By Christine McGourty
Science Correspondent, BBC News
On his hands and knees, hammering nails into the poolside decking, Piers Sellers looks like any other dad battling to keep on top of the home repairs.
But this is the humid and mosquito-ridden Clear Lake area of Texas, a stone's throw from Nasa's Johnson Space Center; and Piers Sellers' day job leaves little time for DIY at the moment.
A crew member assigned to the next space shuttle launch, British-born Sellers is due to make his second spaceflight on Saturday.
With two, possibly even three, spacewalks scheduled, his role on the mission is a gruelling one. But relaxing indoors, with a drop of wine and the company of wife Mandy, you would not guess the man behind the beaming smile would soon be dropping through a hatch in the shuttle and lowering himself into space.
It is, he says, one of the things he is looking forward to most.
"You can train in water and that gives you some idea of what it's like being weightless. And we use virtual reality, where you put the goggles on and it looks like you're outside the space station," he explains.
"All of this helps a lot, but it's not the same as actually opening the hatch in the floor, sticking your head out and falling into space, with the Earth rolling by underneath your body at five miles a second, and the Sun blazing down on you.
"It's a strange, rather harsh and very beautiful environment."
And though every minute of the spacewalks is carefully planned in advance, there is sometimes an opportunity just to enjoy the moment.
"Sometimes they say, 'just sit there for two minutes while we figure something out', and you get two minutes to watch Africa go by between your toes, or watch the Sun come up over Asia, or see thunderstorms flashing for 4,000 miles around the equator.
Sellers has practised the mission on a simulation system (Image: P.Francis)
"It is remarkable, to see all that, with your own eyes, through a visor - remarkable."
For a moment, you can almost imagine why a very clever, grown man with a wonderful wife and two teenage children would risk everything for a ride into space. Since the Columbia disaster and Challenger before that, it has been impossible to ignore the fact that this is not a risk-free activity.
But the trademark smile of this former Sussex man turns to astonished laughter when I suggest there might be just a possibility that he will not return safely from the mission.
Failure, it seems, is programmed out of the minds of Nasa's astronauts.
He concedes, though, that Nasa has failed to solve completely the foam problem that has dogged it for years - it was a suitcase-sized piece of foam falling from the fuel tank on lift-off that damaged Columbia's wing and caused the heat-shield to fail as it came through the Earth's atmosphere at the end of the mission three years ago.
"I'm pretty confident some foam will come off the tank," says Sellers. "We always lose a little bit and we expect to lose a little bit this time. But I'm pretty confident no large pieces will come off in places where we don't want them to come off. That is, we won't have pieces that will be likely to strike the shuttle."
Back on track
If there was a critical foam strike once again, the task of fixing the problem could fall to Sellers. With spacewalk partner Mike Fossum, he'll be testing the shuttle's boom-arm system - to see whether it can act as a stable platform from which astronauts could do any crucial repairs to the shuttle's exterior.
They will also be testing some repair material - though it's only suitable for minor cracks or nicks.
When I joined them training for the spacewalks in Nasa's giant swimming pool - the so-called Neutral Buoyancy Laboratory - it crossed my mind that this might not have been what Sellers had in mind when he first dreamed of being an astronaut, inspired by the Moon missions of the Apollo era.
DISCOVERY SHUTTLE FLIGHT
Mission known as STS-121
Discovery's 32nd flight
18th orbiter flight to ISS
Lift-off: 1549 EDT, 1 July
Location: Kennedy Space Center, Launch Pad 39B
Objective: To test new safety equipment and procedures
Payload: Cargobay has 12.75t of equipment and supplies
Crew: Lindsey, Kelly, Fossum, Nowak, Wilson, Sellers, Reiter
Sellers was flat on his back, struggling to squeeze into the legs of his spacesuit, with fellow crew members Stephanie Wilson and pilot Mark Kelly lending a hand.
It took the best part of half an hour before he was ready to be lowered into the pool - where a full-scale replica of the shuttle's payload bay and the International Space Station were submerged below.
With the flight delayed more than once, there has been no shortage of time for extra training and the crew is now more than ready to get this crucial flight underway.
STS-121, as it is designated, is just the second shuttle flight since Columbia and the final so-called Test Flight, intended to prove the safety of the shuttle so that regular service can resume again in the autumn.
Some 16 launches are required over the next few years to complete construction of the space station before the shuttle is finally retired in 2010. Any serious glitches on this latest mission and the orbiter might never fly again.
Sellers is confident of a successful mission, but admits that nothing is guaranteed: "We've made the space shuttle system as safe as we think we can for the time being.
"We've driven down a lot of risks; but at the end of the day, it's a dangerous business and it always has been. I don't think any of us are under any illusions that there is still some risk there."