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.

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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.

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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.

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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.

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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.

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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.

[This is part two of a two-part article.]

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Astrophysicist Neil deGrasse Tyson is the director of the Hayden Planetarium at the American Museum of Natural History. His latest book, co-authored with Donald Goldsmith, is Origins: Fourteen Billion Years of Cosmic Evolution (W.W. Norton, 2004).