spacecraft

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 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.

In daily life you rarely need to think about propulsion, at least the kind that gets you off the ground and keeps you aloft. You can get around just fine without booster rockets—simply by walking, running, rollerblading, taking a bus, or driving a car. All those activities depend on friction between you (or your vehicle) and Earth's surface.

When you walk or run, friction between your feet and the ground enables you to push forward. When you drive, friction between the rubber wheels and the pavement enables the car to move forward. But try to run or drive on slick ice, where there's hardly any friction, and you'll slip and slide and generally embarrass yourself as you go nowhere fast.

For motion that doesn't engage Earth's surface, you'll need a vehicle equipped with an engine stoked with massive quantities of fuel. Within the atmosphere, you could use a propeller-driven engine or a jet engine, both fed by fuel that burns the free supply of oxygen provided by the air. But if you're hankering to cross the airless vacuum of space, leave the props and jets at home and look for a propulsion mechanism that requires no friction and no chemical help from the air.

One way to get a vehicle to leave our planet is to point its nose upward, aim its engine nozzles downward, and swiftly sacrifice a goodly amount of the vehicle's total mass. Release that mass in one direction, and the vehicle recoils in the other. Therein lies the soul of propulsion. The mass released by a spacecraft is hot, spent fuel, which produces fiery, high-pressure gusts of exhaust that channel out the vehicle's hindquarters, enabling the spacecraft to ascend.

Propulsion exploits Isaac Newton's third law of motion, one of the universal laws of physics: for every action, there is an equal and opposite reaction. Hollywood, you may have noticed, rarely obeys that law. In classic Westerns, the gunslinger stands flat-footed, barely moving a muscle as he shoots his rifle. Meanwhile, the ornery outlaw that he hits sails backward off his feet, landing butt first in the feeding trough—clearly a mismatch between action and reaction. Superman exhibits the opposite effect: he doesn't recoil even slightly as bullets bounce off his chest. Arnold Schwarzenegger's character the Terminator was truer to Newton than most: every time a shotgun blast hit the cybernetic menace, he recoiled—a bit.

Spacecraft, however, can't pick and choose their action shots. If they don't obey Newton's third law, they'll never get off the ground.

Realizable dreams of space exploration took off in the 1920s, when the American physicist and inventor Robert H. Goddard got a small liquid-fueled rocket engine off the ground for nearly three seconds. The rocket rose to an altitude of forty feet and landed 180 feet from its launch site.

But Goddard was hardly alone in his quest. Several decades earlier, around the turn of the twentieth century, a Russian physicist named Konstantin Eduardovich Tsiolkovsky, who earned his living as a provincial high school teacher, had already set forth some of the basic concepts of space travel and rocket propulsion. Tsiolkovsky conceived of, among other things, multiple rocket stages that would drop away as the fuel in them was used up, reducing the weight of the remaining load and thus maximizing the capacity of the remaining fuel to accelerate the craft. He also came up with the so-called rocket equation, which tells you just how much fuel you'll need (assuming you won't be stopping at any filling stations en route) for your journey through space.

Nearly half a century after Tsiolkovky's investigations came the forerunner of modern spacecraft, Nazi Germany's V-2 rocket—V for Vergeltungswaffen, or Vengeance Weapon. The V-2 was conceived and designed for war, and was first used in combat in 1944, principally to terrorize London. The brainchild of Wernher von Braun and hundreds of other scientists and engineers working with the Nazis, the V-2 was the first ballistic missile and the first rocket to target cities that lay beyond its own horizon. Capable of reaching a top speed of about 3,500 miles an hour, the V-2 could go a few hundred miles before plummeting back to Earth's surface in a deadly free fall from the edge of space.

To achieve a full orbit of Earth, however, a spacecraft must travel five times faster than the V-2, a feat that, for a rocket of the same mass as the V-2, requires no less than twenty-five times the V-2's energy. And to escape from Earth orbit altogether, and head out toward the Moon, Mars, or beyond, the craft must reach 25,000 miles an hour. That's what the Apollo missions did in the 1960s and 1970s to get to the Moon—a trip requiring at least another factor of two in energy.

And that represents a phenomenal amount of fuel.

Because of Tsiolkovsky's unforgiving rocket equation, the biggest problem facing any craft heading into space is the need to boost excess mass in the form of fuel—most of which is the fuel required for transporting the fuel it will burn later in the journey. And the spacecraft's weight problems grow exponentially. The multistage vehicle was invented to soften this problem. In such a vehicle, a relatively small payload—such as the Apollo spacecraft, an Explorer satellite, or the space shuttle—gets launched by huge, powerful rockets that drop away sequentially or in sections when their fuel supplies become exhausted. Why tow an empty fuel tank when you can just dump it and possibly reuse it on another flight?

Take the Saturn V, a three-stage rocket that launched the Apollo astronauts toward the Moon. Designed by von Braun (among others), it could almost be described as a giant fuel tank. The Saturn V and its human cargo stood thirty-six stories tall, yet the three astronauts returned to Earth in an itty-bitty, one-story capsule. The first stage dropped away about ten minutes after liftoff, once the vehicle had been boosted off the ground and was moving at about 9,000 feet per second (more than 6,000 miles per hour). Stage two dropped away about ten minutes later, once the vehicle was moving at about 23,000 feet per second (almost 16,000 miles per hour). Stage three had a more complicated life, performing several episodes of fuel burning: the first to accelerate the vehicle into Earth orbit, the next to get it out of Earth orbit and head it toward the Moon, and a couple more to slow the craft down so that it could pull into lunar orbit. At each stage, the craft got progressively smaller and lighter, which means that the remaining fuel could do more with less.

Since 1981, NASA has used the space shuttle for missions in low-Earth orbit—a few hundred miles above our planet. The shuttle has three main parts: a stubby, airplanelike orbiter that holds the crew, the payload, and the three main engines; an immense external fuel tank that holds more than half a million gallons of self-combustible liquid; and two solid rocket boosters, whose two million pounds of rubbery aluminum fuel generate 85 percent of the thrust needed to get the giant off the ground. On the launchpad the shuttle weighs four and a half million pounds. Two minutes after the launch, the boosters have finished their work and drop away into the ocean, to be fished out of the water and reused. Six minutes later, just before the shuttle reaches orbital speed, the now-empty external tank drops off and disintegrates as it reenters Earth's atmosphere. By the time the shuttle reaches orbit, 90 percent of its launch mass has been left behind.

Now that you're launched, how about slowing down, landing gently, and one day returning home? Fact is, in empty space, slowing down takes as much fuel as speeding up.

Familiar, earthbound ways to slow down require friction. On a bicycle, the rubber pincers on the hand brake squeeze the wheel rim; on a car, the brake pads squeeze against the wheels' rotors, slowing the rotation of the four rubber tires. In those cases, stopping requires no fuel. To slow down and stop in space, however, you must turn your rocket nozzles backward, so that they point in the direction of motion, and ignite the fuel you've dragged all that distance. Then you sit back and watch your speed drop as your vehicle recoils in reverse.

To return to Earth after your cosmic excursion, rather than using fuel to slow down you could do what the space shuttle does: glide back to Earth unpowered, and exploit the fact that our planet has an atmosphere, a source of friction. Instead of using all that fuel to slow down the craft before reentry, you could let the atmosphere slow it down for you.

One complication, though, is that the craft is traveling much faster during its home stretch than it was during its launch. It's dropping out of an 18,000-mile-an-hour orbit and plunging toward Earth's surface—so heat and friction are much bigger problems at the end of the journey than at the beginning. One solution is to sheathe the leading surface of the craft in a heat shield, which deals with the swiftly accumulating heat through ablation or dissipation. In ablation, the preferred method for the cone-shaped Apollo-era capsules, the heat is carried away by shock waves in the air and a continuously peeling supply of vaporized material on the capsule's bottom. For the space shuttle and its famous tiles, dissipation is the method of choice.

Unfortunately, as we all now know, heat shields are hardly invulnerable. The seven astronauts of the Columbia space shuttle were cremated in midair on the morning of February 1, 2003, as their orbiter tumbled out of control and broke apart during reentry. They met their deaths because a chunk of foam insulation had come loose from the shuttle's huge fuel tank during the launch and had pierced a hole in the shield covering the left wing. That hole exposed the orbiter's aluminum dermis, causing it to warp and melt in the rush of superheated air.

Here's a safer idea for the return trip: Why not put a filling station in Earth orbit? When it's time for the shuttle to come home, you attach a new set of tanks and fire them at full throttle, backward. The shuttle slows to a crawl, drops into Earth's atmosphere, and just flies home like an airplane. No friction. No shock waves, No heat shields.

But how much fuel would that take? Exactly as much fuel as it took to get the thing up there to begin with. And how might all that fuel reach the orbiting filling station that could service the shuttle's needs? Presumably it would be launched there, atop some other skyscraper-high rocket.

Think about it. If you wanted to drive from New York to California and back again, and there were no gas stations along the way, you'd have to drag along a fuel tank as big as a tanker truck. But then you'd need an engine strong enough to pull a tanker, so you'd need to buy a much bigger engine. Then you'd need even more fuel to drive the car. Tsiolkovsky's rocket equation eats your lunch every time.

In any case, slowing down or landing isn't only about returning to Earth. It's also about exploration. Instead of just passing the far-flung planets in fleeting flybys, a mode that characterized an entire generation of NASA space probes, the spacecraft ought to spend some time getting to know those distant worlds. But it takes extra fuel to slow down and pull into orbit. Voyager 2, for instance—launched in August 1977—has spent its entire life coasting. After gravity assists from both Jupiter and then Saturn (the poor man's propulsion mechanism), Voyager 2 flew past Uranus in January 1986 and past Neptune in August 1989. For a spacecraft to spend a dozen years reaching a planet and then spend only a few hours collecting data on it is like waiting two days in line to see a rock concert that lasts six seconds. Flybys are better than nothing, but they fall far short of what a scientist really wants to do.

On Earth, a fill-up at the local gas station has become a pricey activity of late. Plenty of smart scientists have spent plenty of years inventing and developing alternative fuels that might one day see widespread use. And plenty of other smart scientists are doing the same for the world of propulsion.

The most common forms of fuel for spacecraft are chemical substances: ethanol, hydrogen, oxygen, monomethyl hydrazine, powdered aluminum. But unlike airplanes, which burn fuel by drawing oxygen through their engines, spacecraft have no such luxury; they must bring the whole chemical equation along with them. So they carry not only the fuel but an oxidizer as well, kept separate until valves bring them together. The ignited, high-temperature mixture then creates high-pressure exhaust, all in the service of Newton's third law of motion.

Bummer. Even ignoring the free lift a plane gets from air rushing over its specially shaped wings, pound for pound, any craft whose agenda is to leave the atmosphere must carry a much heavier fuel load than does an airplane. The V-2's fuel was ethanol and water; the Saturn V's fuel was kerosene for the first stage and liquid hydrogen for the second stage. Both rockets used liquid oxygen as the oxidizer. The space shuttle's main engine, which must work above the atmosphere, uses 385,000 gallons of liquid hydrogen and 143,000 gallons of liquid oxygen.

Wouldn't it be nice if the fuel itself carried more punch than it does? If you weigh 150 pounds and you want to launch yourself into space, you'll need 150 pounds of thrust under your feet (or spewed forth from a jet pack) just to weigh nothing. To actually launch yourself, anything more than 150 pounds of thrust will do, depending on your tolerance for acceleration. But wait. You'll need even more thrust than that to account for the weight of the unburned fuel you're carrying. Add more thrust than that, and you'll accelerate skyward.

The space mavens' perennial goal is to find a fuel source that packs astronomical levels of energy into the smallest possible volumes. Because chemical fuels use chemical energy, there's a limit to how much thrust they can provide, and that limit comes from the stored binding energies within molecules. So, given those limitations, physicists and engineers have been looking into innovative alternatives.

After a vehicle rises beyond Earth's atmosphere, propulsion need not come from burning vast quantities of chemical fuel. In deep space, the propellant can be small amounts of ionized xenon gas, accelerated to enormous speeds within a new kind of engine. A vehicle equipped with a reflective sail can be pushed along by the gentle pressure of the Sun's rays, or even by a laser stationed on Earth or on an orbiting platform. And within ten years or so, a perfected, safe nuclear reactor will make nuclear propulsion possible—the rocket designer's dream engine. The energy it generates will be orders of magnitude more than chemical fuels can produce.

While we're getting carried away with ourselves, making the impossible possible, what we really want is the antimatter rocket. Better yet, we'd like to arrive at a new understanding of the universe, to enable journeys that exploit shortcuts in the fabric of space and time. When that happens, the sky will no longer be the limit.

Launching a spacecraft is now a routine feat of engineering. Attach the fuel tanks and rocket boosters, ignite the chemical fuels, and away it goes.

But today's spacecraft quickly run out of fuel. In fact, by the time a craft exits Earth orbit, there's no fuel left in its main tanks—which, no longer needed, have dropped back to Earth. Only tiny tanks remain, permitting only mild midcourse corrections. All the spacecraft can do is coast to its destination.

And what happens when it arrives?

Without the benefit of filling stations or sizable tanks of spare fuel, the craft cannot be made to slow down, stop, speed up, or make serious changes in direction. With its trajectory choreographed entirely by the gravity fields of the Sun, the planets, and their moons, the craft can only fly by its destination, like a fast-moving tour bus with no stops on its itinerary—and the riders can only glance at the passing scenery. That's what happened with the Pioneer and Voyager spacecraft in the 1970s and 1980s: they simply careened from one planet to the next on their way out of the solar system.

If a spacecraft can't slow down, it can't land anywhere without crashing, which is not a common objective of aerospace engineers. Lately, however, engineers have been getting clever about fuel-deprived craft. In the case of the Mars Rovers, their breakneck speed toward the Red Planet was slowed by aerobraking through the Martian atmosphere. That meant they could land with the help of nothing more than parachutes and airbags.

Today, the biggest challenge in aeronautics is to find a lightweight and efficient means of propulsion, whose punch per pound greatly exceeds that of conventional chemical fuels. With that challenge met, a spacecraft could leave the launchpad with fuel reserves onboard, and use them much later. Scientists could think more about celestial objects as places to visit than as planetary peep shows.

Fortunately, human ingenuity doesn't often take no for an answer. Legions of engineers are ready to propel us and our robotic surrogates into deep space with ion thrusters, solar sails, and nuclear reactors. The most efficient engines would tap energy from a nuclear reactor by bringing matter and antimatter into contact with each other, thereby converting all their mass into propulsion energy, just as Star Trek's antimatter engines did. Some physicists even dream of traveling faster than the speed of light, by somehow tunneling through warps in the fabric of space and time. Star Trek didn't miss that one either: the warp drives on the starship USS Enterprise were what enabled Captain Kirk and his crew to cross the galaxy during the TV commercials.

In October 1998, an eight-foot-tall, half-ton spacecraft called Deep Space 1 launched from Cape Canaveral, Florida. During its three-year mission, Deep Space 1 tested a dozen innovative technologies, including a propulsion system equipped with ion thrusters.

Acceleration can be gradual and prolonged, or it can come from a brief, spectacular blast. Only a major blast can propel a spacecraft off the ground. You've got to have at least as many pounds of thrust as the weight of the craft itself. Otherwise, the thing will just sit there on the pad. After that, if you're not in a big rush—and if you're sending cargo rather than crew to the distant reaches of the solar system—there's no need for spectacular acceleration. And that's when ion thrusters work best.

Ion-thruster engines do what conventional spacecraft engines do: they accelerate propellant (in this case, a gas) to very high speeds and channel it out a nozzle. In response, the engine, and thus the rest of the spacecraft, recoils in the opposite direction. You can do this science experiment yourself: While you're standing on a skateboard, let loose a CO₂ fire extinguisher (purchased, of course, for this purpose). The gas will go one way; you and the skateboard will go the other way. This equivalence of action and reaction is a law of the universe, first described by Isaac Newton in the late seventeenth century.

But ion thrusters and ordinary rocket engines part ways in their choice of propellant and their source of the energy that accelerates it. Deep Space 1 used electrically charged (ionized) xenon gas as its propellant, rather than the liquid hydrogen-oxygen combo burned in the space shuttle's main engines. Ionized gas is easier to manage than explosively flammable chemicals. Plus, xenon happens to be a noble gas, which means it won't corrode or otherwise interact chemically with anything. For 16,000 hours, using less than four ounces of propellant a day, Deep Space 1's foot-wide, drum-shaped engine accelerated xenon ions across an electric field to speeds of twenty-five miles per second and spewed them from its nozzle. As anticipated, the recoil per pound of fuel was ten times greater than that of conventional rocket engines.

In space, as on Earth, there is no such thing as a free lunch—not to mention a free launch. Something had to power those ion thrusters on Deep Space 1. Some investment of energy had to first ionize the xenon atoms and then accelerate them. That energy came from electricity, courtesy of the Sun.

For touring the inner solar system, where light from the Sun is strong, the spacecraft of tomorrow can use solar arrays—not for propulsion itself, but for the electric power needed to drive the equipment that manages the propulsion. Deep Space 1 has folding solar "wings." Fully extended, they span almost forty feet—about five times the height of the spacecraft itself. The arrays on them are a combination of 3,600 solar cells and more than 700 cylindrical lenses that focus sunlight on the cells. At peak power, their collective output was more than 2,000 watts, enough to operate only a hair dryer or two on Earth but plenty for powering the spacecraft's ion thrusters. And last I heard, the radio was still on.

Other, more familiar spacecraft—such as the now-disintegrated Soviet space station Mir and the nearly seven-year-old International Space Station (ISS)—have also depended on the Sun for the power to operate their electronics. A work-in-progress orbiting about 250 miles above Earth, the ISS will eventually carry more than an acre's worth of solar panels. For about a third of every ninety-minute orbit, as Earth eclipses the Sun, the station orbits in darkness. So by day, some of the collected solar energy gets channeled into storage batteries for later use during dark hours.

Although neither Deep Space 1 nor the ISS uses the Sun's rays to propel itself, direct solar propulsion is far from impossible. Consider Cosmos 1, an engineless, 220-pound spacecraft that will be propelled (once it achieves Earth orbit) solely by the pressure of sunlight. In fact, Cosmos 1 is a solar sail. By the time you read these words, it may have entered its initial intended orbit, 500 miles above Earth. The project is a privately funded collaboration between U.S. and Russian space scientists, led by The Planetary Society. This summer's launch will culminate nearly five years of work by rocket scientists who would rather collaborate than contribute to mutual assured destruction (aptly known as MAD).

Shaped like a supersize daisy, this celestial sailboat folds inside an unarmed intercontinental ballistic missile left over from the Soviet Union's cold war arsenal, and then launches from a Russian submarine. Cosmos 1 has a computer at its center and eight reflective, triangular sail blades made of Mylar reinforced with aluminum. When unfurled in space, each blade extends fifty feet yet is only 0.0002 inch thick—much thinner than a cheap trash bag—and can be individually angled to steer and sail the craft.

Once aloft, the solar sail will accelerate because of the continual, collective thrust of the Sun's gazillion photons, or particles of light, hitting its blades and bouncing off the reflective surfaces. As they bounce, the photons will give rise to a gazillion little recoils in the opposite direction. No fuel. No fuel tanks. No exhaust. No mess. You can't get greener than that.

Having entered space, a lightweight solar sail could, after a couple of years, accelerate to 100,000 miles an hour. Such a craft escapes from Earth orbit (where it was deposited by conventional rockets) not by aiming for a destination but by cleverly angling its blades, as does a sailor on a ship, so that it ascends to ever-larger orbits around Earth. Eventually its orbit could become the same as that of the Moon, or Mars, or something beyond.

Obviously a solar sail would not be the transportation of choice if you're in a hurry to receive supplies, but it would certainly be fuel efficient. If you wanted to use it as, say, a low-cost food-delivery van, you could load it up with freeze-dried veggies, ready-to-eat breakfast cereals, Cool Whip, and other edible items of extremely high shelf life. And as the craft sailed into sectors where the Sun's light is feeble, you could help it along with a laser, beamed from Earth, or with a network of lasers stationed across the solar system.

Speaking of regions where the Sun is dim, suppose you wanted to park a space station in the outer solar system—at Jupiter, for instance, where sunlight is only 1/27 as intense as it is here on Earth. If your Jovian space station required the same amount of solar power as the completed International Space Station will, your panels would have to cover twenty-seven acres. So you would now be laying solar arrays over an area bigger than twenty football fields. I don't think so.

To do complex science in deep space, to enable explorers (or settlers) to spend time there, to operate equipment on the surfaces of distant planets, you must draw energy from sources other than the Sun.

Since the early 1960s, space vehicles have commonly relied on the heat from radioactive plutonium as a power supply. Several of the Apollo missions to the Moon, Pioneer 10 and 11 (now more than 8 billion miles from Earth, and headed for interstellar space), Viking 1 and 2 (to Mars), Voyager 1 and 2 (also destined for interstellar space and, in the case of Voyager 1, farther along than the Pioneers), and Cassini (now orbiting Saturn), among others, have all used plutonium for their radioisotope thermoelectric generators, or RTGs. An RTG is an inefficient but long-lasting source of nuclear power. Much more efficient, and much more energetic, would be a nuclear reactor that could supply both power and propulsion.

Nuclear power in any form, of course, is anathema to some people. Good reasons for this view are not hard to find. Inadequately shielded plutonium and other radioactive elements pose great danger; uncontrolled nuclear chain reactions pose an even greater danger. And it's easy to draw up a list of proven and potential disasters: the radioactive debris spread across northern Canada in 1978 by the crash of the nuclear-powered Soviet satellite Cosmos 954; the partial meltdown in 1979 at the Three Mile Island nuclear power plant on the Susquehanna River near Harrisburg, Pennsylvania; the explosion at the Chernobyl nuclear power plant in 1986 in what is now Ukraine; the plutonium in old RTGs currently lying in (and occasionally stolen from) remote, decrepit lighthouses in northwestern Russia. The list is long. Citizens' organizations such as the Global Network Against Weapons and Nuclear Power in Space remember these and other similar events.

But so do the scientists and engineers who work on NASA's Project Prometheus.

Rather than deny the risks of nuclear devices, NASA has turned its attention to maximizing safeguards. In 2003 the agency charged Project Prometheus with developing a small nuclear reactor that could be safely launched and could power long and ambitious missions to the outer solar system. Such a reactor would provide onboard power and could drive an electric engine with ion thrusters—the same kind of propulsion tested in Deep Space 1.

To appreciate the advance of technology, consider the power output of the RTGs that drove the experiments on the Vikings and Voyagers. They supplied no more than a hundred watts, about what your desk lamp uses. The RTGs on Cassini do a bit better: they could power your thousand-watt microwave oven. The nuclear reactor that will emerge from Prometheus should yield as much as 200,000 watts of power, equivalent to the energy needs of a small school—or a single SUV. To exploit the Promethean advance, an ambitious scientific mission has been proposed: the Jupiter Icy Moons Orbiter, or JIMO. Its destinations would be Callisto, Ganymede, and Europa—three of the four moons of Jupiter discovered by Galileo in 1610. (The fourth, Io, is studded with volcanoes and is flaming hot.) The lure of the three frigid Galilean moons is that beneath their thick crust of ice may lie vast reservoirs of liquid water that harbor, or once harbored, life.

Endowed with ample onboard propulsion, JIMO would do a flyto, rather than a flyby, of Jupiter. It would pull into orbit and systematically visit one moon at a time, perhaps even deploying landers. Powered by ample onboard electricity, suites of scientific instruments would study the moons and send data back to Earth via high-speed, broadband channels. Besides efficiency, a big attraction would be safety, both structural and operational. The spacecraft would be launched with ordinary rockets, and its nuclear reactor would be launched cold—not until JIMO had reached escape velocity and was well out of Earth orbit would the reactor be turned on. As of this writing, however, plans for JIMO are on hold: a series of simpler missions will more expeditiously test the new Promethean propulsion systems.

Someday there might be wackier ways to explore within and beyond our solar system. The folks at NASA's now-defunct Breakthrough Propulsion Physics Project, for instance, were dreaming of how to couple gravity and electromagnetism, or tap the zero-point energy states of the quantum vacuum, or harness superluminal quantum phenomena. Their inspiration came from such tales as From the Earth to the Moon, by Jules Verne, and the adventures of Buck Rogers, Flash Gordon, and Star Trek. It's okay to think about this sort of thing from time to time. But, in my opinion, though it's possible not to have read enough science fiction in one's lifetime, it's also possible to have read too much of it.

My favorite science-fiction engine is the antimatter drive. It's 100 percent efficient: put a pound of antimatter together with a pound of matter, and they turn into a puff of pure energy, with no by-products. Antimatter is real. Credit the twentieth-century British physicist Paul A.M. Dirac for conceiving of it in 1928, and the American physicist Carl D. Anderson for discovering it five years later.

The science part of antimatter is fine. It's the science-fiction part that presents a small problem. How do you store the stuff? Behind whose spaceship cabin or under whose bunk bed would the canister of antimatter be kept? And what substance would the canister be made of? Antimatter and matter annihilate each other on contact, so keeping antimatter around requires portable matterless containers, such as magnetic fields shaped into magnetic bottles. Unlike the fringe propulsion ideas, where engineering chases the bleeding edge of physics, the antimatter problem is ordinary physics chasing the bleeding edge of engineering.

So the quest continues. Meanwhile, next time you're watching a movie in which a captured spy is being questioned, think about this: The questioners hardly ever ask about agricultural secrets or troop movements. With an eye to the future, they ask about the secret rocket formula, the transportation ticket to the final frontier.