By Christine McGourty
BBC science correspondent
Could the Discovery astronauts, if they had to, repair the kind of damage that led to the destruction of Columbia?
Nasa has invested great effort in designing different techniques
Over two years on, and despite a huge research and development effort by Nasa, the answer to the question remains unclear.
Though the space agency would have liked one, there is no reliable, proven "shuttle repair kit".
But on day five of Discovery's mission, the crew's spacewalkers Soichi Noguchi and Steve Robinson will test two repair methods in space for the first time.
In the shuttle's cargo bay is a large box containing a selection of damaged shuttle parts - cracked white tiles as well as a sample of the Reinforced Carbon-Carbon (RCC) panels that protect the leading edge of the spacecraft's wing during re-entry.
On the first spacewalk of the mission, Noguchi will open this and attempt to repair the tiles using a sticky, thick grey substance called "emittance wash".
Robinson will then test a crack repair technique using a material referred to as Noax, for Non-Oxide Adhesive Experimental.
Lora Bailey, the engineer leading the effort to develop tile repair techniques, described the method of applying the emittance wash as a simple "shoe polish applicator method".
The material is housed in a reservoir inside the handle and dispensed out through a foam tip.
"You just apply it by dabbing it on the tile," says Bailey.
The emittance wash is intended primarily to repair relatively shallow cracks. But it could also be used as a primer, prior to application of some other repair material - such as the silicon-based material that became known colloquially as the "goo gun" and which Nasa has been developing to fix more extensive damage.
Initial tests with the "goo gun" were disappointing, though, and that is not being tested on this mission.
SHUTTLE RETURN TO FLIGHT
Mission known as STS-114
Discovery's 31st flight
17th orbiter flight to ISS
Payload: Raffaello Multi-Purpose Logistics Module
Lift-off: 13 July, 1551 EDT
Location: Kennedy Space Center, Launch Pad 39B
Discovery crew: Collins, Kelly, Noguchi, Robinson, Thomas, Lawrence and Camarda
For the emittance wash test, Noguchi will have 25 minutes to attempt to fill a crack measuring 10cm by 10cm (four inches by four inches); plus another half hour to clear up the expected mess.
On the RCC sample, there are four cracks and two "gouges" and Robinson is expected to try to fix one of each.
This technique is designed to fix the type of damage caused by small pieces of foam coming off the redesigned external tank - though Nasa hopes any pieces of debris would be far smaller this time.
Other repair techniques have been developed by Nasa but will not be tested on this mission.
These include not just the "goo gun" but mechanical, as opposed to chemical, repair methods.
Carbon-silicon carbide plates could be fitted to a damaged part of the wing's leading-edge and the Noax material might also be used as a sealant once the plate was fitted.
The astronauts have openly expressed their reservations. Mission commander Eileen Collins commented earlier this year that some of the techniques Nasa had been developing "are more mature than others; some are more promising than others".
She added that in her view a repair technique should be "tested in space, brought back home and run through some kind of test facility" before any moves to use it for real damage.
She described some of the repair techniques as "immature" and said the mission was "an experimental flight".
The safety of Eileen Collins and her crew depend on the shuttle's thermal protection
"I don't believe they're certified in my opinion for us to fly back on," she said. Adding, though, that she was hopeful the techniques would be ready in the "not too distant future".
And fellow astronaut Charlie Camarda said the task of developing an on-orbit repair kit should not be underestimated: "It's a very, very daunting problem and people have spent an awful long time trying to solve it.
"We never thought it would be possible to actually repair a vehicle in space. It's a very extreme environment."
One problem, he said, was that what seemed a good repair might be destroyed during the heat of re-entry.
"Depending on the type of damage, whether its on the belly of the vehicle or whether it's on the wing leading edge makes a big difference as to whether you're going take the risk and say this repair is safe or it isn't safe."
In tests, the Noax material in particular has caused concern with its tendency to expand and start bubbling.
Nasa has decided not to go with the goo gun this time
The bubbles can be scraped down, but after the material stops bubbling it becomes too hard to apply, so the astronaut has to find the "sweet spot" , when the bubbling has stopped but the material is still workable, according to Cindy Begley, Nasa's officer for the mission's spacewalks.
Though uncertainties such as this remain, Nasa's Steve Poulos, manager for the orbiter project office, described the repair techniques as "contingency capability hardware".
He added: "Some things cannot be tested on the ground. Everything we can do on the ground we have done. We're learning as we go. "