The International Space Station has cost $75-80 billion. Gerstenmaier argues that it teaches us about space exploration. Credit: © NASA

Bill Gerstenmaier from NASA's Johnson Space Center in Houston, Texas, has been manager of the International Space Station (ISS) programme for just over two years. Here, he talks to about the successes, failures and future of the ISS at a time when it is threatened by shifting priorities at NASA - and by a faulty oxygen generator.

Is the United States still firmly committed to the ISS?

It's part of [President Bush's] vision for space exploration. The first priority is to return the shuttle safely to flight, and the second is to complete the ISS.

We have to make sure we don't get so enamoured of the new vision that we forget about the space station. [In the United States], there's always a push for the new thing, and the station isn't as glamorous as the other programmes. We're learning tremendous lessons for exploration there: how to maintain a craft with very limited supplies, how to organize an international project, and how to make best use of the crew.

Once construction of the station is finished in 2010, the shuttle will be decommissioned, so how will you actually keep it running?

If you asked now, "Would you invest that much?" No. But if you walk away from it now, you?ve wasted a tremendous resource. Bill Gerstenmaier , NASA's Johnson Space Centre, Houston

We're looking at that pretty closely. The Soyuz can carry astronauts up to the station, and there are other resupply vehicles. We've also put out a request to US industry to see if they have an interest in resupplying the station. Frankly, we don't know the answer right now, but we have a year or so to get all this figured out.

How long can the ISS survive after that?

I have 28 shuttle flights that I can use to finish the station. The last few are just going to put spares on the station, effectively filling up the pantry. We think we can last three to four years beyond 2010 without any resupply. From a hardware point of view, we can easily fly ISS to at least 2016. We could still get 10 years of science out of the station.

Do you believe that the ISS was oversold by NASA, promising far more than it could ever deliver?

I think that it was. We promised more than we could deliver, and we kept seeing cost overruns. Now we've scaled back, I think we've learned to be more realistic.

Wouldn't it be better to just cut your losses at this point?

We've invested a tremendous amount already. Since the early 1980s, if you include all the redesigns, $75 billion to $80 billion has gone into this. If you asked now, "Would you invest that much?" No. But now that we're here, if you walk away from it, you've wasted a tremendous resource.

Is it really worth all that money?

I don't think any government is thrilled about the amount they have committed to ISS because they don't really see a return on their investment. And I can't say that they have got a fiscal return from the station. But that's part of research; we're building a foundation that will eventually deliver improvements in space-faring.

One of the problems with the station is that it has too many reasons for being. None of the single reasons is significant enough that they capture the imagination of the public, but if you sum all the pieces together it's worthwhile.

At the moment a crew of three does less than ten hours of science a week. Will there ever be a full-time scientist on board?

We plan to upgrade the crew from three to six in January 2009. We'll get more dedicated research time from that. And research isn't just peer-reviewed papers, it's engineering lessons and operational lessons learned. I think those are just as valuable.

Have there been any scientific, rather than operational, discoveries on the ISS?

We have an ultrasound unit on the station that we were using to look at cardiovascular function in zero gravity. We wondered if you could detect broken bones with it, because we didn't have an X-ray machine on board. And it turned out you could.

Now, in Detroit, Michigan, the Redwings ice-hockey team are using ultrasound so they can quickly find out if as player has a fracture, and if not, put him straight back out on the ice. A lot of science can come out of that sort of serendipity.

The station's on-board oxygen generator has been broken for almost a month now. What are your plans?

There's a Soyuz launch on 14 October, so we're busy working out if there are any parts we need. Even if it's not repaired, we can still keep a crew in orbit. I have two other oxygen generating systems: solid chemical 'candles', and a substantial amount of air in the airlock tanks.

A Progress craft can bring up more oxygen, and then potentially another [generator] unit in spring. But this is exactly the same kind of problem you're going to face in exploration, and the way we handle it is paving the way for going to the Moon or Mars.

Will there ever be another space station after the ISS?

There may be another station, but it won't be nearly as complex as ISS. There could be a space station that serves some commercial purpose: tourism, or perhaps materials processing. I think that lunar bases are more likely, serving as a staging post for Mars.