America's abandonment of a manned return-mission to the Moon by 2020 raises the old question of whether it is better to put people into space, with all the huge safety costs that incurs, or to rely on relatively inexpensive machines, such as the robotic rovers that have performed so well exploring the surface of Mars.
It was Barack Obama's judgement yesterday that the $81bn Constellation programme, which had the ultimate aim of a human landing on Mars by the middle of the century, could not be justified. But the arguments for manned space missions are as much to do with national kudos as scientific necessity. Human beings can of course make quicker and better decisions than any robot, but their presence in space also gives a country a sense of pride and international position – membership of an exclusive club of rich nations.
It is interesting that one of the first people to comment on President Obama's announcement was the former head of Nasa, Michael Griffin, who said that the United States is effectively abandoning the field of manned spaceflight to competitors such as Russia and China. "It means that essentially the US has decided that they're not going to be a significant player in human space flight for the foreseeable future," Mr Griffin said.
Fear of what other countries could do in space has always been a driver for manned exploration. The first and only manned mission to the Moon, the Apollo programme, was inspired by this kind of competitive spirit, which was not altogether friendly given that it was carried out at the height of a cold war between the two nuclear superpowers.
America won that race, but only after it had diverted huge amounts of resources to the effort. Many people have questioned what good it did in practical terms.
There is of course an argument for sending astronauts back to the Moon based on something bigger than mere national pride. It is about exploring the frontiers of knowledge with real people. The existence of men and women with the "right stuff" would act as inspiration for millions of children who would otherwise fail to be interested in science.
But it could also be argued that inspiring the next generation might be better done by spending the trillions of dollars of a manned Moon mission on better schools and infrastructure here on Earth – the real place where we actually live.
That's the calculation President Obama has had to make.