Countdown starts for space shuttle sequel

Alan Boyle Science editor • Profile • E-mail

A year ago, the shuttle flight known as STS-121 might have been seen as little more than a rerun of NASA's first post-Columbia mission — a second chance to test the safety modifications made after the 2003 tragedy.

But when shuttle Discovery lifted off for last summer's "return to flight" mission, NASA's managers were surprised to see a piece of foam insulation breaking off the external fuel tank and flying past the shuttle's wing. NASA believes it was just such a mishap that led to Columbia's destruction — and so STS-121 was grounded for months more while the tank went through another round of redesign.

Today, with NASA beginning the countdown toward STS-121's scheduled Saturday launch to the international space station, the mission is much more than a summer rerun. If anything, this year's flight of Discovery is even more crucial than last year's flight of Discovery.

Testing the changes made over the past year is the top task, of course. After extensive wind-tunnel testing, NASA engineers decided to remove the source of last year's foam-debris problem — a 34-pound (15-kilogram), hand-applied section of insulation known as the protuberance air load ramp.

NASA put components and scale models of the redesigned tank through weeks of extensive aerodynamic testing in wind tunnels.

"The results just came back, and the conclusion is, we have good margin and we're good to fly with it," Discovery commander Steve Lindsey told NBC News earlier this month. "But nevertheless, it's probably the largest change to the exterior of the vehicle, the whole shuttle system, we've made since the beginning of the program."

NASA has also addressed some of the other issues that caused so many headaches last year — throwing out a glitch-prone batch of fuel-tank sensors, replacing 15,000 potentially loose gap fillers wedged in between Discovery's protective tiles, and even fixing rubber seals on the windows.

But shuttle program manager Wayne Hale admitted that "we still have areas to improve," particularly when it comes to the flyaway foam problem. In fact, several safety officials recommended holding off on Discovery's launch until engineers could finish fixing another potential problem area: 34 aerodynamically shaped foam ramps meant to protect the supercooled tank's fittings from flying chunks of ice.

Taking a risk on a flight test
In the end, NASA Administrator Mike Griffin sided with those who said the changes made so far needed to be tested sooner rather than later — a decision that met with the Discovery astronauts' approval.

"In my opinion, we made this big change — we ought to go flight-test that change as we develop the new change for the ice frost ramp," said Lindsey, a veteran test pilot.

Part of Griffin's reasoning was that even if flying foam did so much damage to Discovery that it couldn't return home safely, Discovery's crew could shelter themselves on the space station for the several weeks it would take to launch a sister shuttle, Atlantis, on a rescue mission. But the NASA chief admitted that such a scenario could deal a fatal blow to the shuttle program — even if no lives were lost.

"If we were to lose another vehicle, I would tell you right now that I would be moving to figure out a way to shut the program down," Griffin told reporters earlier this month. "I think at that point we're done."

Failure vs. success
If STS-121 is a failure, that would raise grave questions about the future of the international space station as well: Since the Columbia tragedy, the job of resupplying the station has largely fallen to unmanned Russian cargo ships — but currently, only the shuttle can bring up large pieces of the station that still have to be installed, such as the Japanese and European laboratory modules.

FLIGHT STATUS Earliest opportunity for launch is 3:48 p.m. ET Saturday. Forecasters say there's a 40 percent chance of favorable weather for a Saturday launch. Ten-minute launch windows are available daily July 1-19. Video of the launch and other mission highlights will be streamed live by MSNBC.com. • MSNBC.com video | • NASA TV on Web

But if STS-121 is a success, that would clear the path for an ambitious schedule of 16 space station construction missions over the next four years — and perhaps a final servicing stop at the Hubble Space Telescope, which experienced a worrisome glitch just this month. NASA officials say they would continue to make safety improvements in the shuttle system all the way up to the fleet's retirement, now scheduled for 2010.

So how do you judge success? It's not just a matter of getting up to orbit and back down to Earth in one piece. NASA's agenda for Discovery's 12-day mission also includes trying out some safety procedures that aren't yet quite ready for prime time, resupplying and repairing the space station, doing a bit of space science — and generally adjusting to the "new normal" for post-Columbia shuttle flights.

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