Space Shuttle Discovery executed a triumphal return to Earth today, capping a letter-perfect mission that began with thunderstorm delays and safety concerns, but ultimately showed what NASA at its best can achieve.

The mission high points included multiple confirmations that the orbiter was unharmed in its blue-sky Fourth of July launch, and with solid progress achieved in three spacewalks (extra vehicular activities, or EVA).

While NASA has suffered setbacks including fatal incidents during its recent history, this mission is burnishing the image of the agency, in much the same way that NASA gained from unmanned missions to Mars featuring live videos of the red planet landscape. There were few glitches in the Discovery mission, except for a tiny auxilliary power unit fuel tank leak.

NASA Administrator Michael Griffin termed it “an enormously successful flight … as good a mission as we’ve ever flown,” adding that he “never saw a vehicle look this clean,” or undamaged, upon landing.

But Griffin warned against overconfidence, noting that the space station is only half-built, so that NASA needs to have the 16 remaining shuttle flights succeed. He added, though, that future flights will be driven by safety and other concerns, not by the schedule.

“We don’t have any slack,” he said. “We have just enough shuttle flights left” to finish building the space station, “so we can’t mess up.”

While the successful Discovery flight countered safety concerns that prompted two key NASA safety and engineering officials to argue against the flight in a pre-mission conference, Griffin said he still welcomes free expression and arguments, and hopes they continue.During the Discovery mission, with astronauts wearing space suits instead of hard hats, they formed a galactic construction crew to work on the ISS for hours at a time.

Floating weightless in the void, at times standing upside down hundreds of miles above their home planet, Piers J. S . ellers and Michael E. Fossum labored on the ISS.

They helped to repair a miniature railway along the outside of the ISS and performed other work toward an eventual end to construction of the space station, a task that was delayed for years. That delay was caused when the Space Shuttle Columbia was damaged during a launch so that it disintegrated in the subsequent reentry, with all crew members lost on Feb. 1, 2003.

Foam insulation ripping loose from the Columbia external fuel tank damaged a heat shield on the leading edge of a wing, permitting blistering-hot gases of reentry to intrude inside the orbiter, and it failed structurally.

Meticulous multiple microscopic examinations of the Discovery orbiter exterior revealed that no such damage occurred, as expected. Cameras running during launch and ascent showed that only small pieces of foam came loose, and in any event that occurred at a point too late in the flight to damage the orbiter.

Examples of goals achieved included laying a new cable and installing a replacement cable drum so that work on the ISS construction could proceed.

As they worked moving heavy items, the astronauts could be heard grunting from the effort they had to exert.

The duo labored for hours, bolting and unbolting items, and finally finished their spacewalk almost on time.

One of the many things that went right is that the shuttle crew members were cleared to extend their mission by a day, to a total 13 days. They include astronauts Steven Lindsey, commander; Stephanie Wilson, Lisa Nowak, Sellers, and Fossum, all mission specialists; and Mark Kelly, pilot.

Another of the three spacewalks had a serious goal: finding ways to repair a space shuttle if it is damaged.

With the next launch set for Aug. 28, when Space Shuttle Atlantis will soar heavenward, NASA officials have said that just because Discovery launched without a problem, that doesn’t guarantee that there never can be another mishap damaging an orbiter.

On the one hand, because officials after the Columbia disaster know that such damage could occur, there is no chance that a damaged shuttle orbiter would attempt reentry, so another Columbia-style disaster won’t happen.

But an orbiter is a very expensive piece of hardware, and a vitally needed asset if the ISS program is to continue. An average of more than four shuttle launches per year is planned for the remainder of this decade.

So a key question now is, if a shuttle is damaged during a launch, can it be repaired in space?

The answer may be yes.

In this EVA, Sellers–a British subject by birth–and Fossum did a takeoff of “This Old House” by applying a type of repair putty to the exterior of the shuttle. It’s called NOAX, or non-oxide adhesive experimental.

However, even if the NOAX works well in sealing cracks, it wouldn’t be able to repair a hole in an orbiter wing of the size that was punched in the wing on Columbia.

Another success in this by-the-book mission was more prosaic: the shuttle brought hundreds of pounds of fresh supplies to the ISS, sorely needed after the dearth of shuttle missions since the Columbia incident. And, of equal import, Discovery became a sky-high, high-tech trash truck, taking a huge number of discarded items back to the home planet.

Eventually, the shuttles will be replaced by a Crew Exploration Vehicle (CEV).

The CEV, which wouldn’t appear until the next decade, could be a key component in the U.S. move to explore space. President Bush has advanced a vision of manned flights returning to the moon, then venturing to Mars, and thence onward into the solar system.

But a key Senate panel raised questions as to whether the money will be made available to turn such an expansive, and expensive, vision into reality. (Please see full story in this issue.)