Mars, and Step on It | Space | Air & Space Magazine
To travel from star to star, ships could surf a wave in space-time itself. Since the 1990s, theories of interstellar flight have focused as much on gravity, electromagnetism, and the properties of space-time as on propulsion systems. (Paul DiMare)

Mars, and Step on It

When it’s not the journey but the destination that counts.

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Few stories distill the question of human destiny as neatly as the 2000 movie Cast Away. In it, Chuck Noland (Tom Hanks), the sole survivor of an airplane crash in the South Pacific, ends up on an uncharted island. As days become weeks, he slides into despair, realizing that no one is coming for him. The years pass, and the permanence of his isolation sinks in, continually pounded home by the muffled roar of the surf breaking on the island’s encircling reef.

Noland can be thought to represent humanity on planet Earth. Will we ever get off the island? We swam out to the reef a handful of times during the Apollo program, arguably our greatest technical achievement as a species. But ever since, we’ve been back on the beach. Can we build the craft we need to break out?

Voyager 1, launched on an outer planet tour in 1977, is now the most distant object humans have sent into space, having left the solar system after 32 years of voyaging at a little better than 38,000 mph. At this rate, Voyager could get from New York to San Francisco in three minutes and 55 seconds, but wouldn’t reach the nearest star, Proxima Centauri, for 73,000 years. (It’s not headed there; it’s instead wandering toward the constellation Camelopardalis, where it will drift past its first star in about 300,000 years.) But if Voyager were traveling at the speed of light, a little more than 670 million mph, it would take only 4.2 years, a journey almost imaginable.

“We can see the theoretical possibilities of these things happening, but we just can’t get the engineering there,” says Bob Frisbee, an engineer at NASA’s Jet Propulsion Laboratory in California. Frisbee is one of dozens of U.S. scientists and engineers who are studying how humans could cross the vast gulfs of interstellar space in some meaningful time frame. Frisbee is known for his work on a propulsion system that uses the energy released from collisions of matter and anti-matter. His design uses a superconducting magnet as a nozzle to direct charged particles—produced by annihilating protons and anti-protons—to produce thrust.

“Philosophically, this is the kind of brainstorming stuff that people were doing about how to get to the moon,” says Frisbee. “What velocity? What kind of engine? What do we need to bring? And golly, they did it.”

Frisbee, who, in his day job studies how electric propulsion could be used for future robotic missions, is also a member of the Tau Zero Foundation, a group of scientists, engineers, and laypeople who stroll the distant shoals of theoretical spaceflight. Marc Millis, a physicist at NASA’s Glenn Research Center in Cleveland, Ohio, created the foundation and maintains a Web site (www.tauzero.aero) for the members, who have agreed to work together toward practical interstellar flight and to use this quest to teach people about science, technology, and our place in the universe. From 1996 to 2002, Millis ran what was, for NASA, an unusually future-oriented program called the Breakthrough Propulsion Physics Project. The program acknowledged the limitations of rocketry as we know it and encouraged studies of faster-than-light travel using the properties of matter—gravity and electromagnetism, for example—and of space and time. “All in all, we’re getting smart enough to ask the right questions,” says Millis.

The field gained momentum in the 1990s, shortly after the publication of three papers, all analyzing geometrical properties of space-time, the coordinate system containing both spatial and time dimensions rooted in Albert Einstein’s general theory of relativity. The papers proposed that through the manipulation of space-time, objects could get around the universe’s speed limit—the speed of light. The first two papers offered mathematical equations describing shortcuts for getting from one place in the universe to another: so-called wormholes. The third transformed the concept of warp drive from an element of science fiction, made famous by the TV series “Star Trek,” into a serious topic among theoretical physicists.

In that 1994 paper, Mexican physicist Miguel Alcubierre offered a mathematical proof that faster-than-light travel is possible within the constraints imposed by Einstein’s general theory of relativity. A spacecraft, Alcubierre theorized, would not dart across interstellar space; instead, it would ride a wave in the fabric of space-time, traveling inside a “warp bubble” like a person standing on a moving sidewalk. Some not-yet-defined force would work to condense the space-time ahead of the spacecraft and stretch out the space-time behind.

Millis has compiled the results of the Breakthrough Propulsion Physics Project and related work into a book of technical articles, The Frontiers of Propulsion Science, and members of the Tau Zero Foundation continue to exchange ideas and debate strategies, though a program no longer exists to fund experiments and observation. “We talk among ourselves and encourage each other to launch into projects,” says Millis, “and we’ve been very successful with that, even without money.” One member, for example, is revisiting the British Interplanetary Society’s Daedalus program. Daedalus is a 1970s effort to invent a practical starship powered by nuclear fusion, the process in which extreme pressures and temperatures cause the nuclei of atoms to join, releasing energy. The society is holding a symposium this month to reconsider the idea in light of the advances in relevant technologies made over the past 30 years.

“I think back to the era of Dirac and Schrödinger and Einstein,” says Millis of the great theoretical physicists of the early 20th century, Paul Dirac and Erwin Schrödinger, who shared a Nobel Prize in 1933 for groundbreaking work in quantum mechanics. “When they were having their pivotal meetings and sometimes heated debates, they weren’t being funded for that work. They were just doing it because that’s what they did. And they made significant advances.

“And I’m thinking to myself, Well, it would be great if we got funding, but even if we don’t, when we talk amongst ourselves and debate things and encourage each other to write papers, we’re going to make progress.”

Millis estimates that, based on the energy needs as we understand them now, the first true interstellar mission won’t happen for another two centuries. And he chuckles at a common paradox voiced in his field: It’s useless to launch an interstellar mission, because future spacecraft, benefitting from technological advances, will overtake those launched earlier.
But he’s far from cynical.

“I grew up watching Apollo, and the systematic and well-thought-out march to that. And they did it. When you look into pioneering topics, there are those people who don’t want to touch it because it’s too far out there. But if it’s mature enough for you to at least start asking the right questions, and you do an honest job, then you can be a pioneer.”

For now, traveling to the stars will have to wait; the challenges close to home are big enough. On a clear night, we glimpse five planets with the naked eye: Saturn and Jupiter, bitter cold gas giants with poisonous atmospheres; Mercury and Venus, furnaces that would incinerate us; and Mars, a planet of extremes. At least they are Earth-like extremes: desert barrenness, as frigid in winter as Antarctica. It is also wrapped in a thin atmosphere of carbon dioxide, but humans have to bring their own environments wherever they travel in space. And it’s close—roughly 36 million miles away at its closest approach.

With rockets fed by liquid oxygen and liquid hydrogen, it would take us a little longer than a year just to get to Mars and back—200 days each way. The long-term exposure to radiation on such lengthy missions could endanger the astronauts.

Bill Emrich, a propulsion engineer at NASA’s Marshall Space Flight Center in Huntsville, Alabama, is one of the people pondering how to get to Mars a lot faster. In 2003, the Marshall center started work on the Propulsion Research Laboratory, where Emrich could investigate powerplants that would cut the transit time to Mars from 200 days to 100.

Rocket scientists rate the efficiency of rocket engines in “specific impulse,” a measure like miles per gallon, but with a time component: the number of seconds that a pound of rocket fuel will create a pound of thrust. For chemical engines, the best figure barely tops 450 seconds, paltry compared to ideas on the drawing board. A chemically propelled ship would exhaust much of its fuel at the beginning of a trip to Mars, like a drag racer that floors it, then coasts the rest of the way. To slow enough to enter orbit around Mars, it would depend on friction with the atmosphere as a brake. Unlike Earth’s atmosphere, thick and fluffy as a down comforter, with some room for error, the Martian atmosphere is a bed sheet.

“I’d love to go to Mars, but not on that ship,” says Emrich. “You’re going down to just a few thousand feet above the surface. It would be a very scary ride.” Come in too steep, he says, and you plow into the ground. Come in too shallow, and you skip off the atmosphere to become the next Voyager. “Very little room for error,” he says. “You get one crack at it.”

His solution: a nuclear thermal rocket. It would produce thrust the way chemical rockets do: by heating a propellant—in this case, hydrogen—and ejecting the expanded gas through a nozzle. Instead of heating hydrogen through combustion, however, the nuclear rocket vaporizes it through the controlled fission, or splitting of atomic nuclei, of uranium. Because nuclear fuel has a greater energy density, it lasts a lot longer than chemicals, so you can keep the engine running and continue to accelerate for half the trip. Then, with the speedometer clicking off about 15 miles per second—twice the speed reached by returning Apollo astronauts—you’d swing the ship around to point the other way and use the engine’s thrust to decelerate for the rest of the trip. Even when factoring in the weight of the reactor, a nuclear engine would cut the transit time in half.

In a program called NERVA (Nuclear Engine for Rocket Vehicle Applications), NASA built a nuclear thermal rocket in the 1960s. It delivered a specific impulse of 850 seconds—twice the efficiency of the best chemical rockets—and could have been tweaked to deliver up to 1,000 seconds. As NASA prepared to follow the moon missions with human voyages to the planets, the nuclear thermal rocket was a serious candidate to replace chemical engines in the Saturn V launch vehicle’s upper stages. Instead, despite more than 20 successful test firings in the Nevada desert, NERVA died in the mid-1970s.

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. 

Michael Klesius is an associate editor at Air & Space/Smithsonian and a fan of high-speed space travel ideas.
 

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