By Brian Palmer
Monday, January 24, 2011; 9:00 PM

Humankind is looking deeper and deeper into the cosmos and seeing incredible things that were totally unknown just a few years ago. In the past three decades, astronomers have described the nature of gamma ray bursts, proved that the expansion of the universe is accelerating, and helped land rovers on Mars. And yet, while our telescopes can see Earthlike planets more than 20 light-years into the intergalactic abyss, we humans can't seem to get past the moon, a sterile rock orbiting just 238,000 miles from Houston. What's taking so long?

The first problem is fuel. The distance between Earth and Mars changes, since our orbits are elliptical, but the minimum distance is 34 million miles. Fueling a spacecraft over that distance poses a challenge, because as you add fuel, you add weight. As you add weight, you have to add more fuel to propel that weight. At certain distances, the cycle becomes totally unmanageable: Traveling on conventional fuel to Proxima Centauri, the next closet star after our sun, for example, would require a tank larger than the visible universe.

Mars isn't nearly that far, but it's far enough that NASA will have to get creative with fuel management. One strategy NASA is considering involves sending pieces of the Mars Transport Vehicle into space, in a series of seven launches, then assembling them in low-Earth orbit. That way, NASA won't have to find a single heavy launch vehicle capable of hauling all those fuel tanks, plus the fuel needed to get to Mars and back, out of the Earth's powerful gravitational field.

Another benefit of in-space assembly is that engineers wouldn't have to worry about aerodynamics, since air resistance isn't a concern in the vacuum of space. Instead of boasting a sleek, Apollo-like design, the Mars vehicle might look more like a modular house.

NASA scientists are also pondering whether a nuclear reactor or a solar-electric engine would be better than the chemical propellants that took us to the moon. The fuel-free electric option is particularly innovative. While scientists say it would provide less acceleration, it could eventually achieve the same top speeds.

"Think of it as a stock portfolio," says NASA's Bret G. Drake, one of the scientists tasked with constructing a vision for putting humans on Mars. "It builds up slowly over time."

The journey to Mars would probably last seven or eight months. To save on cargo weight, the astronauts would rely on dehydrated snacks. The shuttle might also carry a few tabletop gardening devices to provide fresh greens as a periodic treat.

Scientists say landing on Mars may require some fuel and skillful piloting. When astronauts reenter Earth's atmosphere, they use air resistance to decelerate the craft, flying in a complicated pattern to maximize that braking effect before touching down. Mars has an atmosphere, but it's much thinner than Earth's, so the pilots would likely have to use some fuel to decelerate.

We would have a live communications link to astronauts stepping onto the Martian surface, but it would take between eight to 20 minutes for their historic first words to reach Earth.

For those first words, Bill Nye, executive director of the Planetary Society, favors "For the joy of discovery." His selection may merit special consideration, since he's partially responsible for the first written message on Mars. Nye pushed to have the Mars rovers carry the inscription "To those who visit here, we wish a safe journey and the joy of discovery."

Mars would be able to supply astronauts with some useful resources, according to scientists. For example, NASA would send technology enabling them to extract breathable oxygen from the carbon-dioxide-heavy Mars atmosphere. They can also combine carbon dioxide with hydrogen to make water. (NASA assumes that any water the astronauts find is unlikely to be drinkable.)

The astronauts would spend about 18 months on Mars, living in a modular home on the surface of the planet. The timeframe is fairly rigid, because they would have to return when Earth and Mars are close together in their orbits.

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