What cancelled/proposed space project do you find most interesting?

[shynung]

Aquarius rocket.

Cheapskate thing for launching food, fuel, and duct tapes, that can blow itself up 1 out of 3 times without breaking the bank. SSTO, too.

[sp1989]

Singularity Research Initiative

http://forum.kerbalspaceprogram.com/threads/69923-Singularity-Research-Initiative-dev-thread

[gooddog15]

project A119, as it was going to blow up nuclear weapons on the lunar surface just to show superiority over the soviet union

[Comrade Jenkens]

Venture Star

I think that if they'd just persevered rather than cancelling at the very first setback space travel would be very different today.

[tychochallenge]

Terrestrial planet finder, real bummer

[SargeRho]

Cancelled: Venture Star and DC-X.

Proposed: Mars Direct.

[djnattyd]

The cancellation of the JIMO (Jupiter Icy Moons Orbiter) program still makes me cry a little bit inside every time I think of it.

Not only was it the first serious (more-than-imaging) exploration of Jupiter's moons, it was very ambitious. Why just explore Europa when you can take a look at Callisto and Ganymede, too? There were also a number of new technologies, such as a deep-space fission reactor plant, high-impulse Hall thrusters, ice-penetrating radar, and high-bandwidth interplanetary data transmission. Much of this was enabled by the unprecedented power availability provided by the power plant.

Despite all this, it was very do-able and had even started preliminary mission planning and acquisition contracts before it was cancelled. The rationale at the time was that manned space missions were more important (thanks, Constellation). Every NASA mission since then seems incredibly dull, unambitious, and risk-averse.

This, especially as the whole "manned space missions" thing hasn't happened either. Other than Curiosity NASA hasn't really done anything ambitious in years. Obviously the blame lies with the US government or, more precisely, Congress rather than NASA themselves but I feel that science and the potential reward of extraterrestrial life on Europa are worth the costs involved.

[Skyler4856]

http://en.wikipedia.org/wiki/Project_HARP

While it wasn't canceled, it's still really bad***