HOW big is the Earth? Any encyclopedia will give you an answer: its equatorial diameter is 12,756km, or, for those who prefer to think that way, 7,926 miles. Ah, but then there is the atmosphere. Should that count? Perhaps the planet's true diameter is actually nearer 13,000km, including all its air. But even that may no longer be an adequate measure. For the Earth now reaches farther still. The vacuum surrounding it buzzes with artificial satellites, forming a sort of technosphere beyond the atmosphere. Most of these satellites circle only a few hundred kilometres above the planet's solid surface. Many, though, form a ring like Saturn's at a distance of 36,000km, the place at which an object takes 24 hours to orbit the Earth and thus hovers continuously over the same point of the planet.

Viewed this way, the Earth is quite a lot larger than the traditional textbook answer. And viewed this way, the Space Age has been a roaring success. Telecommunications, weather forecasting, agriculture, forestry and even the search for minerals have all been revolutionised. So has warfare. No power can any longer mobilise its armed forces in secret. The exact location of every building on the planet can be known. And satellite-based global-positioning systems will guide a smart bomb to that location on demand.

Yet none of this was the Space Age as envisaged by the enthusiastic “space cadets” who got the whole thing going. Though engineers like Wernher von Braun, who built the rockets for both Germany's second-world-war V2 project and America's cold-war Apollo project, sold their souls to the military establishment in order to pursue their dreams of space travel by the only means then available, most of them had their eyes on a higher prize. “First Men to a Geostationary Orbit” does not have quite the same ring as “First Men to the Moon”, a book von Braun wrote in 1958. The vision being sold in the 1950s and 1960s, when the early space rockets were flying, was of adventure and exploration. The facts of the American space project and its Soviet counterpart elided seamlessly into the fantasy of “Star Trek” and “2001: A Space Odyssey”. Other planets may or may not have been inhabited by aliens, but they, and even other stars, were there for the taking. That the taking would begin in the lifetimes of people then alive was widely assumed to be true.

No longer. It is quite conceivable that 36,000km will prove the limit of human ambition. It is equally conceivable that the fantasy-made-reality of human space flight will return to fantasy. It is likely that the Space Age is over.

Bye-bye, sci-fi

Today's space cadets will, no doubt, oppose that claim vigorously. They will, in particular, point to the private ventures of people like Elon Musk in America and Sir Richard Branson in Britain, who hope to make human space flight commercially viable. Indeed, the enterprise of such people might do just that. But the market seems small and vulnerable. One part, space tourism, is a luxury service that is, in any case, unlikely to go beyond low-Earth orbit at best (the cost of getting even as far as the moon would reduce the number of potential clients to a handful). The other source of revenue is ferrying astronauts to the benighted International Space Station (ISS), surely the biggest waste of money, at $100 billion and counting, that has ever been built in the name of science.

The reason for that second objective is also the reason for thinking 2011 might, in the history books of the future, be seen as the year when the space cadets' dream finally died. It marks the end of America's space-shuttle programme, whose last mission is planned to launch on July 8th (see article, article). The shuttle was supposed to be a reusable truck that would make the business of putting people into orbit quotidian. Instead, it has been nothing but trouble. Twice, it has killed its crew. If it had been seen as the experimental vehicle it actually is, that would not have been a particular cause for concern; test pilots are killed all the time. But the pretence was maintained that the shuttle was a workaday craft. The technical term used by NASA, “Space Transportation System”, says it all.

But the shuttle is now over. The ISS is due to be de-orbited, in the inelegant jargon of the field, in 2020. Once that happens, the game will be up. There is no appetite to return to the moon, let alone push on to Mars, El Dorado of space exploration. The technology could be there, but the passion has gone—at least in the traditional spacefaring powers, America and Russia.

The space cadets' other hope, China, might pick up the baton. Certainly it claims it wishes, like President John Kennedy 50 years ago, to send people to the surface of the moon and return them safely to Earth. But the date for doing so seems elastic. There is none of Kennedy's “by the end of the decade” bravura about the announcements from Beijing. Moreover, even if China succeeds in matching America's distant triumph, it still faces the question, “what next?” The chances are that the Chinese government, like Richard Nixon's in 1972, will say “job done” and pull the plug on the whole shebang.

No bucks, no Buck Rogers

With luck, robotic exploration of the solar system will continue. But even there, the risk is of diminishing returns. Every planet has now been visited, and every planet with a solid surface bar Mercury has been landed on. Asteroids, moons and comets have all been added to the stamp album. Unless life turns up on Mars, or somewhere even more unexpected, public interest in the whole thing is likely to wane. And it is the public that pays for it all.

The future, then, looks bounded by that new outer limit of planet Earth, the geostationary orbit. Within it, the buzz of activity will continue to grow and fill the vacuum. This part of space will be tamed by humanity, as the species has tamed so many wildernesses in the past. Outside it, though, the vacuum will remain empty. There may be occasional forays, just as men sometimes leave their huddled research bases in Antarctica to scuttle briefly across the ice cap before returning, for warmth, food and company, to base. But humanity's dreams of a future beyond that final frontier have, largely, faded.