, a wide-field camera with a wide spectral range that will open new windows on the universe and, at the same time, restore lost visual performance due to radiation damage in other detectors. In the near ultraviolet, WFC3 will boost discovery efficiency by 40 percent while the near infrared detector will allow much faster surveys.

"The thing WFC3 has that's particularly exciting is sensitivity into the near infrared," Margon said. "The reason that's important is, once again, the red shift. If you want to look at the distant universe, it gets redder and redder as you look farther and farther away. Hubble is a general purpose telescope, it will look at everything, planets, stars, galaxies, all that. But the problem that probably excites people the most right now is this issue of the dark energy, which is accelerating the expansion of the universe. And that's a problem that didn't exist when Hubble was launched.

"The status of this dark energy now is, everybody agrees it's there, which is itself pretty astonishing, and that the dark energy that is responsible for accelerating the expansion is actually 75 percent of the matter/energy budget of the universe. So not only is it there, but it's the overwhelming form of stuff, even though (10) years ago we had not even a glimmer that it existed."

Hubble has played a major role in the ongoing search for answers, by finding distant Type 1A supernovas, stellar explosions thought to occur when a compact white dwarf in a binary star system accumulates enough mass from its companion to reach a critical density. At that point, the quantum mechanical property that had been resisting the inward crush of gravity is overwhelmed, triggering a catastrophic collapse and explosion.

Because the explosions occur at the same mathematical point - the moment the star's mass exceeds roughly 1.44 times that of the sun - astronomers believe their energy output is roughly the same. Thus, the light output of a Type 1A supernova can be used as a so-called "standard candle," a mileage marker, in effect, that can be used to determine the distance to objects farther back in space and time than otherwise possible.

The apparent brightness of an object drops off with distance from the observer in a precise way and observations in the late 1990s showed Type 1A supernovas in remote galaxies were dimmer than expected. The most obvious explanation, assuming the supernovas really do behave like standard candles, was that the universe had expanded more - and that the supernovas were more distant - than would be expected if the cosmic expansion was slowing down.

Astronomers believe the dark energy driving that acceleration has been present since the big bang, but it was overshadowed by gravity through the first five billion years or so of the cosmic expansion. But as the universe thinned out and its density dropped, dark energy began reversing what to that point had been a gravity-driven deceleration. And so, the universe began accelerating and flying apart faster and faster.

Hubble has found the most distant Type 1A supernovas, helping scientists confirm the idea of dark energy. The problem is, Margon said, "nobody knows what it is, nobody has any clue as to why it's there, what its form is, it's just there. The next thing you want to ask is what the hell is it? Is it Einstein's cosmological constant, is it something else?"

"It turns out, an extremely sensitive test of what form the dark energy is in is to just ask how does this 'oomph' change with cosmic time? How does its importance change with cosmic time? And Einstein's cosmological constant, this repulsive gravity, it doesn't change at all with cosmic time. But if, for example, we're part of a multi-dimensional universe and there are other dimensions pushing on us and stuff like that, those things change with cosmic time.

"So the way you can actually probe that, of course, is to simply look backwards and look at distant objects, because then you're testing the geometry of the universe in the past. And if you ask how far back do I need to look to start to make a difference amongst the different ideas about what dark energy is, it turns out to be a red shift that corresponds very, very nicely to the reddest sensitivity of Wide Field Camera 3. Mostly by good luck, I've got to say!

"So if you can just continue to map out the deviations from the Hubble diagram (classical expansion) of very distant galaxies out in the reddest band where Wide Field 3 works, you should be able to differentiate between models of the dark energy. ... That, I think, is really exciting."

The Cosmic Origins Spectrograph, twice as sensitive as STIS and 10 to 20 times more sensitive than earlier instruments in medium and high resolution spectroscopy, offers equally exciting science.

COS was designed to study the large-scale structure of the universe, the intergalactic medium, the origin of the elements, the formation and evolution of galaxies, the interstellar medium and the formation of stars and planets.

"We now understand that the universe has sort of three slices of the pie," Margon said. "There's dark energy, which is about 75 percent. There's dark matter, which is about 20 percent. And then there are atoms (of normal matter), which is just about 5 percent. But something that there have been glimmers of for about 50 years and now we're finally quite certain of, is that in the atoms-we-know category, most of them are not contained in stars and galaxies, but are rather contained in a very dilute gas in between galaxies.

"The original naive picture of the way the universe was put together was that galaxies were the building blocks and in between galaxies there was essentially a perfect vacuum. Gradually, creeping up over 50 years, the picture is actually reversed. It turn out that probably more than 50 percent of all normal atoms are between galaxies, rather than inside them. Which, of course, continues to drive the Earth, sun and things we know to more of a footnote."

So how does one study the intergalactic medium, or IGM? By looking at distant objects like quasars and figuring out how that light was affected by its passage through the IGM on its way to Earth.

While COS is a general purpose instrument and will be used by astronomers to study a variety of targets, "sort of the motivating design problem was to look at very distant quasars, just as background targets, and your line of sight to them will have to traverse a huge number of these atoms in the intergalactic medium," Margon said.

"It turns out that given the conditions in the intergalactic medium, the only place they will interfere with light from those distant quasars is in the ultraviolet. ... The critical diagnostics cannot be reached from ground-based telescopes. And again, because you need to observe in the UV, there's no future clever technological development from ground-based telescopes that will overcome that. Nobody's going to invent some device to observe light that doesn't arrive.

"So characterizing the state of this intergalactic medium, where most atoms reside, is kind of the father problem for the Cosmic Origins Spectrograph. That's why it's called 'cosmic origins.' Because that dilute medium is the medium out of which galaxies and stars eventually collapsed. But it turns out what has been left behind is, in fact, the majority of the atoms in the universe. It's probably 90 percent hydrogen and 10 percent helium. Everything else, with the exception of just very trace amounts of lithium and deuterium have been built up later in stars."

It is not yet clear how uniform the IGM might be - the degree to which it is lumpy, filamentary or smoothly distributed - but COS may help find the answer.

"As we see absorptions, as we see interference in the spectra of background objects caused by the intergalactic medium, those pieces of matter will have characteristic red shifts depending on how far away they are," Margon said. "And so COS will take these ultraviolet spectra of very distant objects and will ask, are there discrete interruptions of the spectra that correspond to discrete red shifts, in which case it would be very lumpy. Or are there just kind of absorptions everywhere through the spectrum, in which case it might be more uniform. Nobody really knows."

But the answer, Margon said, "actually has very profound cosmological data in it."

"The lumpiness bears an imprint of conditions very early on in the big bang because there's essentially nothing to change it later," he said. "So aside from probing the majority of atoms in the universe, you also end up getting fundamental cosmological information about what were the conditions the instant after the big bang."

If the Atlantis astronauts are successful, Hubble will be more capable than at any point in its history.

"This will be an absolutely, jaw dropping, superb set of scientific capabilities that we're going to be providing the astronomical community," said Lekrone. "You've heard it said many time before, and it's absolutely true, if this mission goes nominally, if we're able to accomplish everything we're setting out to do, then Hubble will be at the apex of its capabilities after the astronauts leave it, it will be better than it's ever been before. And the possibilities that engenders in one's mind are endless, and exciting, and just hard to fathom."