Astronaut, scientists dreaming up space shuttles for Mars fligh

WEST LAFAYETTE, Ind. -- Though it will likely be decades before a human sets foot on the Martian surface, former astronaut Buzz Aldrin and scientists at Purdue University already are working on a shuttle they hope will one day take people from Earth to the Red Planet and back.

The interplanetary bus line would involve two massive spacecraft that could house up to 50 people on their six-month shuttles between the two planets. The craft would continuously cycle between Earth and Mars using gravity as their primary power source, with an occasional shot from a booster rocket.

"If we're going to go to Mars with human beings, we need to do it in an evolutionary way, so that we can continue doing it," said Aldrin, the second man to walk on the moon.

James Longuski, a professor of aeronautics and astronautics at Purdue, agrees that it doesn't make sense to land the first people on Mars without knowing how to land more.

"We want to know what the transportation setup would look like before we've even set foot on Mars," Longuski said.

While it may sound like putting the cart a few light-years before the horse, Jim Garvin, NASA's lead scientist for Mars exploration, said these ideas must be considered now, even though human travel to Mars is a distant goal.

$500 million a year

Garvin said NASA will spend about $500 million a year over the next 10 years on Mars exploration.

"We're in a decade of discovery for Mars," Garvin said. "It will fill in the homework we need, so come 2010, maybe sooner, we'll know enough to start asking the question, 'Now what do we do if we want to insert humans as the explorers?'"

Aldrin first came up with the idea of an Earth-to-Mars cycler in the mid-1990s, and he's been working on it ever since. Along with Longuski at Purdue, researchers at the Massachusetts Institute of Technology and the University of Texas are involved in the project.

All the research so far is theoretical, and Longuski said the earliest he could imagine cyclers being used is 2018.

The craft would never actually stop at either planet. A smaller "taxi" vehicle would dock with the cycler on its sweep past, unloading passengers and transporting them to the surface.

"These cyclers, they just whiz on by," Longuski said. "It's like a bus that never stops. Passengers just have to jump on as it's going past."

Once a cycler is launched on a specific trajectory, gravity would keep it circling the planet. This is what's known as a gravitational "slingshot" effect.

As a spacecraft travels near a planet, the gravity of that planet pulls the craft toward it, then whips it around the other side, boosting its speed. This is the same phenomenon that in 1970 helped NASA safely bring home the crippled Apollo 13, using the moon's gravity to slingshot the craft and crew back to Earth.

The trick is finding the right path for these cyclers to follow because the orbits of Earth and Mars are out of sync.

"It's like a game of celestial billiards," Longuski said. "Tell me which shot will knock all the balls into the pockets."