Two astronauts currently assigned to the International Space Station (search) are making preparations to greet the first shuttle flight since Columbia crashed in 2003 as it was returning to Earth.

One of them is American John Phillips (search), who said his mission has changed from conducting science experiments to focusing on the safety of future shuttle flights.

Phillips, along with Russian Sergei Krikalev (search), will be making sure space shuttle Discovery is safe to return to Earth.

Discovery is scheduled to lift off on Wednesday.

"It's a big moment for NASA to prove if new safety measures will work. And Sergei and I ... will be a big part of that. We're going to be mapping the outside of the vehicle ... looking for any damage to the thermal protection," Phillips told FOX News.

For Phillips, the mission has meant a bit of a career change. Before the Columbia tragedy, he was supposed to go up to the ISS in a space shuttle. But the disaster in February 2003 changed all that.

Three months ago, he found himself in the flight engineer seat of the Russian Soyuz rocket as it made its way from Earth to the ISS.

"I had to go through full flight engineer training, and that adds a whole huge dimension to it. It adds a whole lot of time here training but also adds some satisfaction. I frankly didn't like the right seat and not knowing what the two guys to my left were doing, because I hadn't had enough training to understand it," Phillips said, recounting the training he received in Moscow.

The 54-year-old former Navy pilot said not only did his ride change, so did his mission from essentially conducting science experiments to preparing future shuttle missions for the return flight.

Phillips and Krikalev are excited about their new mission of welcoming shuttle flights for the next six months.

But the mission is about more than safety. The Russians who built many of the modules for the space station note the space station originally designed to be built in 1988 now probably won't be completed until 2010.

The return of the space shuttle program means the construction process is back on track.

"We are all looking forward to seeing it because we need it. Technically, the program was developed for the assembly period based on the shuttles' capabilities. It cannot be replaced by any existing vehicles in any country," Alexei Krasnov, director of manned space flight in Russia, said of the new space station program.

Krasnov said the next generation of spacecraft won't be less complex ... people are talking about the next flights to go to the moon and then Mars.

But more missions in more complex spacecraft mean more risk, too.

"The best of all worlds isn't the world we live in. We are resource limited, and so you have to accept a certain level of risk, and I'm not sure it's some sort of sliding scale. If you want to accept no risk you just never go," said Phillips.

FOX News' Dana Lewis contributed to this report.