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:
