NASA is considering proposals submitted last month from a handful of companies for a spacecraft propulsion system that could refuel on Mars and other planets. Some of the companies that submitted bids include Alliant Techsystems Inc. and Northrop Grumman Corp., which both employ hundreds of people in Maryland. The designs center on an engine fueled by a mix of liquid oxygen and liquid methane, the components of which are found naturally on Mars. The propellants, which NASA dubs “LOX/methane,” are nontoxic and safer than those that power the space shuttle. And astronauts would not have to carry heavy fuel for the return trip but could fill up at an outpost in deep space. “If we go to Mars at some point, it will be a method of living off the land,” said Mark D. Klem, project manager for NASA’s Propulsion and Cryogenic Advanced Development project, which seeks out technologies that could be developed for future NASA missions.