Last week's federal budget proposal put the nail in the coffin for Constellation, NASA's expensive system of rockets and capsules that was to replace the ageing space shuttles and send humans back to the Moon. Influenced by the bleak assessment of a high-level advisory panel last year (see Nature 461, 153; 2009), NASA is abandoning its Moon mission even though it has already spent US$9 billion on the rockets. For now, the idea is to nurture competition between commercial space companies with the aim of lowering the ever-increasing cost of putting payloads and people into space.

At a press briefing by NASA administrator Charles Bolden in Washington DC last week, the sight of the seven men and women clustered in front of him could not help but evoke memories of the 1959 introduction to the Mercury Seven, the first US astronauts to rocket into space. Except that this time, the seven people were not astronauts, but executives of commercial space companies.

With the death of Constellation, Bolden has little choice but to put his faith in these executives. President Barack Obama's budget proposal would offer $6 billion of incentives to privately owned launch companies over five years, to encourage the development of crew capsules to sit atop the rockets that many of the companies are already developing to ferry cargo to the International Space Station. One of the main contenders, SpaceX of Hawthorne, California, says that it can produce a seven-seat capsule for its Falcon rocket in three years. The company estimates the cost of trips to the station at $20 million — a bargain compared with the $50 million the United States now pays Russia for rides in its Soyuz capsules, and compared with the roughly $100 million it costs to put an astronaut up on the shuttle.

This is a high-risk strategy for NASA, and the agency will undoubtedly face sharp questions about it in Congress. It is not known whether these private companies can build capsules to the strict standards required for human space-flight in such a short time. It is even less clear whether a space-flight market really exists beyond the US government — and if so, whether it is large enough to push down the cost of rocket launches.

A cautionary tale can be found in the Atlas and Delta rockets that are used to launch most science missions. The current versions of these rockets were developed in the early 1990s when the US military, needing a way to launch reconnaissance and communications satellites, offered incentives that are similar to the ones NASA is now planning. The programme was initially successful — Atlas and Delta rockets were the cheapest and most reliable around. And their manufacturers were happy to offer a discount because they were betting on an emerging commercial market for satellite communications, exemplified by companies such as Iridium and Globalstar. But ground-based cellular phone networks got to the marketplace first. Iridium and Globalstar folded. And with only NASA and the military as customers, prices for Delta and Atlas rockets have since gone up.

Optimists, however, prefer to use a different example — one from 1925, when Congress authorized the US post office to use private air companies to carry the mail. Several of the companies that were formed to take advantage of that opportunity have since evolved into today's commercial airlines.

If commercial space-flight turns out to work like the airlines and not satellite communications, this high-risk strategy could have a high pay-off — not least for space science. The cancellation of Constellation has already made room in the NASA budget for an 11% boost to space science, mostly in Earth-observation programmes, such as the replacement Orbiting Carbon Observatory for monitoring global carbon dioxide levels. Soon, regular commercial flights could make it not only cheaper to send experiments into space, but easier for scientists to go up and perform their experiments in person (see page 716). And eventually, a fleet of capable commercial rockets could make it cheaper, and therefore more common, for scientists to send telescopes into orbit and to deploy robot probes into the Solar System.

Bolden and the Obama administration deserve credit for striking out in a different direction. This was a risk that had to be taken: NASA's old way of doing business — building hugely expensive government-owned rockets for government use only — was going nowhere fast, least of all the Moon.