The 5 Coolest NASA Missions That Never Happened

NASA is full of ambitious dreamers. But those dreams cost money. Ever since the end of the Apollo program, this tension has meant that many of NASA's ideas are killed before they ever progress much beyond concept drawings.
These ideas have ranged from far-fetched fantasies to financially prudent missions. Some were just sketches and equations on paper, while others materialized into models and test materials. Here are some of the most fascinating ideas that never happened.