The robots are out there, dozens of them, going where their soft-bodied, oxygen-breathing creators can’t or won’t anytime soon. They own space. While a handful of humans hunker down in near-Earth orbit in the International Space Station, an aging craft conceived in the Reagan era, unmanned machines at this very moment are orbiting Mercury, trundling across the sands of Mars, even preparing to leave the confines of the solar system.

The space station is a thing of beauty in its way, the apotheosis of Apollo-style technology. But in terms of scientific achievements it suffers in comparison with NASA’s spaceborne fleet of robots—currently 55 strong—especially given the large funding gap that has always existed between the manned and unmanned space programs. NASA’s budget for 2012 provides about $4.5 billion for robotic space science, versus $8.3 billion for human space exploration, almost $3 billion of which goes to the station alone. And that is the outlay for a NASA without shuttle flights or plans to send people back to the moon.

Noble as human exploration may be, we would know very little about anything in the cosmos much more distant than the moon were it not for robotic explorers. Through them we have learned of lava plains on Venus, a buried ocean on Jupiter’s moon Europa, lakes of methane on Saturn’s moon Titan, and salty geysers on another Saturnian moon, Enceladus. And manned missions? Since the Apollo moon landing of 1969, NASA has mostly confirmed what it knew from the outset, which is that hurtling humans deep into space is expensive, dangerous and, for the foreseeable future, beyond reach. The reality is, when it comes to carrying out serious space science, humans simply can’t compete with spacefaring hardware. And that is probably not going to happen in our lifetime.

Sheltered as we are by Earth’s atmosphere and magnetic field, which deflect lethal radiation from space, we are like coddled children who have never ventured into a tough neighborhood. The space station, orbiting less than 250 miles overhead (about the distance between Boston and Philadelphia) hardly qualifies as a space station at all. It is really more of a not-quite-above-Earth’s-atmosphere station, since it, like the shuttles once did, travels within the thin air of the ionosphere.

Real outer space is hazardous, even for robots. Ed Stone, the project scientist for the astonishingly successful twin Voyager missions that flew past Jupiter, Saturn, Uranus, and Neptune in the late 1970s and 1980s, and are now on their way to the stars, says the Voyagers would not have survived their close encounters with Jupiter had it not been for some advance warning from a previous mission.

That precursor mission, called Pioneer 10, flew past Jupiter in 1973. It did not get as close to Jupiter as the two Voyagers would, but it got near enough to find that the massive planet could wreak havoc on spacecraft. “Pioneer 10 discovered that the radiation environment of Jupiter was much more severe than anyone had expected,” Stone says. “It took us nine months to redo the circuitry on the Voyager spacecraft to make it much less susceptible to radiation effects.”

In just a few hours near Jupiter, Voyagers 1 and 2 absorbed blasts of radiation hundreds of times greater than the lethal dose for humans. And surface conditions on some planets are even more murderous. The early Soviet Venera probes, the first spacecraft to touch down intact on another world, were built like tanks. Even so, they succumbed to Venus’s crushing atmospheric pressure and 900 degree Fahrenheit surface temperatures—hot enough to melt lead—within a couple hours of landing.

The sheer harshness of the environment that begins a few hundred miles above our heads, and the huge distances involved in traveling to other planets, bode poorly for any of us who might dream of establishing human outposts much beyond Mars. “I don’t think the stories of science fiction we read in childhood are ever going to happen,” says John Mather, a Nobel laureate who is now the senior project scientist for the James Webb Space Telescope, the $6.5 billion successor to the Hubble telescope that NASA plans to launch in 2018. “People won’t be rocketing around the solar system. I think we’ll be able to send people throughout the inner solar system, to visit asteroids, comets, and Mars—if we want to. Beyond that,” he says, pausing for a moment, “people are fragile, and we cry when they die. Robots are fragile but we don’t cry when they die.”

Perhaps the demise of some robots should be mourned. Right now, the two Voyager spacecraft are poised at the very edge of the solar system; Voyager 2, the more distant, is nearly 11 billion miles from Earth. The spacecraft have traveled farther than any other objects humanity has made, and they are still sending back data 34 years after their launch. Both Voyagers have reached the outer limits of the heliosphere, an enormous, tenuous bubble of charged particles from the sun that surrounds our solar system.

Instead of the relatively stable environment that was expected at the brink of interstellar space, the Voyagers have encountered what some scientists have described as a “magnetic jacuzzi,” a region where the sun’s magnetic field appears to be ripping apart and reconnecting, forming bubbles millions of miles across. It is a place human explorers will not visit in the foreseeable future, although the data on the flux of cosmic rays (energetic particles from deep space) into the solar system will allow engineers to better gauge the risk to both spacecraft and humans during extended space voyages.

The Voyagers are now so far away that the signals from their 23-watt radio transmitters, powered by a radioactive generator, take more than 12 hours to reach Earth. Sometime within the next five years Voyager 1 will become the first spacecraft to cross the boundary of the heliosphere and enter interstellar space. There it is likely to be buffeted by winds of particles from supernovas that will be colder and denser than the solar wind, though still not as dangerous as Jupiter’s radiation belts. The spacecraft should have several years to explore the new realm. “We have enough electrical power to operate fully until 2020,” says Stone, who has been involved with the Voyagers since their inception.