CAPE CANAVERAL ? With the International Space Station nearly complete, the final four shuttle missions hope to ensure it's built to last.
After the shuttle's retirement, a fleet of smaller vehicles -- including two that have flown once and two more that have never flown -- will be counted on to bring up the food and equipment needed to ensure the $100 billion research outpost stays in service for at least another decade.
To give the station the best chance to sustain a failure or late deliveries, NASA is stocking it with large spare parts only flown easily by the shuttle and as much food and other goods as possible.
"We've done as good as we can with the remaining shuttle flights," said Bill Gerstenmaier, NASA's associate administrator for space operations. "We've optimized things, we've put things in place, and we're really kind of set up as good as we can (be)."
The shuttle's work resumes with Monday's 6:21 a.m. blastoff from Kennedy Space Center of Discovery and seven astronauts. Their 13-day mission will haul up more than 17,000 pounds of equipment and supplies and a large coolant tank.
Change of plans
The space station, which extends the length of a football field and weighs about 800,000 pounds, began construction in 1998 with the shuttle's cargo capacity in mind. But plans changed after the 2003 Columbia disaster. President George W. Bush announced plans to retire the shuttle in 2010, a timeline that President Barack Obama has stuck to.
NASA began to carefully analyze which station parts were most likely to fail and prioritize the delivery of spares best carried up by the shuttle.
That planning has taken on more importance, with Congress expected to extend the station's life by five years, to 2020 to further capitalize on the huge investment of time and money that went into building the outpost.
"It's critical that all these missions go off successfully for the long-term sustainment of the station," said Cristina Chaplain, director of acquisition and sourcing management for the U.S. Government Accountability Office. "After the shuttle retires, there will be no comparable vehicle that can take things of the size and weight that the space shuttle can take."
The bulky, 1,800-pound ammonia tank flown by Discovery is an example of the large parts vital to the station's long-term survival.
Astronauts Rick Mastracchio and Clay Anderson will replace an existing tank over the course of three spacewalks, replenishing a system that helps dissipate heat generated by the station's electrical systems.
Other critical parts include dome-shaped gyroscopes, doghouse-sized gas tanks and radiator panels that extend 75 feet.
It's easy to transport bunches of them in the shuttle, which can launch a maximum weight equivalent to two 71-passenger buses driven by Brevard Public Schools. One of those buses could fit in an orbiter's 60-foot payload bay.
But if the shuttle has served as a space truck over the years, its successors are more like vans, wagons or -- in the case of the Russian Soyuz capsule that American astronauts will ride to the station until a new U.S. crew vehicle is ready -- a hatchback.
NASA will rely on five unmanned spacecraft to deliver cargo to the station, including vehicles flown by three international partners. European and Japanese freighters that have less than half the shuttle's capacity have each flown once and are only expected to fly once a year. Russia's more compact and proven Progress spacecraft will make six deliveries this year.
But over the next five years, plans call for more than half of the 182,000 pounds of dry cargo the outpost is expected to need to be carried by spacecraft in development by U.S. companies SpaceX and Orbital Sciences Corp., according to a 2009 GAO report.
SpaceX's Dragon will lead the way, with resupply runs targeted for next May and October. But first the new spacecraft must successfully complete three demonstration flights, planned to start in July.
That timeline appears challenging since the company's Falcon 9 rocket has not yet flown. The inaugural launch could happen next month from Cape Canaveral.
Orbital's Taurus II rocket and Cygnus spacecraft are preparing for a single demonstration flight next March, with later flight dates still to be determined.
?Sense of urgency
"We've always felt there was risk in that (commercial cargo) schedule, by virtue of inherent complexities in developing new rockets and new spacecraft," Chaplain said. "I think those stand."
To reduce that risk, NASA's 2011 budget proposal requests $312 million to bolster the demonstration program with additional tests -- a 62 percent increase over the $500 million originally allocated.
"Anytime you can add more testing to the program, that increases your probability of mission success," said Alan Lindenmoyer, manager of NASA's Commercial Crew and Cargo Program Office.
The flights' urgency depends in part on how well station systems hold up.
If all goes well, Gerstenmaier said a commercial delivery could slip into late 2011 or early 2012 before supplies might begin to run low, threatening the station's ability to sustain a full crew of six.
"Beyond that, it becomes more problematic, depending on what fails," he said. "We need those folks as soon as they're ready to fly."
Their performance will be watched even more closely now that Obama wants to turn the transportation of U.S. astronauts over to commercial companies, too.
Back to earth
The Dragon flights also are seen as critical to the station's science goals.
It's the only spacecraft planned after the shuttle that can return a significant quantity of cargo to the ground, a capability called "downmass" that is critical to some scientists who need experiment samples returned for further study.
Discovery, for example, will bring home 72 glass vials holding jatropha curcas plants that Wagner Vendrame, an associate professor at the University of Florida in Homestead, hopes may be commercially cultivated as an alternative energy crop.
"In order to finalize the experiment and collect the results of what we're performing, we need to do certain analysis that we would not be able to do in space," he said.
David Watson, interim associate director of the National Space Biomedical Research Institute in Texas, said technology improvements such as video monitoring are helping investigators collect data remotely and reduce the quantity of samples needing return.
Prospective station researchers also know that proposals to fly and return large equipment are less likely to win approval.
"Considering all the experiments that could be sent up there, those with less constraints probably have a better chance, because you don't have to bump something else to get a piece of equipment there or to return samples," he said.
NASA scientists are confident the station will flourish as a unique platform for microgravity research. Before Endeavour's February launch, Julie Robinson, the station program scientist, called the post-shuttle transportation plan "very robust."
"We have no concerns about being able to bring home samples for investigators over the life of this extended space station," she said.
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