Instead of focusing on building a better shield, engineers should design spaceships that can hop in and out of passing asteroids, argues study author Gregory Matloff, an adjunct professor of physics at the New York City College of Technology.

The asteroid itself could then block cosmic rays during the voyage—astronauts could pull a Millennium Falcon and park their ship in a crater, or they could use on-board mining tools to tunnel into the rock. (Related: “Obama’s New Plan for NASA: Why Go to an Asteroid?”)

According to Matloff’s calculations, to be published in the March-April 2011 issue of the journal Acta Astronautica, the asteroid “taxi” would need to be about 33 feet (10 meters) wide to provide enough shielding. It would also need to pass close enough to both planets—within a couple million miles—to make the trip feasible.

Already there are five known asteroids that fit the criteria and will pass from Earth to Mars before the year 2100, based on a database of 5,500 near-Earth objects (NEOs), or comets and asteroids whose orbits take them near our planet.

The asteroids 1999YR14 and 2007EE26, for example, will both pass Earth in 2086, and they’ll make the journey to Mars in less than a year. The trouble would be getting home: Because of their wide orbits, it’d be five years before either asteroid would swing around Mars as it heads back toward Earth.

Matloff did find a third space rock that will travel from Mars to Earth—but it makes the journey too early, in 2037. For now it seems a space taxi to Mars would be a one-way ride.

However, the number of NEOs has increased since the database was compiled, Matloff said. There are now more than 7,000 known NEOs, so more potential rock taxis could exist.

Ideally, astronauts would divert an asteroid so that it cycles permanently between Earth and Mars on a well-timed orbit. Humans could nudge an asteroid into the desired path using a solar sail or gentle propulsion. (See “Solar Sail Hybrid Launches From Japan.”)

Once the asteroid is in a stable orbit, Matloff said, “you’d just jump on it. You could store provisions and spare parts on it and use it for shielding. … ”