General Atomics (GA), an American company founded in 1955 to develop smaller and safer nuclear power plants, became well-known only after establishing an aviation division in 1995 that soon became the major and pioneering manufacturer of large UAVs (Unmanned Aerial Vehicles). Now GA has returned to its roots with a new nuclear power system for long range space vehicles. The new GA design is Nuclear Thermal Propulsion (NTP) that uses heat generated from the decay of radioactive isotopes to heat a gas that then produces an efficient, long term propulsion system. The amount of propulsive power is not great enough for launching space ships from earth. But once in space the NTP is much lighter and more efficient than the power powerful chemical rockets that put space vehicles into orbit. For example, an NTP would reduce travel time to Mars by about 25 percent.
These miniature power plants were invented by an American scientist in 1957 and General Atomics was one of the firms that developed practical applications, like the SNAP devices. These nuclear power sources are actually radioisotope thermoelectric generators (RTG). They use a hot (in terms of heat) radioactive isotope (like Plutonium 238) to generate heat, which flows through a thermocouple to a heat sink, producing a few hundred watts of electricity. These power generators are usually designed to last ten years without any maintenance. Some have lasted for decades, especially the ones designed for use in long range space probes.
The new General Atomics NTP device is smaller, lighter and more efficient for high-speed space vehicles that don’t just rely on a gravity type propulsion system where a space vehicle uses small rockets to break out of orbit and then use that orbital speed to get to the moon, Mars or other planets on missions that can take months or years. These space probes sometimes use RTGs to maintain power as they get too far from the sun for their solar powers to yield sufficient power. RTGs are also used for research devices left on the moon or planets to collect information and transmit it back to earth.
Since the 1960s the United States used several dozen RTGs for space satellites and some navigation beacons in Alaska. Russia was much more prolific, and careless, with RTGs. By the end of the Cold War in 1991 Russia had hundreds of RTG nuclear power generators sitting in remote places, posing a risk to the environment, and a potential source of radioactive material for a "dirty bomb", which is explosives surrounded by the radioactive material, that is scattered by the explosion, contaminating a large area. The Soviet Union used over a thousand of these RTGs for space satellites and probes, navigation beacons in remote areas, as well as light houses on its arctic coast. These devices also put out lethal, but short range, radioactivity. Animals who cuddle up to them in the Winter, will die after a few hours’ exposure. Some illiterate (unable to read the warnings printed on the RTG exterior) hunters have suffered the same fate.
Unlike Russia, the United States collected all of its terrestrial RTG units by the mid-1990s, and safely stored or disposed of them. Not so with the Soviet ones, and Russia is reminded of this from time to time, but many of these reactors are still out there, no longer generating much electricity, but still very radioactive.
RTGs and related devices like the NTP are safer to use in space, where there is already a lot of radiation and no small animals looking for a warm place to stay, or illiterate locals who can’t read the warning labels describing the radiation risks.