getting back home alive, for one thing. And then there's the issue of fuel, something long distance trips require a lot of -- but stocking up on here on Earth means potential weight problems at launch. One proposal offered up in the past is space-based fueling stations conveniently located in key spots on the way to a distant destinations like the Moon, Mars, and asteroids. NASA is collecting proposals that can demonstrate the validity of such a plan, including the ability to store liquid oxygen and hydrogen, transfer it, and have a ship approach for fueling. If you think you've got your bases covered -- and can keep it under $200 million -- you've got until May 31st at 11:59 PM EST to hand over a proposal.