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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.

“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.

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