Heading out: to explore deep space—and make stops along the way—spacecraft will need new forms of propulsion

Natural History, July, 2005 by Neil deGrasse Tyson

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 aero-braking 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 C[O.sub.2] 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.