Of course, designing the wings is only half the challenge. The other half is powering them, for which the engineers are developing a "reciprocating chemical muscle," or RCM, expressly for Mars' virtually oxygen-free atmosphere. The RCM consists of two parallel rods that push in opposite directions-hence, "reciprocating"-and contains a chemical propellant and a catalyst (Michelson won't say what they are) which, when combined, produce a high-pressure gas that drives pistons connected to the wings. Researchers will also need to perfect control systems that will keep the Entomopter upright and pointed in the right direction, but Michelson and company are undaunted, as they have the field pretty much to themselves. "To my knowledge," says Michelson,"we're the only flapping-wing vehicle for Mars out there."