Nuclear thermal rockets are limited by the heat tolerance of the uranium fuel and the engine’s structure, so engineers have experimented with new fuel elements and heat-resistant materials. At Marshall, Emrich has constructed a simulator that can test a nuclear rocket’s components by subjecting them to some of the conditions that fission would produce—the temperatures and pressures, though not the radioactivity. Because work on NASA’s Ares rocket, which will boost astronauts to the moon, has taken over the propulsion lab, Emrich is moving his simulator to another facility.
Not far from NASA’s Johnson Space Center in Houston, Franklin Chang Díaz, a former NASA astronaut and veteran of seven space shuttle flights, is developing an alternative to the nuclear thermal rocket. VASIMR, the Variable Specific Impulse Magnetoplasma Rocket, combines features of the high-thrust/low-specific-impulse chemical rocket, and the low- thrust/high-specific-impulse nuclear rocket. VASIMR is a plasma rocket. Instead of a combustion chamber, it uses three staged, magnetic cells that first ionize hydrogen and turn it into a super hot plasma, then further energize it with electromagnetic waves to maximize thrust. Chang Díaz promises his rocket could attain a speed of 31 miles a second, and would reduce a one-way trip to Mars from three months to one. His team has made slow progress on the concept since the late 1980s. Last fall, his VX-200 rocket prototype’s first stage, powered by argon, reached a milestone: a successful, full-power firing in his Webster, Texas lab. Having spent about $25 million from several government sources so far, and with equipment, lab space, and personnel from NASA, Chang Díaz is coming closer to a flight test. NASA is considering testing the rocket on the International Space Station, perhaps as soon as 2011 or 2012, where it may contribute to maintaining the huge laboratory’s orbit.
After VASIMR, the next step up in velocity is a nuclear fusion rocket. Scientists haven’t yet re-created sustained, controlled fusion, the chemical process that powers stars and promises enormous benefits as a power source on Earth, but that hasn’t stopped them from getting a lot of money from governments to try. The International Thermonuclear Experimental Reactor, being built in southern France, is a joint project of the European Union, Japan, China, India, South Korea, Russia, and the United States. The reactor will cost at least $15 billion, is not expected to begin operation until 2018, and is the size of an office building, but scientists hope that once they achieve fusion on the ground, reactors can be downsized for space travel. Fusion gives off more energy and less radiation than fission, and could propel a ship at high speed. In one scenario, its exhaust would be contained by a string of superconducting magnets shaped like huge washers, each perhaps 15 feet in diameter. The string of magnets would reach back from the reactor for the length of several football fields.
“The problem is not so much the amount of energy; you have gobs and gobs of energy,” says Emrich. “The problem is power, which is how fast you get the energy out of the system. A hydrogen bomb releases a huge amount of energy instantly but melts everything in sight.”
By contrast, the superconducting magnets corral the power of all that energy and essentially squirt it out the end. “Magnetic fields don’t melt,” says Emrich.
In theory, the engine could unleash a specific impulse of a million seconds. It would need only 1/10th of that to propel a craft to Mars in two weeks. But Emrich notes that to make a fusion-powered spaceship light enough to reach Mars in two weeks, propulsion experts will need a breakthrough in materials science.
“Mars in 30 days?” he says. “That’s getting closer.”
If and when new materials make that possible, Mars may in fact be too close to Earth for a fusion rocket to truly show what it’s got under the hood. A trip to Jupiter, on the other hand, 366 million miles away at its closest approach, would give the crew of a fusion-powered spacecraft almost 183 million miles of acceleration to the journey’s midpoint. By then, a fusion engine delivering about 30,000 seconds of impulse would have gathered a speed of 50 miles per second—about 180,000 miles an hour. After decelerating for the next 90 days, it would slip into orbit around Jupiter; by then, the trip would have lasted 180 days, only six times as long as a one-way trip to Mars, despite covering 10.5 times the distance. True, while the astronauts are exploring Jupiter, Earth wanders farther away than it was at launch time; however, at these speeds, orbital separation between the planets becomes less of a problem.
“The space program began the day humans chose to walk out of their caves,” says Chang Díaz. “By exploring space we are doing nothing less than insuring our own survival.” Chang Díaz believes that humans will either become extinct on Earth or expand into space. If we pull off the latter, he says, our notion of Earth will change forever.
As for Cast Away’s Chuck Noland, he eventually concludes that it would be better to risk it all and die trying to escape his imprisonment than to waste away on the beach. He builds a coarse raft, says good-bye to the island, and rows out to the reef. In a panicky moment when it seems he’s already failed, he barely surmounts crashing walls of surf. Briefly exuberant, he turns for one long, sobering look at the island, its peaks receding on the horizon. Then he turns his back to it, and paddles out to the Pacific in pursuit of his destiny.