By Pallab Ghosh BBC science correspondent

I would characterise the decision that America made 35 years ago to retreat from further lunar and Mars missions as a mistake Michael Griffin, Nasa administrator

When Dr Michael Griffin took charge of the US space agency (Nasa) last year, it seemed as if he was being handed a poison chalice.

Whereas some of his predecessors had been in charge of heroic expeditions to the Moon - he had inherited a grounded shuttle fleet that was soon to be scrapped and a partially built International Space Station that critics were labelling expensive and pointless.

Since then, he has overseen the shuttle's return to flight last summer.

Though the objectives of testing repair techniques for the shuttle were achieved, it was evident his engineers had failed to totally solve the problem that led to the destruction of the shuttle Columbia.

Large chunks of foam continued to rip off the external fuel tank, posing a deadly threat to the orbiter.

In a rare in-depth interview, I had a chance to talk to Dr Griffin about his challenges and his vision for Nasa.

Number one priority

Dr Griffin's number one priority is to solve the foam problem and get the aging fleet back into service. So how is he doing?

"Very well", he says. "We've made immense progress in the past few months because of Discovery's initial test flight.

Nasa needs to fix the shuttle's foam problem

We're sure that we know what causes the foam to come off and we believe we have some fixes in store that will take care of that."

According to Dr Griffin, the shuttle could be back in service as early as May.

But having spent about �1.5bn on returning the shuttle to flight last August, how could the same problem that killed seven astronauts on Columbia have happened again?

The agency did the best it could, according to Dr Griffin. His engineers couldn't carry out test flights to understand what went wrong, so they had to rely on modelling the problem and then try to fix it for Discovery's mission.

"We did very well on that mission. We dramatically reduced foam loss everywhere except where that large chunk of foam fell off. Rather than look on the negative side, I think we did very well in mitigating the loss of foam in just one mission"

Lost time

If, as Dr Griffin hopes, the shuttle does return to flight by the middle of next year, his next problem will be to finish off the International Space Station.

Construction has been put on hold for nearly three years while the orbiter fleet has been grounded.

The exploration of Mars will be one of the great and challenging human activities for hundreds of years to come

Before the Columbia tragedy, the space station was in trouble - ambitious plans to build a research lab in the sky were being scaled back as its costs began to increase beyond expectations.

Now Nasa has to make up for lost time and somehow finish off the ISS before the remaining shuttle fleet is retired in four years' time.

Dr Griffin claims it can be done, but only by using all the remaining planned shuttle missions to take up and bolt on the outstanding modules.

That'll be at the cost of scrapping the planned scientific research missions. However, critics say, Nasa has cut back on science in order to build a space station of limited scientific value.

"There are many different kinds of science that we do. We have not cut back the space science programmes such as New Horizons. We have, though, cut back on the human-related science to concentrate on building the space station." Once built, the ISS will be available for research, he says.

Stepping stone

Publicly, Dr Griffin defends the International Space Station and the shuttle programme. But my sense was that he regards them as follies from a bygone age.

He will do whatever he needs to in order to meet international commitments; but he's keen to move on to President Bush's programme to send people back to the Moon, Mars and beyond. So I asked him directly - was the shuttle and all that went with it a mistake?

Dr Griffin says it is time for Nasa to move on and do other things

"I wouldn't characterise the space shuttle as a mistake. I would characterise the decision that America made 35 years ago to retreat from further lunar and Mars missions as a mistake.

"The decision to restrict ourselves to low-Earth orbit is not one, in retrospect, I think we would have made. And I think that's the point of President Bush's vision for space exploration.

"It's time to move out of low-Earth orbit and move on to do other things."

Suddenly Dr Griffin's eyes light up. This seems to be where his enthusiasm really lies.

"The Moon is interesting of itself; but it is also a stepping stone as you go to Mars. And the exploration of Mars will be one of the great and challenging human activities for hundreds of years to come, and there is the question of what lies beyond Mars?"

Moon shot

But asked "why go?" the administrator, once categorised as a "dull engineer", acquires a fervour reminiscent of President Kennedy when he set Nasa the challenge to put a man on the Moon:

"We go not for gold and silver but for knowledge and experience, and for the expansion of technology and of human experience. That occurs when we explore. These are the reasons we do these things, and they are a part of what it means to be human."

Cynics claim the vision is too expensive. Dr Griffin agrees that we cannot afford to go to the Moon with today's funding at the pace we did with Apollo in the 60s and 70s.

But by spending a little each year, he believes that there is enough in Nasa's existing budget to get an astronaut on to the Moon by 2020.

"We can't spend all of our nation's fortune [on this], but... the average American is spending 15 cents per day on the space programme.

"I personally spend more than that on chewing gum. So we are not risking our fortunes. We are risking a tiny portion of wealth to explore, to learn and understand."

Dr Griffin's aim is to restore core values to the agency. To enable it to pursue goals that raise the human spirit and to enable Nasa to reach for the stars once again.