Despite the success of NASA's second shuttle flight since the 2003 Columbia tragedy, the decision to launch astronauts to the Hubble Space Telescope remains uncertain as top agency officials debate its safety.

The shuttle Discovery's near flawless STS-121 mission this month completed NASA's return to flight effort and demonstrated that post-Columbia safety improvements appear to be effective.

But NASA chief Michael Griffin said mission managers and engineers must complete a thorough analysis of Discovery's mission - and launch the upcoming STS-115 flight aboard Atlantis in late August - before deciding whether a Hubble spaceflight is safe to fly.

"No one wants to do a Hubble flight more than I," Griffin said after Discovery's six-astronaut crew landed on July 17. "But we do not want to get ahead of ourselves. We want to go about things in the right way."

After initially canceling the fifth - and final - Hubble serving mission in 2004, NASA backpedaled to study possible robotic flights to the space observatory before returning to the original, astronaut-crewed plan.

That plan, if approved, calls for a team of astronauts to launch toward Hubble no earlier than December 2007, possibly using the Atlantis orbiter, Hubble officials have said.

NASA's primary concerns over a Hubble-bound shuttle servicing mission revolve around astronaut safety.

Since the loss of Columbia and its seven-astronaut crew, NASA has spent enormous efforts and funds to address the tragedy's root cause - heat shield damage from external tank foam debris - and demonstrate shuttle inspection and repair techniques.

Unlike the rest of NASA's 15 remaining shuttle flights, which allow astronauts to take refuge aboard the International Space Station (ISS) if their orbiter is too damaged to land safely, a Hubble-bound crew has no such safety net. The telescope flies in a higher orbit, and at a different inclination to the Earth's equator, than the ISS, making it difficult to reach the station.

But NASA has developed some safety measures to reduce general shuttle flight risks and allow a specific, Hubble-related orbiter repairs.

STS-121 spacewalkers Michael Fossum and Piers Sellers found that a 100-foot (30-meter) robotic arm combination is apparently stable enough to serve as a platform for shuttle heat shield repairs. The tool would allow basic fixes to be performed away from the ISS.

"It gives us a really high confidence that we can use this for repair," Discovery's STS-121 lead shuttle flight director Tony Ceccacci, tapped to lead a Hubble shuttle mission if approved, told SPACE.com. "So it's one of the stepping stones to get there."

The mission was a shot in the arm for Hubble scientists who hope that shuttle flights will be deemed safe enough for one last visit to their orbital telescope.

"We're greatly encouraged," Preston Burch, Hubble program manager at NASA's Goddard Space Flight Center, told SPACE.com. "People aren't running around here with their hands in the air, screaming for joy. But this is certainly a very positive development."

A Hubble in need

Burch said no less than five spacewalks will be required to put Hubble on a science path well into the next decade. After 16 years and thousands of hours observing the universe, the space telescope is showing its age.

"These instruments and all the rest of the equipment don't last forever," Burch said. "Hubble's just like your automobile. You've got to take it into the shop once in a while and take care of it."

The primary goals for a possible Hubble mission include:

• The delivery of the new Wide Field Camera-3 to enhance Hubble's all-seeing eye.

• Replacing the telescope's 16-year-old batteries, a broken fine guidance sensor, and some thermal insulation.

• An overhaul of the telescope's six-gyroscope attitude control system (two are now in operation, with two spares and two broken units).

• A first-ever orbital repair of Hubble's Space Telescope Imaging Spectrograph (STIS). Never designed for repair on Earth let alone in space, the fix requires new tools to remove 111 small screws and replace a broken control board.

Left alone, Hubble could pump out science through 2009 and, with some clever engineering, maintain basic operations through 2011, Burch said. But one last servicing mission will add five good years onto the space telescope's lifetime, he added.

"There's still a tremendous amount of science that can come from it," Burch said, adding that the first bits of new Hubble hardware could be sent to NASA's Kennedy Space Center launch site in August 2007. "The telescope today is a far more capable and reliable telescope than when it was launched."

At a Capitol Hill luncheon earlier this month, Griffin assured that no Hubble decision will be made until after NASA's STS-115 astronauts complete their mission.

"If we learn nothing from Columbia but one thing we ought to understand [that the] people who get on these birds and execute these missions in pursuit of what I believe to be a noble enterprise deserve our attention and respect while they are doing it," Griffin said.

Space News Staff Writer Brian Berger contributed to this story from Washington D.C.

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