The massive cement expanses that dot the flat Florida landscape have been launching pads for history: the first US astronauts blasted into orbit, the Apollo missions to the moon and nearly 30 years of space shuttle flights. But human space travel from Kennedy Space Centre will soon come to a halt.

After nearly three decades, the space shuttle programme is set to come to a close in September.

The retirement of the shuttle fleet will leave Russian Soyuz capsules as the only means to get humans into orbit - at least until commercial providers can deliver a series of new spacecraft to serve as taxis that NASA will pay to ferry astronauts aloft.

The shift comes as part of President Barack Obama's plans to overhaul the US space programme, scrubbing existing plans to develop a new spacecraft to return humans to the moon.

The move amounts to a privatisation of spaceflight, moving away from contractors who simply build spacecraft for the government to contractors that both build and operate them, notes Scott Pace, director of the Space Policy Institute at George Washington

University.

But Obama's new space policy, unveiled last month in his 2011 budget proposal, has proven hugely unpopular in Congress, creating a showdown between lawmakers, NASA administrators and the commercial aerospace industry waiting in the wings.

'I believe this budget and the vision it represents would end our human spaceflight programme as we know it and would surrender at least for our lifetime our leadership in the air,' said Senator David Vitter, a Republican who is joined by many members of both parties, particularly those with NASA centres in their districts.

Critics in Congress decry a loss of US prestige, declining technological progress, the lack of a distinct inspirational goal and a fear that emerging powers like China and India will outpace the US. They also worry about loss of jobs in their districts, with an estimated 7,000 shuttle-related jobs to be lost in and near Kennedy Space Centre alone.

'This senator fears the US is going to be on the sidelines,' said Florida's Bill Nelson, 'while other countries continue to make incremental progress toward destinations like the moon.'

But top space officials argue there was never enough money to get back to the moon under the existing plan, which NASA administrator Charlie Bolden called 'living in a hallucination.' Freed of the routine task of ferrying astronauts to the International Space Station, the US space agency could refocus on new and as yet unspecified missions, he said.

Obama's plan must still be approved by Congress, where the next-generation Constellation spacecraft programme was popular, if underfunded. Members of Congress have expressed ire that they were not consulted, and many say the details of commercial spacecraft are too sketchy and potentially unsafe.

The axed Constellation programme, hatched under former president George W. Bush, was so underfunded that a review panel concluded last year it was unsustainable and could not reach its goals with existing spending levels.

The same panel suggested commercial companies could be enlisted to taxi astronauts into low-Earth orbit, including to the International Space Station.

'The United States needs a way to launch astronauts to low-Earth orbit, but it does not necessarily have to be provided by the government,' the review panel's report said. 'As we move from the complex, reusable shuttle back to a simpler, smaller capsule, it is an appropriate time to consider turning this transport service over to the commercial sector.'

While having a commercial option is a worthy goal, the Space Policy Institute's Pace said such a move is 'premature'.

'I would like to see the commercial crew being demonstrated and being functional before I gave up government capability,' he told the DPA.

The Constellation programme had its sights set on the moon, but could have been used as a backup to reach low-Earth orbit if commercial interests were not ready to deliver crews into space.

'Now we've taken that insurance policy away,' Pace said. 'We have no lunar plans. We have no real Mars plans. We're fully reliant on commercial crew and cargo coming online on time. I think that's an okay bet with cargo, but it's not an okay bet with crew.'

Much groundwork already exists for the jump to commercial spaceflight, but the burden of safety concerns for transporting humans will likely take years, observers note.

Private contractors built much of the shuttle and are involved in much of the preparations of getting the reusable spacecraft ready to fly. NASA has already contracted with private firms to supply the International Space Station after the shuttle retires.

One such craft, Space X's Falcon 9 rocket, is already sitting on a launch pad at Kennedy. The California-based firm said it will conduct its first launch within months and is contracted to make at least a dozen deliveries to the ISS under a $1.6 billion contract with NASA.

Shortly after the change of course for manned spaceflight was announced, NASA named five aerospace firms to devise concepts for transporting humans into orbit. NASA awarded a total of $50 million to the companies to study human spaceflight alternatives

after the retirement of the space shuttle later this year.

The money comes from a government stimulus measure already authorised by Congress last year to jump-start the faltering US economy.

Many of the firms are already giants in the aerospace field, such as Boeing, which received $18 million for its work on a transportation system including a seven-person crew capsule.

The largest grant of $20 million went to 20-year space veteran Sierra Nevada Corporation for work on its Dream Chaser seven-person spacecraft, and United Launch Alliance received $6.7 million for an emergency detection system for Atlas V and Delta IV rockets.