The co-founder of a rocket launch firm has proposed an audacious plan to send astronauts on a one-way trek to Mars using a pair of tethered U.S. space shuttles that would parachute to the Martian surface. Inventor Eric Knight, a co-founder of the rocket firm UP Aerospace, detailed the plan - which he's billed "Mars on a Shoestring" - in a thought exercise designed to encourage unconventional thinking for future human spaceflight. "My thought paper is a mental exercise to encourage new ideas," Knight told SPACE.com in an e-mail interview. "I also hope it spurs a re-evaluation of the timeline...