The Big Question: Is Nasa's trillion-dollar space mission worth the money?

Why are we asking this now?

Nasa has just announced details of the plan, first announced by President Bush in 2004, for a settlement on the moon, to be permanently staffed from 2024. The outpost would be a way stage of a programme for a manned exploration of the solar system. The hugely costly project, which would return man to the moon for the first time in three decades, has rekindled arguments about whether the US space agency is value for money.

Do we need Nasa?

If you believe Professor Stephen Hawking, the bottom line answer is simple. Our very survival as a species may depend on space travel, a field in which Nasa is world leader. "The survival of the human race is at risk as long as it is confined to a single planet," Professor Hawkings said last week, warning of the existential peril of a major asteroid collision. But if independent colonies can be established in space, mankind's future should be safe.

Professor Hawking is of course thinking of another star, as there is nowhere like Earth in our solar system. But you agree with him, you have to start somewhere - and the moon and Nasa, for all that organisation's imperfections, are the obvious places. Others would argue that the world has far more pressing things to spend its money on back on earth, than this hugely costly mission.

What would the moon mission cost, and can the US afford it?

Nasa expects other countries to take part in the venture. It has already met officials from the European Space Agency, and from the national agencies of Australia, Britain, France, Germany, Italy, India, China and Russia, and an international conference is planned for next year. But the US will clearly be the lead partner (even though the ESA has its own plan to put a man on Mars by 2030).

The moon programme will thus clearly consume the lion's share of Nasa's overall budget - $17bn (£8.7bn) for fiscal year 2007 and $18bn for next year in the next two decades. The lunar station might cost $100bn or more, scientists say. The price-tag for the entire project, including a human mission to Mars, has been put at $600bn. Given the almost inevitable cost over-runs in a cutting edge technological project of this kind, the final bill could be nearer $1 trillion.

But these sums, though huge, are manageable when they are spread over 20 years or more. By comparison, the gross domestic product of the US is some $12 trillion annually, while the bill for the Iraq war is now $7bn (£3.6bn) a month, or $80bn-plus a year. Also Nasa hopes to recoup some of the cost by attracting commercial interests.

What has Nasa achieved?

A great deal, but it is increasingly easy to forget. The agency was set up in 1958, spurred by US anxiety at the Soviet Sputnik. Since then Nasa has put men on the moon, developed the re-usable shuttle programme, placed the Hubble Space Telescope into orbit, and run hugely successful unmanned missions, such as Mars Pathfinder and the Mars Exploration Rovers which have sent back valuable data on the "red planet" to scientists.

But public interest in space exploration has waned. Shuttle flights are now utterly routine, and the experiments performed aboard are of modest scientific value. They are ignored by the media, except when malfunctions threaten a flight. In any case, the shuttle programme will grounded for good after 2010, while the Hubble, arguably the best value for money of any Nasa initiative, is approaching the end of its life.

Space exploration badly needs to recapture the imagination of the public, particularly of young people. So what better than a moon station, and a manned flight to Mars? Critics, however, say that important scientific programmes will be short-changed, to that the headline-grabbing moon project can be funded.

What is the point of a base on the moon?

It would be a first test of whether a permanent human settlement, in cramped and very difficult conditions, is possible away from Earth. The moon's south pole is likely to be chosen as the site, because it lies in sunlight for most of the time. This would enable solar power collectors to generate electricity - crucial for of Nasa's goal of, as far as possible, "living off the land." The area also has craters that may house valuable gases. The one with most potential is Helium-3, a lighter form of the gas that is very rare on Earth, and which could be used for nuclear fuel. The moon, with its very low gravity, is also seen as launch pad for flights to Mars. Nasa is already developing a new rocket, Ares I, and a reusable orbiting and landing capsule called Orion. But some scientists are sceptical, saying that the technology needed to get to the moon "won't get us beyond the moon."

Why can't we simply build out from the existing International Space Station?

The problem is that the ISS is nearing the end of its life. The project is due to be completed over the remaining four years of the shuttle programme, by which time it may have cost $110bn (£55.5bn). Because of its dependence on the shuttle, it is limited to a low earth orbit, limiting its capabilities. Some say it has yielded little of major scientific value - and its main claim to fame now is as the ultimate exotic tourism destination. A moon station would have far greater potential. Michael Griffin, the chief administrator of Nasa has even implied the shuttle and the space station were mistakes: "It is now commonly accepted that was not the right path. We are now trying to change [it] while doing as little damage as we can."

Do we need manned space missions at all?

Unquestionably they are far more costly, and the most spectacular discoveries have come from unmanned missions. But humans have one vast advantage over machines and artificial intelligence. They can evaluate a situation swiftly and intuitively, and make on-the-spot creative judgements, whereas a machine can only do this after evaluating huge quantities of data. Despite massive publicity given to disasters like Challenger and Columbia, human missions have a far higher success rate than unmanned ones. Most crucially, they generate massive human interest.

Should the US be investing in a permanent station on the moon?

Yes...

* It will be an ideal launching pad for manned missions further into the solar system. To Mars and beyond

* The return of men to the moon will boost public support for space exploration

* It could provide valuable scientific breakthroughs

No...

* Establishment of a lunar station will cost far more money than any results can possibly justify

* It will divert resources away from more valuable programmes that yield results more quickly

* Mars, the next target, is not a viable site for a human colony