Provided by Vox.com The MOLAB could travel at 21 miles per hour. (USGS)

NASA is full of ambitious dreamers. But those dreams cost money. And Congress has to approve them first.

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. But they all share one characteristic: they never happened.

Here are some of the most fascinating ideas concocted over the years.

As the Apollo program made progress toward a crewed moon landing, some NASA scientists made plans for longer human missions to explore and study the moon's surface.

Toward that end, in 1963, NASA contracted with GM to produce an inhabitable lab on wheels that astronauts could live in for weeks at a time as they drove around the moon. It was essentially a lunar RV, powered by an engine from a Chevrolet Corvair (the car that eventually became infamous as the subject of Ralph Nader's book Unsafe at Any Speed).

But after a few successful Apollo landings, plans for longer-term exploration of the moon were cancelled. GM had built a single prototype, and it was eventually loaned to the US Geological Survey (which used it for several projects in the deserts of the southwest).

Following the success of the Apollo program, some scientists began drawing up ideas for enormous space colonies that would be established on stations in Earth's orbit. A 1975 NASA study, for instance, envisioned "a space habitat where 10,000 people work, raise families, and live out normal human lives."

Plans were devised for several different space stations, each of which would have rotated to use centrifugal force to simulate the feeling of gravity. Residents would use soil brought from the moon to grown their own food, purify their own water, and have their own parks, shops, schools, and hospitals. One of the colony's purposes would be industry: "Using solar energy to generate electricity and to power solar furnaces the colonists refine aluminum, titanium, and silicon from lunar ores shipped inexpensively into space," the report noted.

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The 5 coolest NASA missions that never happened