From a source of heat comes power to explore
Radioisotope power systems, or RPS, provide electricity and heat that can enable spacecraft to undertake scientific missions to environments beyond the capabilities of solar power, chemical batteries and fuel cells.
RPS are sometimes referred to as a type of "nuclear battery." While some spacecraft, like Cassini, do run their systems directly off of their RPS, others like the Mars Science Laboratory rover can use the RPS to charge batteries and run their systems and instruments off of stored battery power. In either case, the RPS is attached directly to a spacecraft, much like a power cord being plugged in.
These technologies are capable of producing electricity and heat for decades under the harsh conditions of deep space without refueling. All of these power systems, flown on more than two dozen NASA missions since the 1960s, have functioned for longer than they were originally designed.
The RPS used to power NASA spacecraft are supplied by the U.S. Department of Energy (DOE). NASA and DOE continue to collaborate on maintaining and developing several types of RPS.
The building block
The General Purpose Heat Source module, or GPHS, is the essential building block for the radioisotope generators used by NASA. These modules contain and protect the plutonium-238 (or Pu-238) fuel that gives off heat for producing electricity. The fuel is fabricated into ceramic pellets of plutonium-238 dioxide (238PuO2) and encapsulated in a protective casing of iridium, forming a fueled clad. Fueled clads are encased within nested layers of carbon-based material and placed within an aeroshell housing to comprise the complete GPHS module.
Each GPHS is a block about four by four by two inches in size, weighing approximately 3.5 pounds (1.5 kilograms). They are nominally designed to produce thermal power at 250 watts at the beginning of a mission, and can be used individually or stacked together.
Modules have been subjected to extreme testing conditions that significantly exceeded the intensity of a wide range of potential accidents. Such tests have included simulating multiple reentries for a single module through Earth's atmosphere, exposure to high temperature rocket propellant fires, and impacts onto solid ground.
The enhanced GPHS modules used in the latest generation of radioisotope power systems incorporate additional rugged, safety-tested features that build upon those used in earlier generations. For example, additional material (20 percent greater in thickness) has been added to the graphite aeroshell and to the two largest faces of the block-like module. These modifications provide even more protection to help to contain the fuel in a wide range of accident conditions, further reducing the potential for release of plutonium-238 that might result.
Types of Radioisotope Power Systems
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Radioisotope Thermoelectric Generator, or RTG, provides power for spacecraft by converting heat generated by the radioactive decay of its fuel source into electricity using devices called thermocouples. RTGs have no moving parts. The latest RPS to be qualified for flight, the
Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is powering the Mars Science Laboratory rover, Curiosity (launched in November 2011).
The Advanced Stirling Radioisotope Generator, or ASRG, converts heat from its Pu-238 fuel source into electricity using a moving piston in a device called a Stirling engine (or Stirling converter). An ASRG would operate more efficiently than an RTG, using only one-quarter of the amount of nuclear fuel in a traditional RPS to produce a similar amount of power. This technology is currently being developed to support potential future missions.
Both the MMRTG and ASRG are designed to be used in the vacuum of space as well as within the atmosphere of Mars.
Radioisotope Heaters
A Radioisotope Heater Unit, or RHU, employs a small pellet of Pu-238 to generate heat for spacecraft structures, systems, and instruments, enabling their successful operation throughout a mission. Some missions employ just a few RHUs for extra heat, while others have dozens.
Radioisotope power systems & heaters by mission
| Nimbus III | Two SNAP-19B3 RTGs | Earth atmospheric science (weather) |
Apollo 11
(Early Apollo Surface Experiment Package) | Two RHUs | Lunar surface |
Apollo 12 through 17
(Apollo Lunar Surface Experiment Package) | One SNAP-27 RTG each | Lunar surface |
| Pioneer 10 | Four SNAP-19 RTGs, 12 RHUs | Outer planet flyby at Jupiter |
| Pioneer 11 | Four SNAP-19 RTGs, 12 RHUs | Outer planet flybys at Jupiter & Saturn |
| Viking 1 lander | Two SNAP-19 RTGs | Mars surface |
| Viking 2 lander | Two SNAP-19 RTGs | Mars surface |
| Voyager 1 | Three MHW-RTGs, 9 RHUs | Outer planet flybys at Jupiter, Saturn, plus interstellar space |
| Voyager 2 | Three MHW-RTGs, 9 RHUs | Outer planet flybys at Jupiter, Saturn, Uranus, and Neptune, plus interstellar space |
| Galileo | Two GPHS-RTGs, 103 RHUs on orbiter, 17 RHUs on atmospheric probe | Venus and Earth flybys, Jupiter orbit, probe to Jupiter's atmosphere |
| Ulysses | One GPHS-RTG | Two Jupiter flybys, Solar polar observations |
| Mars Pathfinder Sojourner Rover | Three RHUs | Mars surface |
| Cassini-Huygens | Three GPHS-RTGs, 82 RHUs on orbiter, 35 RHUs on Huygens probe | Venus, Earth and Jupiter flybys, Saturn orbit, Huygens lander to Titan |
| Mars Exploration Rover Spirit | Eight RHUs | Mars surface |
| Mars Exploration Rover Opportunity | Eight RHUs | Mars surface |
| New Horizons | One GPHS-RTG | Pluto/Kuiper Belt flybys |
| Mars Science Laboratory Rover Curiosity | One MMRTG | Mars surface |
Additional nuclear technologies for space exploration
NASA and DOE have explored other types of nuclear power technology over the years, including space nuclear reactors and nuclear propulsion technologies. Continued research and development of these and other related technologies might one day enable space missions to deliver more payloads on cargo missions, achieve faster trip times on piloted missions, or even provide power for crew stations on the surface of Mars or the moon.
