Invaders From Earth

Mars has been sailed by, orbited, crashed into and bounced on, wheeled over, scooped up, shoved around, drilled, cored, and baked. And engineers have dreamed up many more ways of studying the planet than have been tried so far.

Over the decades, many inventive spacecraft have never made it past the blueprint stage. But with federal money tight, these concepts may have found a new lease on life: NASA has called for cheaper ways to robotically explore Mars, and scientists are reevaluating past proposals.

Leonard David is a space journalist living in Golden, Colorado.