Project Longshot is a design for an interstellar spaceship, an unmanned probe intended to fly to Alpha Centauri powered by nuclear pulse propulsion. Developed by the US Navy's Naval Academy and NASA, Longshot was designed to be built at the Space Station Alpha, the much larger precursor to the existing International Space Station. Unlike the somewhat similar Project Daedalus, Longshot was designed solely using existing technology, although some development was required.
Unlike Daedalus' closed-cycle fusion engine, Longshot used a long-lived nuclear fission reactor for power. Initially generating 300 kilowatts, the reactor powered a number of lasers in the engine that were used to ignite inertial confinement fusion similar to that in Daedalus. The main design difference was that Daedalus relied on the fusion reaction being able to power the ship as well, whereas in Longshot the external reactor provided this power.
The reactor was also used to power a laser for communications back to Earth, with a maximum power of 250 kilowatts. For most of the journey this would be used at a much lower power for sending data about the interstellar medium, but during the flyby the engines would be turned off and the entire power dedicated to communications at about 1 kilobit per second.
The baseline Longshot massed 6.4 metric tonnes, which includes a 3.4 tonne allocation for a large flywheel energy storage system. The rest of its mass was fuel.
Beals, K. A., M. Beaulieu, F. J. Dembia, J. Kerstiens, D. L. Kramer, J. R. West and J. A. Zito. Project Longshot: An Unmanned Probe To Alpha Centauri. U S Naval Academy. NASA-CR-184718. 1988.