More than a year has passed since the space shuttle Columbia broke into pieces over central Texas. This past January President Bush announced a long-term program of space exploration that would return human beings to the Moon, and thereafter send them to Mars and beyond. As this magazine goes to press, the twin Mars Exploration Rovers, Spirit and Opportunity, are wowing the scientists and engineers at the rovers’ birthplace—NASA’s Jet Propulsion Laboratory (JPL)—with their skills as robotic field geologists. JPL’s official rover Web site (marsrovers.jpl.nasa.gov) is being stampeded by visitors.

The confluence of these and other events resurrects a perennial debate: with two shuttle failures out of 112 missions, and the astronomical expense of the manned space program, can sending people into space be justified, or should robots do the job alone? Or, given society’s sociopolitical ailments, is space exploration something we simply cannot afford to pursue? As an astrophysicist, as an educator, and as a citizen, I must speak my mind on these issues.

Modern societies have been sending robots into space since 1957, and people since 1961. Fact is, it’s vastly cheaper to send robots: in most cases, a fiftieth the cost of sending people. Robots don’t much care how hot or cold space gets; give them the right lubricants, and they’ll operate in a vast range of temperatures. They don’t need elaborate life-support systems, either. Robots can spend long periods of time moving around and among the planets, more or less unfazed by ionizing radiation. They do not lose bone mass from prolonged exposure to weightlessness, because, of course, they are boneless. Nor do they have hygiene needs. You don’t even have to feed them. Best of all, once they’ve finished their jobs, they won’t complain if you don’t bring them home. So if my only goal in space is to do science, and I’m thinking strictly in terms of the scientific return on my dollar, I can think of no justification for sending people into space. I’d rather send the fifty robots.

But there’s a flip side to this argument. Unlike even the most talented modern robots, a person is endowed with the ability to make serendipitous discoveries that arise from a lifetime of experience. Until the day arrives when bioneurophysiological computer engineers can do a human-brain download on a robot, the most we can expect of the robot is to look for what it has already been programmed to find. A robot—which is, after all, a machine for embedding human expectations in hardware and software—cannot fully embrace revolutionary scientific discoveries. And those are the ones you don’t want to miss.

In the old days, people generally pictured robots as a hunk of hardware with a head, neck, torso, arms, and legs—or maybe some wheels to roll around on. They could be talked to, and would talk back (sounding, of course, robotic). The standard robot looked more or less like a person. The fussbudget character C3PO, from the Star Wars movies, is a perfect example. Even when a robot doesn’t look humanoid, its handlers might present it to the public as a quasi-living thing. Each of NASA’s Mars rovers, for instance, is described in JPL press packets as having “a body, brains, a ‘neck and head,’ eyes and other ‘senses,’ an arm, ‘legs,’ and antennas for ‘speaking’ and ‘listening.’” On February 5, 2004, according to the status reports, “Spirit woke up earlier than normal today . . . in order to prepare for its memory ‘surgery.’” On the 19th the rover remotely examined the rim and surrounding soil of a crater dubbed Bonneville, and “after all this work, Spirit took a break with a nap lasting slightly more than an hour.”

In spite of all this anthropomorphism, it’s pretty clear that a robot can have any shape: it’s simply an automated piece of machinery that accomplishes a task—either by repeating an action faster or more reliably than the average person can, or by performing an action that a person, relying solely on the five senses, would be unable to accomplish. Robots that paint cars on assembly lines don’t look much like people. The Mars rovers look a bit like toy flatbed trucks, but they can grind a pit in the surface of a rock, mobilize a combination microscope-camera to examine the freshly exposed surface, and determine the rock’s chemical composition—just as a geologist might do in a laboratory on Earth.

It’s worth noting, by the way, that even a human geologist doesn’t go it alone. Unaided by some kind of equipment, a person cannot grind down the surface of a rock; that’s why a field geologist carries a hammer. To analyze a rock further, the geologist deploys another kind of apparatus, one that can determine its chemical composition. Therein lies a conundrum. Almost all the science likely to be done in an alien environment would be done by some piece of equipment. Field geologists on Mars would schlep it on their daily strolls across a Martian crater or outcrop, where they might take measurements of the soil, the rocks, the terrain, and the atmosphere. But if you can get a robot to do the schlepping and deploy all the same instruments, why send a field geologist to Mars at all?

One good reason is the geologist’s common sense. Each Mars rover is designed to move for about ten seconds, then stop and assess its immediate surroundings for twenty seconds, move for another ten seconds, and so on. If the rover moved any faster, or moved without stopping, it might stumble on a rock and tip over, becoming as helpless as a Galápagos tortoise on its back. In contrast, a human explorer would just stride ahead; people are quite good at watching out for rocks and cliffs.

Back in the late 1960s and early 1970s, in the days of NASA’s manned Apollo flights to the Moon, no robot could decide which pebbles to pick up and bring home. But when the Apollo 17 astronaut Harrison Schmitt, the only geologist (in fact, the only scientist) to have walked on the Moon, noticed some odd, orange and black soil on the lunar surface, he immediately collected a sample. It turned out to be minute beads of volcanic glass. Today a robot can perform staggering chemical analyses and transmit amazingly detailed images, but it still can’t react, as Schmitt did, to a surprise. By contrast, packed inside the 150-pound mechanism of a field geologist are the capacities to walk, run, dig, hammer, see, communicate, interpret, and invent.

And of course when something goes wrong, an on-the-spot human being becomes a robot’s best friend. Give a person a wrench, a hammer, and some duct tape, and you’d be surprised what can get fixed. After landing on Mars this past January 3, did the Spirit rover just roll right off its lander platform and start checking out the neighborhood? No, its airbags were blocking the path. Not until January 15 did Spirit’s remote controllers manage to get all six of its wheels rolling on Martian soil. Anyone on the scene on January 3 could have just lifted the airbags out of the way and given Spirit a little shove.

Let’s assume, then, that we can agree on a few things: People notice the unexpected, react to unforeseen circumstances, and solve problems in ways that robots cannot. Robots are cheap to send into space, but can make only a preprogrammed analysis. Cost and scientific results, however, are not the only relevant issues. There’s also the question of exploration.  The first troglodytes to cross the valley or climb the mountain ventured forth from the family cave not because they wanted to make a scientific discovery but because something unknown lay beyond the horizon. Perhaps they sought more food, better shelter, or a more promising way of life. In any case, they felt compelled to explore. The drive to explore may be hardwired, lying deep within the behavioral identity of the human species. To send a person to Mars who can look under the rocks or find out what’s down in the valley is the natural extension of what ordinary people have always done on Earth.

Many of my colleagues assert that plenty of science can be done without putting people in space. But if they are between forty and sixty years old, and you ask what inspired them to become scientists, nearly every one (at least in my experience) will cite the high-profile Apollo program. It took place when they were young, and it’s what got them excited. It’s that simple. In contrast, even if they also mention the launch of Sputnik I, which gave birth to the space era, very few of those scientists credit their interest to the numerous other unmanned satellites and space probes launched by both the United States and the Soviet Union shortly thereafter.

So if you’re a first-rate scientist drawn to the space program because you’d initially been inspired by astronauts rocketing into the great beyond, it’s somewhat disingenuous of you to contend that people should no longer go into space. To take that position is, in effect, to deny the next generation of students the thrill of following the same path you did: enabling one of our own kind, not just a robotic emissary, to walk on the frontier of exploration.

Whenever we hold an event at the Hayden Planetarium that includes an astronaut, I’ve found there’s a small but noticeable uptick in attendance. People invariably seek the astronaut’s autograph. This celebrity status holds even for astronauts most people have never heard of. Any astronaut will do. The one-on-one encounter makes a difference in the hearts and minds of Earth’s armchair space travelers—whether retired science teachers, hardworking bus drivers, thirteen-year-old kids, or ambitious parents.

Of course, people have been excited about robots lately, too. From January 3 through January 5, 2004, the NASA Web site that tracks the doings of the Mars rovers got more than half a billion hits—506,621,916 to be exact. That’s a record for NASA.

The solution to the quandary seems obvious to me: send both robots and people into space. Space exploration needn’t be an either/or transaction, because there’s no avoiding the fact that robots are better suited for certain tasks, and people for others. One thing is certain: in the coming decades, the U.S. will need to call upon multitudes of scientists and engineers from scores of disciplines, and astronauts will have to be extraordinarily well trained. The search for evidence of past life on Mars, for instance, will require top-notch biologists. But what does a biologist know about planetary terrains?  Geologists and geophysicists will have to go, too. Chemists will be needed to check out the atmosphere and sample the soils. If life once thrived on Mars, the remains might now be fossilized, and so perhaps we’ll need a few paleontologists to join the fray. People who know how to drill through kilometers of soil and rock will also be must-haves, because that’s where Martian water reserves might be hiding.

Where will all those talented scientists and technologists come from? Who’s going to recruit them? Personally, when I give talks to students old enough to decide what they want to be when they grow up, but young enough not to get derailed by raging hormones, I need to offer them a tasty carrot to get them excited enough to become scientists. That task is made easy if I can introduce them to astronauts looking for the next generation to share their grand vision of exploration and join them in space. Without such inspiring forces behind me, I’m just that day’s entertainment. My reading of history tells me that people need heroes. Nobody ever gave a ticker-tape parade for a robot.

Twentieth-century America owed much of its security and economic strength to its support for science and technology. Some of the most revolutionary (and marketable) technology of the past several decades has been spun off the research done under the banner of U.S. space exploration: kidney dialysis machines, implantable pacemakers, corrosion-resistant coatings for bridges and monuments (including the Statue of Liberty), hydroponic systems for growing plants, collision-avoidance systems on aircraft, digital imaging, infrared hand-held cameras, cordless appliances, athletic shoes, scratch-resistant sunglasses, virtual reality. And that list doesn’t even include Tang.

Although solutions to a problem are often the fruit of direct investments in targeted research, the most revolutionary solutions tend to emerge from cross-pollination with other disciplines. Medical investigators might never have known of X rays, since they do not naturally occur in biological systems. It took a physicist, Wilhelm Conrad Röntgen, to discover them—light rays that could probe the body’s interior with nary a cut from a surgeon.

Here’s a more recent example of cross-pollination. Soon after the Hubble Space Telescope was launched in April 1990, NASA engineers realized that the telescope’s primary mirror—which gathers and reflects the light from celestial objects into its cameras and spectrographs—had been ground to an incorrect shape. In other words, the billion-and-a-half-dollar telescope was producing fuzzy images.

That was bad.

As if to make lemonade out of lemons, though, computer algorithms came to the rescue. Investigators at the Space Telescope Science Institute in Baltimore, Maryland, developed a range of clever and innovative image-processing techniques to compensate for some of Hubble’s shortcomings. Turns out, maximizing the amount of information that could be extracted from a blurry astronomical image is technically identical to maximizing the amount of information that can be extracted from a mammogram. Soon the new techniques came into common use for detecting early signs of breast cancer.

But that’s only part of the story.  In 1997, for Hubble’s second servicing mission (the first, in 1993, corrected the faulty optics), shuttle astronauts swapped in a brand-new, high-resolution digital detector—designed to the demanding specs of astronomers whose careers are based on being able to see small, dim things in the cosmos. That technology is now incorporated in a minimally invasive, low-cost system for doing breast biopsies, the next stage after mammograms in the early diagnosis of cancer.

So why not ask investigators to take direct aim at the challenge of detecting breast cancer? Why should innovations in medicine have to wait for a Hubble-size blunder in space? My answer may not be politically correct, but it’s the truth: when you organize extraordinary missions, you attract people of extraordinary talent who might not have been inspired by or attracted to the goal of saving the world from cancer or hunger or pestilence.

Today, cross-pollination between science and society comes about when you have ample funding for ambitious, long-term projects. America has profited immensely from a generation of scientists and engineers who, instead of becoming lawyers or investment bankers, responded to a challenging vision posed in 1961 by President John F. Kennedy. “We intend to land a man on the Moon,” proclaimed Kennedy, welcoming the citizenry to aid in the effort. That generation, and the one that followed, was the same generation of technologists who invented the personal computer. Bill Gates, co-founder of Microsoft, was thirteen years old when the U.S. landed an astronaut on the Moon; Steve Jobs, co-founder of Apple Computer, was fourteen. The PC did not arise from the mind of a banker or artist or professional athlete. It was invented and developed by a technically trained workforce, who had responded to the dream unfurled before them, and were thrilled to become scientists and engineers.

Yes, the world needs bankers and artists and even professional athletes. They, among countless others, create the breadth of society and culture. But if you want tomorrow to come—if you want to spawn entire economic sectors that didn’t exist yesterday—those are not the people you turn to. It’s technologists who create that kind of future. And it’s visionary steps into space that create that kind of technologist. I look forward to the day when human beings travel the solar system as if it’s our own backyard—not only with robots, but with real live people, guided by our timeless and boundless need to explore.

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Astrophysicist Neil deGrasse Tyson is the Frederick P. Rose Director of the Hayden Planetarium in New York City. He was recently appointed by President Bush to serve on the nine-member President’s Commission on Implementation of United States Space Exploration Policy.