The Canadian astronaut who spoke Welsh in space and put Gareth Edwards' international rugby cap in orbit is training for his second mission.
A Welsh rugby cap floated in orbit on Dr Williams' first mission
Dafydd Rhys Williams, whose father was from Bargoed, south Wales, is due to undertake three spacewalks on a mission to the International Space Station.
But Nasa has grounded the shuttle fleet until May 2006 following problems during Discovery's recent flight.
Dr Williams' first mission was in 1998 when he flew the Welsh flag in space.
Speaking to the BBC Wales news website, he said that if everything goes to plan and the next launch takes place in May, his second mission could be in March 2007, possibly aboard Endeavour.
But it is possible that could be delayed as Nasa tries to solve the problem with shuttles shedding foam from the fuel tanks - and there are four other missions planned before his.
"We are looking at this mission by mission," he said.
"Hopefully...we're not going to have a problem with the foam again, a lot of it remains to be determined on how each flight goes."
The ill-fated shuttle Columbia took Dr Williams into orbit the first time
His first mission was aboard Columbia in 1998 and his second was to have taken place in November 2003, again aboard Columbia.
He lost close friends and Nasa classmates when the spacecraft burnt up on re-entry to earth in February 2003, killing all seven astronauts.
"It's really hard when you lose people who are that close to you," he said.
"And I was one of the first groups of astronauts to go into the field to participate in the recovery efforts so that made it very difficult as well.
"But I think out of all of this though we have a strong commitment to continue the legacy of human space exploration - for the crew of Columbia, also for the crew of Challenger and for the crew of Apollo 1.
Challenger exploded a minute into its flight in January 1986, killing the crew of seven, while a fire on Apollo 1 in 1967 claimed the lives of three astronauts.
Dr Williams said although such tragedies brought home the hazards, he felt the missions were now "probably safer than they have been in the past".
On his next mission, he will carry out three spacewalks, and is training underwater to install an extension to the International Space Station.
Dr Williams flew the flag for Wales in tribute to his late father Bill
For the first time since the Challenger disaster, a teacher who is also an astronaut is to fly on his mission and Dr Williams is keen to involve Welsh schoolchildren in the educational programme.
He said: "We're proposing flying plant growth chambers in space. Students on the ground could compare the growth rate of the terrestrial plants with the space plants.
"What would be exciting is to see if we can make the plant growth chambers that we're going to make available in north America available to students in Wales so that they can have their classroom projects going on at the same time."
He is also hoping to visit Wales again before his mission, and, having learned a few words of Welsh which he broadcast from orbit in 1998, said he wanted to brush up on the language to speak it in space again.
Dr Williams added: "I'm very excited about the chance to be able to go back and fly in space again - all of us in the astronaut programme are really excited to be back flying the shuttle."