Twenty years ago, the loss of the US shuttle Challenger dealt an enduring blow to confidence in manned space flight yet also helped open up a golden era of exploration by machine.

As the US space agency NASA mourns the 1986 disaster this Saturday, the contrast in fortunes between human and unmanned missions in space has never seemed more acute.

In the past year alone, the US orbiter Cassini has sent home stunning images of distant Saturn and a European suicide probe has landed on its moon, Titan.

Delving into the origins of the Solar System, a NASA spacecraft has smashed open a comet and another has returned home with a cargo of stardust.

On Mars, two US rovers are patiently scouring for signs of water and life, helped by a clutch of orbiters overhead.

Other metal-and-silicon pioneers are en route for the Mercury, Venus and Pluto, while perhaps the most ambitious of all, a European lab is heading to a rendezvous with a distant comet... in 2014.

As robots take on the role of doughty explorers, manned missions in contrast have remained confined to the Earth's backyard and, since Challenger, coloured by safety worries.

"Challenger showed how sophisticated this technology was and how fragile it was, and our dreams were tied to it," said Roger Launius, chair of the Space History Division at the Smithsonian Institution.

The most ambitious manned project, the International Space Station (ISS), and the vehicle on which it depends, the shuttle, are deep in crisis.

Billed as mankind's orbital outpost and a platform for exhilarating zero-gravity science, the ISS may never be completed or even manned other than by a skeleton crew, yet its final cost is widely put at around US$100 billion.

A complex design 30 years old, the shuttle has been crippled by flaws that have cost 14 lives in 114 missions and turned it into a "hangar queen" where an army of engineers pore over every piece before worriedly letting the vehicle fly again.

Initially touted as a cheap form of renewable transport, the shuttle has cost US$145 billion, or around US$1.3 billion per trip, says Roger Pielke, Director of the Centre for Science and Technology and Research at the University of Colorado.

In contrast, NASA's two Martian rovers, Spirit and Opportunity, have cost some US$900 million. Built to operate for only 90 days, they are still going strong two years later.

The shuttle's problems are so bad that NASA is to retire the fleet in 2010, and build a new craft a bigger, updated version of the old Apollo capsule that would ride aloft on shuttle boosters.

With only three shuttles left in service after the loss of Columbia in 2003, and with 18 construction missions needed to bring the ISS to an acceptable state of completion, NASA is wrestling with a real headache.

Unless it can get the shuttle flying again at quick intervals, the agency will have to delay or even abandon deployment of ISS science modules built by Europe and Japan at an overall cost of at least US$1.3 billion.

Apart from the ISS, the other big event in manned flight has been the emergence of China, which in 2003 became the third power to place a man in orbit, and of entrepreneurs, after Space Ship One in 2004 became the first private craft to touch the fringes of space.

The European Space Agency (ESA), for instance, devotes only an eighth of its budget to manned space flight and has not bothered to develop its own spaceship, choosing instead to place its handful of astronauts on US or Russian craft.

Instead, the lion's share of ESA's budget goes into launchers, science satellites in the Earth orbit or space probes that deliver greater value for money. Japan thinks similarly.

NASA, thirsting to revive the can-do reputation that made it such an inspiring agency in the 1960s and early 70s, is pushing hard to take humankind back out of the Earth orbit.

This week, its top officials were to meet with international partners to discuss US President George W. Bush's vision for humans to return to the Moon by 2018 and, later, possibly venture forward to the Mars.

But enthusiasm outside the United States seems notably absent, given the grim experience of the cash-gobbling ISS and doubts about the whole point of Bush's plan.

"The lunar trip is of course interesting, but from the European perspective, it makes scientifically more sense to go directly to Mars," said Jean-Jacques Favier, a former astronaut and trained geologist who is in charge of strategic thinking at France's National Centre for Space Research.

Even then, a Martian mission will depend on faster rockets to reduce the time of interplanetary travel and on scout probes, rovers and other machines to do the spadework for humans.

Source: China Daily