In the 1950s, astronomer Fred Whipple theorized that comets were made of ice, with some rocks and dirt mixed in, and that their trademark long tails were made of vapor created by the sun's heat. To put Whipple's "dirty snowball" idea to the test, scientists sent unmanned ships to get a closer look. The European spaceship Giotto got to within 200 kilometers of Halley's comet in 1986, and since then several probes have passed close to other comets. Now a U.S. ship, Deep Impact, launched in January, has taken the closest look yet. Last week it hurled a hunk of copper at the comet Tempel 1 and recorded the impact, obtaining new data about the comet's composition that may give scientists clues about the early solar system.

It's not a job for the frail--which is to say, it's not a job for humans. These days the most successful space missions seem to be the ones that involve no astronauts. Instead, robotic spaceships, unencumbered by costly life-support equipment and the prying eyes of a safety-conscious public, are sent to do scientists' bidding. Deep Impact is only the latest in a string of astoundingly successful robotic missions that would seem to settle once and for all the perennial question: do we really need humans to explore space? Consider what robot probes have accomplished in the past couple of years. Rovers have made extensive forays across the Martian surface. The Cassini probe is sending back data about Saturn's system of moons and rings. Earlier this year the Huygens probe penetrated the murky haze of Saturn's biggest moon, Titan, and gathered data that may yield clues to the origin of life on Earth. In the meantime, NASA's space shuttle Discovery sits in a hangar at the Kennedy Space Center--a May launch, which would have been the first since the Columbia accident in February 2003, was postponed to July 13--and the International Space Station has been treading water with a skeleton crew of two.

From this recent history it would be reasonable to think that the powers that be are shifting their emphasis away from exorbitantly costly human-spaceflight programs to cheaper and ultimately more enlightening robotic ones. Exactly the opposite is happening. In January 2004, President George W. Bush responded to NASA's woes by setting the United States on an ambitious course to send humans back to the moon and on to Mars. The Europeans are keen to follow suit; they're already funding an unmanned Mars program, which they hope will serve as a precursor to a manned visit, and are now looking for money to contribute to any moon venture the United States embarks on. With budgets tight, funds for manned spaceflight will have to come at the expense of robotic missions.

For NASA, the emphasis on manned spaceflight has taken on nationalistic tones reminiscent of the 1960s moon shot. Instead of Sputnik, the impetus this time is the launch in October 2003 of China's first astronaut and Beijing's promise to colonize the moon. "As a matter of what it takes to be a great nation in the 21st century, I do not believe that we would wish to see a situation where the United States is dependent on any partner for human access to space," NASA administrator Michael Griffin recently told the U.S. Congress. "We need our own capabilities. Two nations [Russia and China] have now put people into space since the United States has last done so. I do not like that." Europe, loath to be left on the sidelines, is looking for a way to develop a spaceship that could transport astronauts without NASA's help--giving Europe what space agency officials call "guaranteed access."

Europe has a powerful rocket in the Ariane 5, but lacks a "crew transport vehicle"--a capsule or ship that would sit on top of Ariane and house humans. Without such a vessel, European astronauts have had to hitch their rides from the United States or Russia. With the shuttle program stalled, though, Europe is mulling a Russian proposal to jointly build an ambitious new spacecraft, the Clipper, that would be launched on a Russian rocket--a third-generation Soyuz II--from French Guiana. The Clipper would be capable of carrying six astronauts into orbit and allowing them to glide back to Earth.

The fate of the Clipper hangs on a request for 50 million euro that the European Space Agency plans to make at a gathering of European government ministers in December; the project may ultimately cost Europe 150 million euro per year over the next decade. An alternative would be for ESA to take its Autonomous Transfer Vehicle--a cargo ship it developed for the ISS that's expected to go up on Ariane 5 later this year--and adapt it to carry a handful of astronauts. ESA officials won't say how much money such a retrofit would require, but it could easily exceed the cost to Europe of the Clipper project.

Europe hasn't yet cannibalized its robotic missions to enable such manned projects. It still plans to launch a spate of probes in the next few years: BepiColombo will be launched toward Mercury in 2011, Gaia will head off to map the stars in the same year, and LISA (the Laser Interferometer Space Antenna) will begin its search for gravity waves in 2013. But in the United States, Bush's vision is already putting a financial burden on NASA. Plans to send a human crew to the Hubble Space Telescope to carry out essential repairs were scuttled, despite an outcry from scientists. For the past 15 years Hubble has given scientists their best view yet of the universe. It's sent back more than 700,000 images of gigantic gas clouds, dying stars and galaxies from billions of years ago. Astronauts had been expected to bring fresh batteries and gyroscopes to the telescope later this year in an effort to extend its useful life another few years. NASA claims that the repairs would be too risky. But critics say that the agency's reluctance is due to a lack of funds. (NASA is considering a cheaper plan to send robots to service the telescope.)

Hubble isn't the only casualty. Budget cuts have closed at least 14 facilities and eliminated several thousand jobs (15 per--cent of NASA's work force). Six Earth-science programs, five orbiting telescopes and seven space probes have either been canceled or delayed.

To be sure, national prestige isn't the only argument for putting humans in space. Many tricky tasks may arise during a long space voyage that only humans can accomplish, experts say. Robots have a hard time with tasks that require manual dexterity. They can't improvise. They don't cope well with the unexpected. "I can program a robot to go down the hall and get a Coke for me from the machine," says John Logsdon, director of the Space Policy Institute at George Washington University. "But if it runs across a $100 bill it will just roll over it."

On a Mars mission that lasts several years, a robot could be stymied by logistical problems that would be a cinch for an astronaut to solve. Steven Squyres, an astronomer at Cornell University and principal investigator of the Mars Exploration Rover, once set up a competition between robots and his graduate students to collect and examine rocks on a field trip in the Mojave Desert. The contest wasn't even close. "It would take our rover one day to accomplish what the humans could do in 35 to 40 seconds," he says. "As capable as robots are, it will take the application of on-the-ground human intelligence to explore our solar system," says NASA's Griffin.

And even if NASA could build a robot that did everything an astronaut could do, would it succeed in capturing the public's imagination? The conventional wisdom has always been that only humans can inspire the public to support space exploration--it wouldn't have meant as much, for instance, if a robot had landed on the moon in 1969 instead of Neil Armstrong. But the recent success of the robotic missions, together with the Internet's ability to bring the latest images to a wider public, have called that reasoning into question. Huygens's compelling images of Titan--they show what appear to be an ice lake at the south pole and a volcano that may release methane--were a big hit with Europe's sci-fi lovers. The Mars Exploration Rovers attracted tens of millions of Web hits a day to NASA's site from people eager to see the latest images of the Red Planet. "That's bigger than eBay," says NASA chief scientist Jim Garvin.

The current thinking at NASA and ESA is that robots will act as astronaut handmaidens of a sort--scouting ahead of manned missions and performing tasks that astronauts don't want to do, or can't. In 2008, for instance, NASA plans to send the Lunar Reconnaissance Orbiter to chart the moon's surface in preparation for a manned mission. The same strategy will hold for a Mars mission. NASA's deputy administrator, David Gregory, told U.S. lawmakers earlier this year that the space agency might send robots to Mars, but only in "anticipation of eventual human visits." NASA is even developing Robonaut, an anthropomorphic robot--it has arms and legs, and even speaks--that could act as an astronaut's assistant. It would be able to make basic repairs to a ship, for instance, or at the very least hand tools to an astronaut. Or be the one to crash-land on a ball of dirty ice.