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.

1) GM's giant moon truck

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).

2) NASA's 1970s space colonies

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.

Of all the concepts on this list, though, this is by far the most fantastical. We're still coming to grips with how hard it is to keep a few people alive in space for a year — let alone a giant colony that grows its own food. These sorts of ideas were tossed around at a few conferences in the 1970s, but never came close to happening.

3) Mars and Venus flybys

A more realistic plan for human space exploration following Apollo was based on a number of planetary flyby missions.

The first one, which NASA proposed to launch in 1975 using an Apollo-inspired craft, would have sent astronauts flying past Mars during a two-year journey, with probes descending off the craft to collect rock samples and return them to the lab.

A followup mission would have seen astronauts do a "triple flyby," using the alignment of Earth, Mars, and Venus to fly past Venus, then Mars, then Venus again in one fell swoop.

But these flybys — like many other post-Apollo plans for human space exploration — were killed due to radically reduced funding from Congress after the success of the moon landings. Instead, after the final Apollo launch in 1972, NASA followed up with the much more modest Skylab space station during the 1970s.

4) Project Longshot

In 1987, researchers from NASA and the US Navy concocted Project Longshot: an interstellar probe that would travel to Alpha Centauri B, one of the closest stars to our sun. The craft would have had a nuclear fission reactor on board to power its engine, and would have sent data back to Earth with a laser. If the probe traveled at about 8,300 miles per second (about 4.5 percent the speed of light), the scientists calculated, the journey to Alpha Centauri B would take about 100 years — and it'd take four years for data to travel back to Earth.

The idea never happened for a few different reasons: apart from the difficulty of developing the technology necessary to travel that far at that speed, there's the problem of convincing NASA administrators to fund a mission that won't bear fruit for a century. Right now, the object we've sent farthest from Earth, Voyager 1, is about 12 billion miles away — less than .2 percent of the way to Alpha Centauri B.

5) The Titan Mare explorer

Titan — Saturn's largest moon — is covered by a number of lakes filled with liquid methane. To explore them, in 2009, NASA proposed sending the Titan Mare Explorer: a probe that would have landed in Titan's second-largest lake and cruised around like a boat, in order to study the chemistry of the sea and help us understand how the methane cycle works. Titan is cloudy (ruling out a solar-powered probe), but if selected, the probe would have relied on a new, ultra-efficient plutonium-powered nuclear generator, which would have let it survive for much longer on less fuel than current generators used.

In 2012, however, the Mare Explorer was passed over for funding (in favor of a probe that will land on Mars and study its interior next year) and in 2013, NASA killed the development of the new nuclear generator, effectively finishing off Titan Mare.

Still, we could explore Titan's seas yet. Last month, NASA announced the concept of sending a submarine to Titan instead. At this point, it's just an idea — one that also has to survive many rounds of funding decisions — but it's possible we could see it launch sometime in the 2020s or 30s.

Correction: This article previously said that Project Longshot would travel at 8300 miles per hour, not 8300 miles per second.