Among the 1,500 tonnes of vital equipment and supplies loaded on to the space shuttle Discovery when it blasts off on Wednesday will be a more precious cargo. Personal items owned by the crew of the Columbia, who died when the shuttle disintegrated during re-entry over Texas in February 2003, have been donated by their families and will be carried into space as a memorial to their fallen colleagues.
NASA remains haunted by the disaster but it is desperate to prove, as it did after the 1986 Challenger explosion, that it is still up to the job of putting people into space; ambitious plans to reach the moon and even Mars depend on it. The agency has spent US$1.1 billion to improve safety on its remaining three-strong shuttle fleet since it lost Columbia, most of which has been aimed at preventing the same scenario happening again. Columbia was doomed just seconds after takeoff when a chunk of insulating foam broke free from its giant external fuel tank and smashed a hole through panels on the wing needed to insulate the craft and crew against the searing heat of re-entry.
The accident, pictures of which were beamed around the world, reminded us that spaceflight is a risky venture, just as years of successful missions and progress to build the international space station had made it seemingly routine. No longer. US$1.1 billion buys a significant improvement in safety, but no amount of money can guarantee it.
As NASA administrator Michael Griffin said when he announced Discovery's launch late last month: "We are being as smart about this as we know how to be but we are up against the limits of our human knowledge. If someone wants more, they're going to have to find smarter humans."
The Columbia accident has guaranteed one thing for Discovery: attention. Columbia's launch merited just one mention in the British press at the time, which focused on the possible terrorist threat to a mission that included Israel's first astronaut, Ilan Ramon. Events on July 13, or whenever Discovery shakes the Florida sands during its three-week launch window, will prove that space travel is big news again.
A successful launch cannot come quickly enough for Griffin and NASA. President George W. Bush's announcement that the US is again aiming for the moon -- with a possible mission to Mars to follow -- has piled pressure on the agency to finish building the space station by 2010. This is partly because the station will be used as a staging post and partly because finishing the long-overdue and over-budget project will free much-needed funds.
The shuttle remains the only way to ferry up the giant segments that are bolted together to extend the station in space. To meet the 2010 construction deadline, NASA has set out a punishing schedule for Discovery and the other two orbiters, Endeavour and Atlantis, which must carry out at least another 28 missions between them, at a rate of two a year each.
Some doubt that the aging shuttle fleet is up to the job. Discovery's launch (its 31st) and NASA's much-heralded return to flight has already been delayed once this year because of lingering safety fears that ice could fall from the external fuel tank. The new mission was only given the green light late last month, and that despite NASA failing to implement three of the 15 safety recommendations made by an independent group set up to scrutinize the Columbia disaster.
So what's changed? The shuttle will look much the same on the launchpad, but there are subtle differences. Most of the improvements are intended to stop debris striking the orbiter vehicle during launch. The biggest risk addressed is the foam used to insulate the huge external tank of chilled liquid hydrogen fuel and liquid oxygen, a suitcase-sized chunk of which was Columbia's downfall.
Dislodged ice is also a danger at launch (the foam is used to prevent it forming) and electric heaters have been installed at several key points, such as the mechanism that joins the fuel tank and orbiter.
Discovering damage is also a priority. When the countdown clock reaches zero, a battery of cameras on the shuttle, around the launchpad and along the US Atlantic coast will whirr into action.
NASA experts will pore over the images during the first six days of the mission looking for evidence of possible problems.
To help them, Discovery's wings have been fitted with dozens of sensors to detect the smallest impact and gauge the size of the resulting cracks and holes.
In space, the shuttle will unfold a robot arm (now twice as long) from its cargo bay and use a video camera and laser imager on the end to inspect crucial areas around the craft such as its nose and the leading edges of the wings. And finally, as it approaches its docking point at the space station, the orbiter will perform a graceful turn to expose the fragile heat-protection tiles on its sensitive underbelly to yet more prying electronic eyes.
Every inch of Discovery will be examined. But what if one of the images reveals NASA's worst nightmare, another hole that could allow superheated plasma to penetrate during re-entry?
"There are different scenarios depending on what type of damage you would see," says a spokesman at NASA's Johnson space center in Houston. "We expect to see some damage, but we don't expect to see damage of a critical size."
NASA has always been aware that the shuttle's heat-protection system was likely to prove its achilles heel. The vehicle frequently returns with heat-proof tiles missing and, even after a successful launch, is vulnerable to strikes from space junk and meteorites.
In-space repairs had been considered and rejected before. But following Columbia, engineers at the agency have revived efforts to give the crew the tools they need to save themselves if a large enough hole is discovered.
Discovery's crew will perform several spacewalks to test the new repair techniques on surplus bits of shuttle carried in Discovery's cargo bay. Back on Earth, the in-orbit DIY jobs will be checked to see whether they are good enough to rely on in future.
To repair the thousands of tiles that cover the underside of the shuttle, the astronauts will try out three techniques. The first is a silicone-based material squeezed into damaged areas from two tanks held on the astronauts' back using a pressurized gun. The sticky material is designed to harden and then burn away gradually during re-entry, similar to ablative heat shields -- in which the shield material itself vaporizes and takes the heat away with it as it goes -- used on space vehicles up to the Apollo missions.
One problem with this mid-space grouting task is that the material expands when heated, so astronauts will have to judge the quantities squeezed out to make sure the expanded material does not disturb the way superheated gases flow over the shuttle's surface during re- entry, perhaps making the situation worse.
The second method applies a dark-colored coating of silicon-carbide granules to tiles which have had their black coating knocked off. In this case, changing the color of the tiles could make the difference between life and death by restoring their ability to radiate away heat.
The third method is more of a sticking plaster -- insulating blankets in a variety of shapes and sizes that can be screwed to remaining intact tiles to cover holes. Early tests suggest that these might form problematic bubbles when applied in space.
Techniques to repair the reinforced carbon blankets wrapped around the most vulnerable parts such as the leading edges of the wing are also a combination of chemical and physical. Cracks can be repaired using a putty-like material, though this might need astronauts to take a portable heater with them on their spacewalk. "A selection of hand tools similar to putty knives would be used to smooth the mat them to be folded to meet the contours of the shuttle, these must be a set shape and size, which raises the possibility of astronauts physically drilling out smaller holes caused by impacts to make them big enough to repair.
Because NASA has been unable to prove that such repairs make a shuttle safe to tackle Earth's atmosphere, the safety review board was unable to approve them.
"Until we do it in space we won't know for certain if it works," the spokesman says. "We've done it on the ground, but that's one reason we're going and flying."
If Discovery's crew are forced to try out their puncture repair kit for real, then the decision to let them attempt re-entry will not be taken lightly. Instead, there are plans for an unprecedented rescue mission, codenamed Safe Haven.
"The last resort would be to leave the shuttle at the station and launch Atlantis to retrieve the crew in basically a rescue mission," the NASA spokesman says. "That in itself is a risky endeavor."
NASA experts think that life support systems on the station could keep the nine people alive for about 56 days. Once the order is given, Atlantis could be launched in 31 days.
The sight of a crippled Discovery being jettisoned from the space station to burn up in the atmosphere would be a devastating blow for NASA. And another fatal accident could lead the US public's enthusiasm for astronauts to dwindle so much that the organization which put a man on the moon is forced to focus on robot missions.
Jeffrey Hoffman, who flew on the space shuttle five times from 1985 to 1996, said: "If the [Discovery] flight is not successful, it's more than NASA's reputation at stake. It's the whole scenario of what the US is going to do with the space station and in space in general."
Even if Discovery lands safely, risk analysts think that the loss of another shuttle during the 28 scheduled missions to 2010 could be as high as 43 percent.
"I worked through the Challenger incident," Hoffman says. "It's two-sided. You can't get those images out of your mind. But everybody knew at the time that the return to flight after Challenger was going to be the safest flight we'd ever flown. We all said at the time the real challenge is not flying that next one safely, it's flying the 100 flights down the line."
The Discovery crew face a similar dilemma. Thoughts of the seven astronauts who died two years ago will weigh on their minds, and not just because a picture of the smiling Columbia crew will be on display in Discovery where it can be seen every day.
"They were dear friends to us -- they weren't just colleagues -- and besides thinking about them every single day, both at work and outside of work, on the mission we will commemorate them in a couple of different ways," mission specialist Steve Robinson says.
The Discovery flight's mission patch (an emblem individually designed for each flight and more important to astronauts than it might seem) has been adapted to include seven extra stars, one of which is the star of David.
But the crew are also eager to get on with their job.
"It's time to move on," says Eileen Collins, the first woman to fly the shuttle, and commander on the forthcoming Discovery launch.
"The Columbia astronauts knew what the risks were. We will always remember our friends, but it's time to take what they lived for and what they believed in, space exploration, and move on and get the shuttle flying again. We need to get the space station built and I know that's what they would want us to do," she says.
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