Project Longshot was a conceptual design for an unmanned space probe that would travel to Alpha Centauri, the closest star system to our own. Conceived out of a partnership between the U.S. Naval Academy and NASA, the probe was intended to be built and launched from the planned Space Station Freedom, which later evolved into the International Space Station.
Longshot would have been powered by a traditional nuclear fission reactor that would, in turn, power lasers that would be used to compress and ignite fusion reactions with 264 tonnes of stored Helium-3/Deuterium fuel pellets. These fusion reactions would propel the ship. Engineers predicted that the probe would be able to travel at 4.5% the speed of light, meaning that the trip to Alpha Centauri would take roughly a century.
Sadly, the project never got off the ground. Laser fusion is still unfeasible and there seems to be a lack of political will to think beyond partisan talking points, let alone the solar system.
(Image: Wikimedia Commons)