By Kevin Connolly
BBC News, Washington
In the middle of July 1969, the United States slipped off the surly bonds of Earth and sent Apollo 11 to the Moon.
The US space shuttle programme is due to end in 2010
It was a mission that combined one of the most daring feats of exploration in human history with one of its most dazzling achievements of science and engineering.
The world was left to marvel at the power of American technology and the scale of its ambition.
But this was all 40 years ago - and it was all ruinously expensive.
So one of the most intriguing problems which President Obama has to resolve in the coming months deals not with the instability of General Motors or the transparency of the system of regulation on Wall Street or even the intransigence of North Korea.
It is about the future of human spaceflight.
To put it bluntly - is the Obama administration ready to pay the colossal bills that would be run up if America were to decide to return to the Moon by 2020?
And is the recession-hit America of 2009 really ready to contemplate a mission to Mars to open a new frontier in the heavens?
The White House has ordered a public review of America's options.
It opens at a time when the country's relative standing in space is at an unusually low point.
The space shuttle programme which was launched in 1981 will come to an end next year when the last craft will be cannibalised, mothballed or scrapped.
But Nasa's proposed replacement programme, Constellation, will not be ready until five years after the shuttle is gone - and that will leave America, for so long the leader in the world of human spaceflight, out of the game, at least temporarily.
Jeff Hanley, the Nasa official in charge of Constellation, says the reason for that is simple.
Having the new generation of launch vehicles ready earlier would have cost much more money, and America's appetite for spending big on space has declined sharply over the years.
"At the peak of the Apollo project, Nasa's budget was something like 4% of the federal budget," he told me.
"Today, less than half a penny of every federal dollar goes to human spaceflight."
The first public hearing of the new review committee drew quite a crowd. Many engineers and contractors who work on the space programme were in attendance.
Most of them would have drawn some encouragement from what they heard from John Holdren, President Obama's science adviser.
"This is a president who gets it," he told them. "He understands the importance of human spaceflight. He was clear in his campaign, and since, about his commitment to go back to the Moon and other destinations beyond low-Earth orbit."
But the problem for Mr Obama is simple. It is about money.
America's budget deficit for this year is going to be around $1.7tn (£1.04tn) - these are hardly the circumstances in which you embark on a major new national project, the immediate practical benefits of which may be rather nebulous.
In the 1960s, America underlined the startling power of its economy by simultaneously undertaking two of the largest and costliest projects in its history - the space programme and the Vietnam war.
The days of that kind of boundless prosperity are over and Americans are going to take some persuading that tax dollars should be spent on a long-term attempt to reach Mars - with the creation of some sort of base on the Moon as a proving ground.
If the space community wants to win the argument, it has three possible options.
First, it can argue that there is some kind of economic benefit to the project - either through materials that might eventually be found in space, or through technologies designed to get there.
Will Americans ever again follow in Buzz Aldrin's footsteps?
It is sometimes argued that materials like Teflon and Velcro were created as a result of the space programme (they were not) - but even if they had been, that is a very expensive way to coat a frying pan or fasten a jacket.
Then there is the philosophical argument that the urge to explore the unknown is part of what makes us human and that we have to answer a kind of collective spiritual need to seek new frontiers - a tough argument to sell in a recession.
Finally, there is the strategic dimension.
The US victory in the race to the Moon in the 1960s was a victory over the communist Soviet Union, America's main strategic rival.
It proved the superiority of American technology - and the greater resources of its economy.
If the United States does not commit itself to a further major programme of exploration it might find itself abdicating space to China, which is expanding its own efforts.
There is no point in winning one Cold War over a communist superpower if you go on to lose a second one.
Triumph and disaster
Michael Neufeld, a historian at the Air and Space Museum, says the last of those arguments is the weakest and he does not believe the American people will buy the idea that a Chinese presence in space represents some kind of threat.
"People are going to have to be convinced that the scientific or the exploration benefits of sending humans to the Moon - and maybe later to Mars - are worth it."
That may not be an easy task.
Man has been in space for nearly 50 years now and the truth is that for long parts of that period the public has been bored by the repetition of space travel and stunned by its cost.
Only moments of triumph like the Moon landings and disasters like the losses of the shuttles Challenger and Columbia really seared themselves into the memory.
We will know within a few months whether Mr Obama himself has bought into any of the economic, philosophical or strategic arguments for reaching out to the heavens - and more importantly, the extent to which he will be prepared to commit American taxpayers' money behind them.
We are told he "gets" space.
We will soon find out if the American people still get it too.