In just the last eight years, astronomers have discovered a bewildering variety of worlds around other stars: planets so hot they vaporize like comets, planets so large they nearly shine like stars, twin planets that orbit their star in lockstep rhythm. What we have not found is a planet remotely like our own—our instruments aren't sensitive enough. That should change soon. About 10 years from now, NASA plans to launch a mission called Terrestrial Planet Finder, a space telescope specifically designed to detect another Earth. The odds are good that a survey of 150 or so nearby stars will reveal at least one small, Earth-like planet. A sister Earth would not look like much at first, just a faint speck of light nearly lost in the glare of its nearby star. Still, a speck of light is all we need to analyze the mass, temperature, and composition of the new world. We can scan for likely chemical markers of life, such as an oxygen-rich atmosphere moistened with water vapor and seasoned with methane. If we find what we're looking for, we will suddenly know life on another planet is highly probable and that we may not be alone. The discovery would arguably be the most profound one in human history. But what then? Five hundred years ago, after Columbus brought back word of a new world across the vast Atlantic, explorers from England, France, Spain, and Portugal did not hesitate to sail west. If we find another Earth, our longing for exploration might be stirred as never before. Who could resist wanting to go there and learn more? The technological challenges of that expedition make even a trip to Mars seem easy. Alpha Centauri, the closest star system and a plausible place to find an Earth-like world, lies 4.4 light-years away—3,000 times farther than any space probe has ever traveled. The star 55 Cancri, which has three large planets similar to those in our solar system, is another 10 times more distant. Crossing the cosmic void will require superfast spacecraft, far more advanced than anything built today but not beyond possibility. "The physics is not out of reach," says Robert Frisbee, an engineer who directs advanced propulsion concepts studies at NASA's Jet Propulsion Laboratory in Pasadena, California. His job, and his lifelong dream, is to find a way to master interstellar travel. He is studying five distinct propulsion technologies that could get an astronaut from here to Alpha Centauri in less than 50 years. "What we're talking about here is not fantasy," Frisbee says. "It's only science fiction until someone does it." A trip to another Earth would require a research and engineering effort at least as intense as the push behind the Apollo program. But Frisbee argues that a similar level of commitment could result in the launch of our first starship in the same time frame it took us to get to the moon—a decade. It would be the most expensive undertaking in the history of humankind. It would also be the most extraordinary.
Nigel Packham, an engineer at Lockheed Martin, spent 15 days sealed in this chamber, breathing oxygen produced by the wheat plants.Photograph courtesy of NASA.
Atomic RocketsWhen we reach for the stars, we will have to retire our chemistry sets
In 1903 Russian physicist Konstantin Tsiolkovsky discovered the great impediment to interstellar travel: A rocket's ultimate speed is limited to about twice the velocity of its nozzle exhaust. The space shuttle blows out its exhaust at less than three miles per second, so it cannot exceed about six miles per second. At that rate, it would take 120,000 years to reach Alpha Centauri. To get there in a human lifetime, a rocket would have to travel at least 3,000 times faster than current propellants, such as liquid hydrogen and kerosene, can thrust. So Robert Frisbee suggests tapping into the enormous energy of nuclear reactions, which could be done three different ways:
Nuclear FissionPros: near-term feasibilityCons: very heavy; needs processed fuel; requires massive radiation shielding; has limited top speed and range
Nuclear fission engineers have 60 years of experience working with fission, the process that powers atomic bombs and nuclear reactors. When the center of a radioactive atom is split apart, the resulting charged atomic fragments fly away at 3 percent of the speed of light, about 5,000 miles per second. Researchers led by George Chapline of Lawrence Livermore National Laboratory have designed a conceptual "fission fragment" reactor to harness those high-speed particles. Their reactor resembles a stack of vinyl records rotating into a cylindrical tower. Each "record" consists of graphite covered with radioactive fuel, such as plutonium or americium. When the fuel spins into the tower, it encounters additional radioactive material inside and triggers a controlled fission chain reaction. Powerful magnets around the reactor corral the resulting nuclear fragments so that they fly away in one direction, producing an exhaust that could accelerate a rocket to 6 percent of the speed of light. To surpass 10 percent of the speed of light, Frisbee proposes building two fission rockets and staging them one on top of the other. The second stage effectively doubles the rocket's top speed, so the expanded version could zip along at 12 percent of the speed of light. Add another two stages to slow everything down by the end of the trip and you could pull into an orbit around a sister Earth in the Alpha Centauri system in 46 years. More-distant voyages would take more than a human lifetime, however, even using additional stages. To keep weight to a minimum, the fission rocket would require a fast-decaying nuclear fuel such as americium. Americium is not a naturally occurring element, so it would have to be processed from spent nuclear fuel. A mission to the next star would require roughly 2 million tons of americium, not to mention a considerable amount of radiation shielding. Using cheaper uranium or plutonium would drive the fuel mass even higher. But the fundamental technology is ready to go.
AntimatterPros: most efficient rocket; best top speed; longest range Cons: heavy; antimatter technology undeveloped
Albert Einstein's famous equation
shows that mass is a concentrated form of energy. Fission and fusion reactions convert just a fraction of 1 percent of their mass into energy. But there is a way to convert matter into energy at nearly 100 percent efficiency: Combine matter with antimatter, its mirror twin. Physicists have created tiny quantities of antimatter by smashing subatomic particles together at near-light speeds. Scientists at CERN, the European physics facility in Switzerland, recently managed to create 1 million antihydrogen atoms—about 10^-15 of a pound. It would make a great fuel for an interstellar rocket. Scaling up the engineering to make a rocket-load of antimatter is a daunting concept. "But it is fairly straightforward because we've already got the bits and pieces. We already make the tanks, the magnets, the radiators, and the particle beams you'd need," Frisbee says. In an antimatter rocket, a dose of antihydrogen would mix with an equal amount of hydrogen in a combustion chamber. The mutual annihilation of a half pound of each, for instance, would unleash more energy than a 10-megaton hydrogen bomb, along with a shower of subatomic particles called pions and muons. These particles, confined within a magnetic nozzle similar to the type necessary for a fission rocket, would fly out the back at one-third of the speed of light. That fast exhaust would translate to a top speed of 66 percent of the speed of light. "This is by far the most powerful rocket we can make," Frisbee says. A two-stage antimatter rocket to Alpha Centauri would need some 900,000 tons of fuel and would arrive in about 41 years. A four-stage version (two to speed up, two to slow down) on a longer voyage would show the advantages of antimatter to better effect. According to Frisbee's calculations, it would need 38 million tons of antimatter fuel, but it would cut the trip to 55 Cancri, 41 light-years away, to an almost manageable 130 Earth years. The same trip would take 400 years using a fusion engine.
Nuclear FissionPros: lighter than a fission engine; less radiation; possibly refuelable Cons: heavy; has a limited range; technology not yet practical
A fusion engine—one that draws power from smashing atomic nuclei together rather than blasting them apart—would be far preferable to a fission engine, Frisbee says. Fusion reactors potentially produce less unwanted radiation, and they should be easier to fuel: They run on deuterium (heavy hydrogen) and helium 3 (a lighter version of ordinary helium), both of which exist in large quantities on the surface of the moon and in the atmosphere of Jupiter. A fusion-powered mission might visit a fueling station in our solar system before setting out for another star. The catch is that engineers have not yet built a workable fusion reactor, despite decades of effort. We know how to build a runaway fusion reaction in an H-bomb, but controlling that energy has proven elusive. Fusion test beds, such as the National Spherical Torus Experiment in Princeton, New Jersey, and the Joint European Torus in England, confine nuclei of deuterium in a magnetic bottle and heat it to millions of degrees; as the nuclei smash together, some fuse and release energy. These types of experiments currently consume nearly twice as much energy as they produce from fusion reactions, but the ratio has been improving. Frisbee is optimistic that the technology is now within reach. Once scientists achieve break-even fusion, they could harness the charged particles generated by these reactions and pipe them through a magnetic nozzle. The spray of particles from a fusion reactor could be used to make a two-stage rocket that would top out at 12 percent of the speed of light. Travel times using fusion power would be similar to those for the fission rocket: fast enough to reach the nearest star but probably not much beyond. A fusion rocket would also require about 2 million tons of fuel but could make do with less radiation shielding. As an added bonus, developing a fusion rocket could spur the perfection of fusion power plants on Earth.
Beyond RocketryThe best way to reach the stars is to leave the fuel back home
The trouble with conventional rocketry, even antimatter rocketry, has been on display at every Mercury, Gemini, Apollo, and space-shuttle launch: The spacecraft is dwarfed by its supply of propellant, so most of the rocket's thrust is used simply to move its own fuel. That crude approach is acceptable for a trip into Earth orbit or a brief excursion to the moon, but many space engineers argue that voyaging to other stars will require innovative propulsion systems that are lighter, more versatile, and swifter than any rocket—nearly as fast as light itself. One such concept has been under development for years and is about to be tested. Another seems as far off as Alpha Centauri.
Laser SailPros:high speed; no fuel on board; near-term technology Cons: massive laser infrastructure; can only go where laser points
In a landmark 1984 paper, the late Robert Forward, then a physicist at Hughes Aircraft's research laboratories, proposed a twist on the ancient technology of sailing. Just as wind can drive a canvas sail across the ocean, a powerful laser can push a huge sail through space. When photons in the laser beam strike the sail, they transfer their momentum and push the sail onward. The spaceship gradually but steadily builds up speed and races off to distant worlds while the laser that propels it stays put in our solar system. Frisbee regards this as the most likely technology to ferry the first spacecraft to another star. Engineers have already built a simple space sail, one that rides on light from the sun rather than a beam of photons from a laser. Within the next few months, the Planetary Society, a private space-enthusiast organization, plans to launch its pioneering solar sail. Cosmos 1, a 50-pound, 100-foot-wide pinwheel of aluminized Mylar-like sheets, will be launched from a Russian submarine in the Barents Sea. Once in space, the sail will be pushed by sunlight to reach a higher orbit. Hoppy Price, the lead engineer for solar sails at NASA's Jet Propulsion Laboratory, thinks this type of fuel-free propulsion will allow entirely new types of planetary missions. But the strength of sunlight falls off exponentially with distance, so solar sails do not work far away from the sun.
A focused beam of laser light, in contrast, could push a craft to Alpha Centauri and beyond because a laser beam does not spread and weaken over distance nearly as much as sunlight does. Frisbee has sketched a design for a trip to 55 Cancri based on Forward's concept. His spacecraft would be powered by a 600-mile-wide aluminum-foil sail with a crew cabin at its center. A powerful laser in Earth orbit or on the lunar surface would bounce off a 600-mile-wide flexible mirror, which would focus the beam so it could drive the sail. The laser would have to pump out light for several years until the spacecraft reached its cruising speed; another couple of years of beaming would allow the craft to slow down (see below). Frisbee's sail needs to be large in part to dissipate heat from the enormous energy of the incoming laser beam. Aluminum melts at a modest 1,220 degrees Fahrenheit. If the sail were manufactured in space, engineers could switch to lighter, more resilient materials. Geoffrey Landis at NASA's Glenn Research Center in Ohio is investigating thin films made of niobium (which melts at 4,490°F) or diamond (which breaks down into graphite at 3,270°F)—"like a soap bubble in thickness," he says. High-temperature substances could withstand a smaller, more intense laser beam. A diamond-film sail with the same capabilities as Frisbee's aluminum sail could allow faster acceleration and decrease the total trip time. More daunting, perhaps, is the laser power needed to drive that sail to 55 Cancri: By Frisbee's estimation, it would take a steady flow of about 17,000 terawatts, or 1,200 times all the energy consumed on Earth at any moment. To meet such an enormous demand, he suggests using a solar-pumped laser, a device that gathers sunlight and transforms it into a focused, coherent beam. Physicists Roland Winston and Joseph O'Gallagher of the University of Chicago have demonstrated a system that can concentrate light to 84,000 times its normal intensity. "Just working with solar sails, we'll wind up solving the major problems of building the laser-sail system," Frisbee says. If we can master the technology, we will no longer need to worry about how far our fuel supply can carry us. A simple design twist would even allow the laser beam to slow the ship to a halt at the other end. And the top speed of a laser sail is limited only by the velocity of light itself. In Frisbee's study, a laser sail accelerates to half light speed in less than a decade. He calculates that by using a sail 200 miles in diameter we could reach Alpha Centauri in just 121/2 years. A 600-mile-wide sail could rendezvous with a planet around 55 Cancri in 86 years.
Fusion RamjetPros:near-light speeds; unlimited interstellar travel in any directionCons:requires major advances in physics and engineering knowledge
A dream spacecraft would blend the best attributes of a laser sail and a conventional rocket: You could steer it wherever you wanted to go, and it would never need to carry any propellant. In 1960 physicist Robert Bussard conceived a technology that could do just that. He called it a fusion ramjet. It uses a huge magnet to create a magnetic funnel tens of thousands of miles in diameter. The funnel scoops up interstellar hydrogen and dumps it into a reactor as fuel. Without any fuel tanks to weigh it down, a fusion ramjet could approach the speed of light and roam almost anywhere in the galaxy.
Frisbee cautions that the ramjet concept remains immature. "At this point, it's the least tied down," he says. It would probably operate like a fusion rocket until it hit about 4 percent of the speed of light, at which point the magnetic maw would take in enough hydrogen to keep the engines humming. Frisbee's rough projections show a travel time of 25 years to Alpha Centauri and 90 years to 55 Cancri. There are two obvious engineering problems with the ramjet. The first is drag: The fusion particles coming out the back shove the ship forward, but the interstellar hydrogen piling up in front acts to slow it down. Passing through the denser regions of the galaxy, the spacecraft might grind to a near halt. In fact, Robert Zubrin, an engineer who runs Pioneer Astronautics, has suggested using a similar magnetic field as a brake to decelerate an interstellar spaceship without expending additional fuel. The second difficulty is that deuterium and tritium, used in today's experimental fusion reactors, are rare in space. Most interstellar hydrogen is the regular variety with a single proton, which is more finicky. "No one has a clue how to make pure hydrogen fuse," Frisbee says. On the other hand, we know it happens every day: "A star can do it."
Personnel IssuesCan we keep an astronaut alive for 40 years?
The human equation may prove just as thorny as the rocket equation. Although we have learned how to keep astronauts healthy aboard a space station, we generally limit their tour of duty to a few months, and we send them a steady stream of supplies from the home planet. A voyage to the closest Earth-like planet in another solar system is likely to involve decades of travel with no support at all. Before the Apollo era, the difficulty of keeping a human alive and healthy in space for such time periods might have seemed insurmountable. Since then, however, life-support science has been hurtling forward as rapidly as rocket design. Donald Henninger, head of NASA's Integrity Project and a 17-year veteran of life-support sciences at the Johnson Space Center in Houston, does not even crack a smile when asked if it can be done. "Sure, it's possible," he says.
Closing the loopEarth sustains 6 billion humans with billions of cubic miles of atmosphere, hundreds of millions of cubic miles of water, and billions of acres of arable land. Henninger calls that huge system a "buffer" against the cruel realities of the universe. But only a minuscule fraction of Earth's air and water cycles through humans at any one time—which is fortunate, because a spaceship cannot offer anything close to the resources of an entire planet. Instead, a comparatively modest supply of water, oxygen, and food on board must be recycled over and over with an almost 100 percent recovery rate. Space scientists call it closing the loops: Keeping the recycling process going for "three years, 30 years—it doesn't much matter, once you close all the loops," Henninger says.
Food A continuous supply of food for space voyagers would require growing and harvesting plants. "The task is not extremely difficult," Henninger says. "It's just a question of how efficiently we can do it." He and his colleagues are experimenting with plants such as wheat and potatoes that maximize caloric yield and compress the growing cycle. Most plants, research has shown, grow faster if they receive high doses of carbon dioxide—conveniently breathed out by astronauts. "Give me reasonable mass and power limitations and we can do it from a life-support point of view," Henninger says. "There are no intractable problems here." Reasonable power and mass should be no problem on an interstellar craft. The space station weighs 179 metric tons. Crew quarters weighing 10 times as much would increase the mass of an antimatter rocket by less than 10 percent. "A drop in the bucket," Frisbee says.
Air supply Astronauts inhale oxygen and exhale carbon dioxide. Mechanical scrubbers could separate the carbon dioxide from the ambient air; chemical processes could then split the bond between the two oxygen atoms and the carbon atom, recovering the O2 part of CO2. "We're close to closing the oxygen loop," Henninger says, and scientists expect to do it on the International Space Station before long.
Water Closing the water loop means purifying shower water, crop water, urine, even sweat. Astronauts on the space shuttle get clean water as a by-product from the onboard fuel cells, which combine hydrogen and oxygen to generate electricity. That is not a long-term solution. But NASA experiments with isolation chambers on the ground have successfully recycled water for up to 90 days by condensing vapor in the air and reprocessing wastewater and urine. On an interstellar voyage, an antimatter rocket or high-powered laser would provide plenty of energy to do the job.
Gravity After a few months of weightlessness, astronauts develop a form of osteoporosis because bones grow stronger in response to constant pressure from Earth's gravity. There is a simple way to simulate gravity on a spaceship: Construct a circular crew compartment that spins like an oversize hamster wheel. Centripetal force presses the astronauts against the outer walls—their floors—giving them the sensation of weight.
Radiation shielding The solar wind, an extended atmosphere blowing out from the sun, forms a magnetic cocoon around the solar system. No spacecraft has yet traveled past its influence, so we have no clear picture of the interstellar environment. It certainly contains cosmic rays, high-speed subatomic bits that could be deadly. Fission, fusion, or antimatter rockets also generate their own radiation. Radiation shielding—perhaps lead, and even some of the onboard fuel—would be needed to protect the crew.
Debris shielding Although interstellar space is extremely empty, even a microscopic dust particle traveling at 50 percent of the speed of light relative to a spacecraft could cause a cataclysmic impact. A more thorough census of interstellar space will tell us how much shielding we will need and what kind.
Psychology There will be no return trip for the first interstellar voyagers. It will take most of a lifetime to get there, and the current designs do not allow enough fuel (or, for the laser sail, a second laser beam) to get back. What kind of astronaut has the right stuff for a one- way mission? How many people should the ship carry? Should children or reproducing couples be sent so that, on arrival, someone young and able could do the exploring? And how do you manage all the disagreements, personality conflicts, and simple claustrophobia that are sure to erupt during a decades- long journey?
Why not just send a robot? A mission to another star cannot rely on controllers back home. A desperate message from Alpha Centauri would take 4.4 years to reach Earth, and our response would take another 4.4 years to return. If the spacecraft is run by a robot, it would need a level of autonomy that baffles Steve Chien, the head of artificial intelligence at NASA's Jet Propulsion Laboratory. "Your system has to be robust enough to survive all sorts of novel problems on its own," he says. Then, if it manages to arrive intact, the robot would have to perform a detailed scientific exploration without guidance from Earth. "We would hope that it would find an alien city interesting because it's unusual—with straight lines and an organizing pattern—and not throw that data away," Chien says. A human would probably recognize an alien city in an excited heartbeat. An artificial intelligence that matches the flexibility of the human brain has proved nearly as elusive as cold fusion. We have no idea how to build such a thing—and even if we did, we would still probably want the inspiring, poetic perspective of a human copilot.
But will we go?Some managers at NASA, frustrated that we have not even planned a manned trip to Mars, bristle at talk of voyaging to other solar systems. Get to Mars first and then the outer planets, they say. We can worry about the stars later. Frisbee counters that "stretch goals" inspire greatness. They certainly defined NASA's shining moment, when President John F. Kennedy laid down the challenge of putting a man on the moon. At the time, in 1961, only one American had flown in space, and only for about 15 minutes. No one had ever built a rocket powerful enough to launch three humans beyond Earth's orbit. No one had managed to soft-land a spacecraft on the moon, let alone get it back home. All the intermediate steps happened only because Kennedy insisted we go. Had he thought smaller, we might never have reached the moon. To Frisbee, traveling to another star is as full of wonder as building the Egyptian pyramids or sailing around the globe for the first time. "Human beings can do amazing things when they want to," he says.
Nigel Packham, an engineer at Lockheed Martin, spent 15 days sealed in this chamber, breathing oxygen produced by the wheat plants.Photograph courtesy of NASA.
Scientists agree that interstellar space travel won't be easy. Marc Millis of NASA's Glenn Research Center has set up a useful "beginner's guide" to interstellar travel. The Web site explains the difficulties of getting to another star and outlines the many techniques—some feasible, some outright wacky—that are being considered: www.grc.nasa.gov/WWW/PAO/warp.htm.
Visit NASA's Jet Propulsion Laboratory Web site for Advanced Propulsion Concepts for more in-depth discussions of the various research avenues for interstellar travel: www.islandone.org/APC.
Information on the Planetary Society's solar sail, Cosmos 1, is available at www.planetary.org/solarsail.
Before we set out for another planet, we'll first need to find it. JPL's Terrestrial Planet Finder Web site details news and background information about the telescope that will help us scope out other Earth-like planets in distant regions of the galaxy: tpf.jpl.nasa.gov.
Read about the Terrestrial Planet Finder project in "Can We Find Another Earth?" by Michael D. Lemonick, Discover, March 2002. This article is available at www.discover.com.
To tweak your imagination and learn how scientists at the European Space Agency envision all aspects of interstellar space travel, including vehicle propulsion and colonization in space and on other planets, visit www.itsf.org.