The pull of gravity
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Three elderly American heroes have been touring US military bases in Europe and Asia this month, telling inspiring tales of space adventures that took place before most people in the audience were born.
But the Apollo astronauts - Neil Armstrong, Gene Cernan and Jim Lovell - were not just living on past glories. They looked at the future of manned space flight and lamented President Barack Obama's decision last month to cancel the Constellation programme under which Nasa would have taken Americans back to the moon by 2020.
"We will go back to the moon, notwithstanding our president and his outlook for the future of space," said Mr Cernan at Ramstein Air Base in Germany . The man who in 1972 was the last to walk on the lunar surface added: "Under the president's proposed budget, it is a mission to nowhere."
The astronauts' intervention is part of a growing backlash against the plans of Mr Obama, who argues that the US cannot afford to build the Ares rockets and Orion crew vehicle that make up Constellation and needs a nimbler development programme led by private companies. His opponents, from both political parties, say the decision jeopardises national security, prestige and commercial interests at a time when other countries are boosting their own space programmes.
When Mr Obama announced the death of Constellation in his 2011 federal budget last month, after the agency had already spent $9bn (£5.9bn, €6.5bn) on it, the political and industrial reaction was initially subdued. "People were so taken aback at the scale of the changes that they were left almost speechless," says Elliot Pulham, chief executive of the Space Foundation , an organisation representing all sectors of the space industry. "But now there is uproar."
Mr Obama did propose a $6bn increase in Nasa's funding over five years, with a new emphasis on paying the private sector to develop spacecraft to carry people and materials into orbit. The administration says this could provide a good service to the International Space Station, which orbits 340km above the Earth, more cheaply than Constellation. Funds are to be transferred from Nasa's in-house facilities and its traditional contractors including Northrop Grumman, Lockheed Martin and Boeing to newer space companies such as SpaceX, Orbital Science and Blue Origin.
At the same time, Nasa would pursue new technologies that could eventually take astronauts beyond Earth orbit to places that humans have never visited, such as asteroids or the moons of Mars. But there is no clear headline destination and no timetable for going further than the $100bn Space Station, which has been assembled over the past 15 years as a collaborative venture grouping the US, Russian, European, Japanese and Canadian space agencies.
Charlie Bolden, a former astronaut recruited by Mr Obama to run Nasa, proclaimed "a bold and ambitious effort to explore new worlds, develop more innovative technologies, foster new industries and increase our understanding of the Earth, our solar system and the universe". But Mr Pulham calls the vision "a dim, lethargic, uninspired view of the future. The administration proposes to strip Nasa of meaningful human exploration goals, and instead turn the agency into a space technology hobby shop with a charter To Boldly Go Where Ever," he says.
At a time of growing polarisation in US politics, space remains a relatively bipartisan issue. In the southern states, notably Florida, Alabama, Texas and California, where Nasa and its contractors are big employers - and high-technology status symbols - Republicans and Democrats are combining to defend their interests.
Florida, the home of manned space flight, probably has most at stake, with 40,000 jobs and $2bn in private income depending on Nasa. The state's entire congressional delegation, Democrat and Republican, has written to Mr Obama expressing concern about his proposal. "This leaves the future of US human space flight in serious doubt, and the highly skilled workforce with the prospect of a major upheaval from which it and our space programme will not have the hope of recovery for many years," the Florida politicians say. They are expected to propose legislation that would preserve a domestic US capability to carry astronauts into orbit.
Such reactions are pushing space up the White House political agenda. Next month Mr Obama will address a "space summit" in Florida, conscious that his plans for Nasa could have a significant impact on the state's delicate political balance.
As things stand, the retirement of Nasa's Shuttle fleet at the end of September will leave Russia's Soyuz spacecraft, based on an ultra-reliable Soviet design from the 1960s, as the only means of conveying people between Earth and the Space Station.
Mr Bolden, the Nasa administrator, says a privately developed US "crew taxi" could be ready by 2016 but many regard his timescale as optimistic. In another "save Constellation" letter, sent to the president this week, the Utah congressional delegation writes : "Since private businesses have never previously developed a low Earth orbit system to transport humans to the International Space Station, one can naturally hypothesise lengthy delays and expensive cost overruns for this novel venture."
While Russia may now be a US ally - at least in space exploration - many Americans with memories of the cold war remain suspicious of the country. They worry too about the rapid progress being made by China and India in space. "There is a direct link between our national defence capabilities and our role as global leaders in space exploration," says Rob Bishop, a Republican congressman from Utah. "Destroy one and we stand to lose our global dominance in the other."
Such comments illustrate the uncertainties about the real point of manned space flight. Its promoters have mixed motivations, some expressed more clearly than others.
One is the human imperative to explore - and conquer. Just as the 19th-century American pioneers believed it was their "manifest destiny" to push west to the Pacific Ocean, Nasa was founded partly on the faith that human destiny calls us to move beyond Earth, first to the moon and then into the solar system and eventually through the galaxy.
This view may be heard less in the early 21st century but is still a powerful lure for some. Linked to it is the more fashionable argument that space exploration is one of the most effective ways of inspiring young people to study and then pursue careers in science and engineering.
National glory and military might were also powerful considerations from the start. They fuelled the original space race between the US and the Soviet Union; since the latter's collapse Nasa has found it harder to command federal dollars.
Then there are commercial and industrial benefits. Space is a large global industry with a worldwide turnover of $257bn, according to the Space Foundation's 2009 Space Report . While a lot of the technology originated in manned space programmes, the commercial space industry today is based largely on building, launching and operating satellites for communications, broadcasting and Earth observation. Space tourism is unlikely to become a significant business for several years.
Scientific curiosity is the final driver of the space programme. But many scientists maintain that unmanned missions offer better value for money. Martin Rees, the cosmologist who heads Britain's Royal Society, supports Mr Obama's decision not to send people back to the moon. "It is very important we pursue science in space," says Lord Rees, "but the case for sending people into space is getting weaker with every advance in robotics and miniaturisation."
In the long run there is a role for people in space, Lord Rees says, but no point in going back to the moon 50 years after Apollo. "If people want to go to Mars, that should be a high-risk, low-cost adventure, not an extremely expensive Nasa-style programme."
While some scientists complain that the huge cost of manned space flight sucks funding from wider space research - pointing out for example that no probes to the outer solar system are currently under construction - industry advocates say that is the wrong way to look at the issue. They argue that the US should increase spending on all space activities, manned and unmanned.
Nasa's spending, at around $19bn a year, accounts for 0.5 per cent of the total federal budget - down from almost 5 per cent at the peak of the Apollo programme. In 1966 the government spent more on Nasa than on health and human services; this year the health and services budget will be almost 50 times greater than Nasa's. While not even the wildest enthusiast advocates a return to expenditure on the Apollo scale, Mr Pulham argues that the US should be willing to devote 1 per cent of the budget to Nasa, as it did in the early 1990s.
That would permit the agency to resume development of Constellation, while encouraging innovative private companies to enter the field.
In reality, given the budgetary constraints, the Obama administration is not going to give the industry the money it wants. But the political pressure on the president may lead to some concessions when he speaks in Florida on April 15. One possibility is to keep the Shuttle fleet flying beyond September. More likely, because the main costs will lie well in the future, is a more specific and inspiring long-term destination in space than Mr Obama has offered so far. This might be Mars or one of the Martian moons.
One day, Americans will probably walk again on a celestial body beyond Earth. But it is unlikely that any of the first group of Apollo astronauts will still be around to celebrate their achievement.
Asians jockey for influence in orbit
When American politicians express fears about other nations overtaking the US in space, they usually mention China and India.
China decided to reach for the stars almost as early as the US and Soviet Union. In 1958, months after the launch of Sputnik, Mao Zedong declared that his nation would seek to match the two superpowers and launch its own satellite.
For the next few decades the country struggled to survive on earth in the turmoil triggered by Mao's ideological campaigns. But today it is a force to be reckoned with in space. In 2003 it became the third nation to send a human into orbit with its own rocket; in 2008, a Chinese astronaut performed the country's first spacewalk.
Beijing's moon exploration programme launched its first lunar probe in 2007. A second unmanned probe is scheduled for October. The plan is to land an astronaut in about 2020.
Niu Hongguang, deputy commander of China's manned space programme, said this month the country hoped to launch its Tiangong 1 space module next year. It will then start practising space docking at the Tiangong, a step towards eventually establishing a manned space station.
One reason for other countries' wariness is Beijing's perceived lack of transparency. Official data are rare but, according to state media, it had spent less than Rmb20bn ($2.9bn, €2.1bn, £1.9bn) on its manned space programme by March 2009. This figure is lower than the estimates of international experts, who assume China is running its programme at about one-tenth of the cost of America's.
India's space programme was until recently focused on practical matters linked to improving the lives of its 1.2bn people, with strong capacity in earth observation, weather and telecommunications satellites. But the Indian Space Research Organisation is pushing deeper into space, reflecting New Delhi's changing sense of its place on the global stage - and its desire to ensure its technical capabilities do not lag behind those of other regional powers, especially China.
India launched a successful unmanned lunar mission in 2008, Chandrayaan-1, and is planning another, with a moon rover to collect samples for analysis, in 2012-13. Most ambitiously, the ISRO has appealed to the government for the green light to send two astronauts into earth's orbit for a week - a venture estimated to cost $2.5bn while unlikely to break scientific ground.
International Space Station
The main focus of manned space flight since its assembly in orbit started in 1998, the station will be essentially complete by the end of this year. The original agreement called for it to operate until 2015 but the international partners say they want to keep it going at least until 2020. The station has a crew of six, who typically spend a few months in orbit, living and working in the complex of a dozen laboratory and accommodation modules. It orbits at 340km, which is 1,000th of the distance to the moon, and goes round the Earth 16 times a day.