Discovery's final countdown: Space shuttle launch signals NASA transition

After 27 years and more than 143 million miles, the space shuttle Discovery is poised for her final countdown.

The orbiter and its crew of six astronauts are scheduled to lift off from the Kennedy Space Center in Florida at 4:50 p.m. local time – with a packed cargo bay.

The ship is carrying an Italian-built cargo carrier re-engineered to provide extra storage space on the International Space Station. In addition, the orbiter is lofting some 5 tons of supplies and Robonaut 2, which designers envision as an eventual humanoid helpmate for future space-station crews.

Discovery's launch marks the first of three curtain calls – one for each of the remaining vessels in the shuttle fleet – as NASA's human spaceflight program enters a period of profound transition.

NASA is handing off the responsibility of ferrying goods and US astronauts to and from the station to private launch companies.

NASA can then focus on preparing to send human explorers beyond the space station's confines in low-Earth orbit, possibly to asteroids, the moon, and Mars.

Such missions, however, likely would be launched in the 2020s, depending on how agreeable successive administrations are to the current blueprint.

Discovery's last launch

As with the end of any long-running production, the cast and support crew behind Discovery's launch are finding this a bittersweet moment.

"She's been an amazing machine; she's done everything we've asked of her," says Michael Leinbach, the shuttle launch director.

For now, the entire team is focused on successfully completing the mission, he says.

But "landing day is going to be tough," he added during a prelaunch briefing Wednesday. "You'll see a lot of people on the runway who will probably choke up some."

Between launch and landing, however, a packed schedule remains. In addition to Robonaut 2, the orbiter is packing what missions payload manager Scott Higgenbotham calls "one big honkin' radiator," to remain at the space station as a spare, should one of the existing radiators – which shed excess heat from the station's interior – fail.

The radiator is bolted to a platform, which astronauts will attach to the outside of the station to serve as storage space for the additional spare parts slated to arrive during the final two missions.

Discovery's false starts

This launch originally was scheduled for early November. But as controllers filled the shuttle's bullet-shaped external fuel tank with liquid oxygen and liquid hydrogen, they detected a hydrogen leak that forced them to scrub the launch.

When technicians returned to the pad after the fuel had been removed to inspect the tank, they found cracks in key structural supports, or stringers, on the tank's outer shell.

Mission managers opted to roll the orbiter back to the cavernous Vehicle Assembly Building, where they discovered more cracked stringers.

Repairs went smoothly, but mission managers continued to postpone the launch until they were convinced both that they understood the reasons for the stringer cracks and that the repairs had addressed that problem.

Investigators traced the problem to stringers that were strong enough to handle the stress that fueling imparts to the empty tank, but not strong enough to handle that stress when coupled with stress unexpectedly added during construction.

Now, "the hardware is ready to fly," says Michael Moses, who heads the shuttle's mission managements team.

As for weather, "We've had some really great weather coming all the way up to launch," says Kathy Winters, shuttle weather officer. Some isolated showers may pop up during the afternoon close enough to the launch site to warrant yet another postponement. But she gives that a 20-percent chance of happening.

Her bumper-sticker forecast? "Good payload, good launch," she says.