SNAP is an acronym for Systems for Nuclear Auxiliary Power. Your friend is spot-on about their applications, although they are not as extensive as it may seem. SNAP reactors are more commonly known as RTGs, or Radioisotope Thermoelectric Generators. As the name (somewhat confusingly) implies, they generate electricity using the natural heat produced during the alpha decay of a radioactive element, usually Plutonium-238 oxide, to precipitate electron flow between two dissimilar metals. Please Register or Log in to view the hidden image! SNAP/RTGs are sealed once activated, upon insertion of their Plutonium cores, and from then on produce a steady amount of power without maintenance or moving parts. As such, they are highly desireable for space flight applications. Many of the larger commercial and military satellites have power demands that cannot be satisfied by solar power alone, so they carry one or two RTGs for supplemental or contingency power. The first RTGs that the DOE ever kicked out produced around 2.7 watts of electricity. The most recently designed system, the GPHS-RTG, or General Purpose Heat Source RTG, generates about 290 watts of electric power. The first RTG system ever put up was a SNAP-3B unit, which provided only partial power for the Navy Transit 4 satellite. That was DOE provided three RTGs for NASA's Cassini mission to Saturn. The Cassini spacecraft, launched to Saturn on October 15, 1997, required three GPHS-RTGs (approximately 870 watts electric). The RTGs are the only source of onboard electric power, because they are plenty potent enough. In spite of all these positives, RTG/SNAP systems are still niche technologies, suitable for only a few esoteric applications. Why? Because they don't scale to meet demand. Especially not in the practical sense. SNAP units, derived even of the technology available today, are still immensely expensive and have an output that is quite meager compared to the average home consumption of 15-something kilowatts - the largest SNAPs around will kick out a maximum of around 500 watts. Homeowners can neither afford, nor be satisfied by, SNAP/RTG power sources. They're still pretty cool though. Oh, and a little bit of preemptive data pertaining to the safety of SNAP/RTGs in general, because it's always been a screeching point of the no-nukes weenies: The DOE has conducted extensive safety testing to assure the power systems would be safe under all accident conditions, including accidents that occur on or near the launch pad, and reentry accidents. The fuel form has been changed from a Pu-238 metal to a more stable pressed oxide. During the three mission accidents that did occur, the RTGs performed as predicted. The Transit 5-BN-3 mission was aborted because of launch vehicle failure. The RTG burned up on reentry as designed with the plutonium dispersed in the upper atmosphere. The RTG design was changed shortly after that to accommodate intact reentry. The next accident was with the Nimbus-B-1 that was aborted shortly after launch by a range safety destruct. The RTG was recovered, with no release of plutonium, and the heat sources were reused in later missions. The Apollo 13 spacecraft carried an RTG to be used on the moon to power a seismic station. The Apollo 13 mission was aborted and the spacecraft returned to Earth. The RTG was attached to the lunar module that broke up on reentry. The RTG heat source reentered the Earth atmosphere intact, with no release of plutonium, and currently is located deep in the Tonga trench in the Pacific Ocean. Extensive testing of RTGs in sea water has been conducted, and there will be no release of plutonium over time from this unit.