- RTGs convert radioactive decay heat into electricity with no moving parts.
- Voyager's RTGs have operated for over 47 years in interstellar space.
- Plutonium-238 provides decades of steady power at roughly 6% conversion efficiency.
A radioisotope thermoelectric generator (RTG) is a device that converts heat from the natural decay of a radioactive isotope into electricity, using thermocouples that exploit the temperature difference between the hot fuel and the cold surrounding environment. RTGs contain no moving parts and require no maintenance, making them the standard power source for spacecraft operating where sunlight is too weak or unreliable for solar panels.
Why It Matters
Key figure
47+ years
Voyager power systems still operating
RTGs have enabled some of the most distant exploration in human history. NASA's twin Voyager probes, launched in 1977, each carry three RTGs that provided 470 watts at launch. Nearly five decades later, both spacecraft continue to return data from interstellar space, powered by the steady decay of plutonium-238.
No other portable power technology has demonstrated that kind of longevity.
On Mars, the Curiosity and Perseverance rovers rely on a newer design called the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). Unlike the solar-powered Spirit and Opportunity rovers, which depended on clear skies and seasonal sunlight, the MMRTG-equipped rovers operate through dust storms and long Martian nights without interruption. That independence from solar conditions allowed NASA to land Curiosity in Gale Crater, a scientifically rich but dimly lit site that would have been off-limits to a solar-powered mission.
RTGs also powered the Cassini orbiter at Saturn, the New Horizons probe that flew past Pluto in 2015, and the Apollo lunar surface experiments that astronauts left on the Moon between 1969 and 1972.
How It Works
The operating principle dates to 1821, when German physicist Thomas Johann Seebeck discovered that joining two different metals in a circuit and heating one junction while cooling the other produces an electric voltage. This is the Seebeck effect, and it remains the basis of every RTG built since.
Key figure
~6%
Thermal-to-electric conversion efficiency
Inside an RTG, pellets of plutonium-238 dioxide generate heat as the isotope undergoes alpha decay. Plutonium-238 is not the weapons-grade plutonium-239 used in nuclear warheads. Its half-life of 87.7 years makes it well suited for missions lasting decades: the power output declines slowly and predictably.
Surrounding the fuel are arrays of thermocouples, solid-state devices made from materials such as silicon-germanium or lead telluride. The large temperature difference between the hot plutonium core (around 1,000 degrees Celsius) and the cold exterior (exposed to space or a planetary surface) drives a continuous electric current through these thermocouples.
The conversion efficiency is low, roughly 6 to 7 percent. Curiosity's MMRTG, for example, converts about 2,000 watts of thermal energy from 4.8 kilograms of plutonium dioxide into approximately 125 watts of usable electricity at the start of its mission.
That low efficiency is an accepted trade-off. RTGs provide power that is constant, reliable, and independent of orientation, distance from the Sun, or atmospheric conditions.
Key Context
The United States launched its first RTG-powered satellite, Transit 4A, on June 29, 1961. Since then, more than 26 U.S. space missions have carried RTGs, along with missions by the Soviet Union (which used strontium-90 RTGs to power remote lighthouses and navigation beacons on Earth) and the European Space Agency.
Plutonium-238 production in the United States effectively stopped in 1988 when the Savannah River Site ceased operations. For years, NASA relied on purchased Russian stockpiles. Oak Ridge National Laboratory resumed domestic production in 2015, initially producing small quantities, with a target of 1.5 kilograms per year to sustain future missions.
FAQ
How is an RTG different from a nuclear reactor?
An RTG generates electricity from the natural radioactive decay of an isotope. It involves no chain reaction and cannot melt down. A nuclear fission reactor, by contrast, sustains a controlled chain reaction to produce far more power but requires active management and shielding systems.
Why do RTGs use plutonium-238 instead of other isotopes?
Plutonium-238 combines a high power density (about 0.57 watts per gram) with a half-life of 87.7 years, which provides decades of steady output. It decays primarily through alpha emission, which is easily shielded. Other isotopes such as strontium-90 and americium-241 have been used or proposed, but each involves trade-offs in power density, radiation type, or half-life.
Can RTGs power a crewed mission to Mars?
RTGs alone would not provide enough electricity for a crewed Mars habitat, which would require tens of kilowatts. NASA's Kilopower project, tested in 2018, explored small fission reactors for that purpose. RTGs remain better suited to robotic missions and small instrument packages.
Are RTGs dangerous to launch?
RTG fuel is encased in iridium cladding and surrounded by graphite impact shells designed to survive launch explosions, atmospheric reentry, and ground impact without releasing plutonium. NASA conducts extensive safety analyses before every RTG launch, and no RTG has ever caused a harmful release during a U.S. mission.
Related Reading
Related Reading
Sources
- Radioisotope Thermoelectric Generators (NASA Science)
- Powering Curiosity: Multi-Mission Radioisotope Thermoelectric Generators (U.S. Department of Energy)
- Multi-Mission Radioisotope Thermoelectric Generator (NASA Mars Exploration)
- An Overview of Radioisotope Thermoelectric Generators (Stanford University)
- Radioisotope Power Systems Timeline (NASA Science)
Fact Check: Claim-by-Claim Verification Verified
All major claims verified against authoritative NASA, DOE, and academic sources. MMRTG wattage corrected from 110W to 125W during drafting.
Sources used for verification
- Radioisotope Thermoelectric Generators - NASA Science
- Powering Curiosity - U.S. DOE
- ORNL Plutonium-238 Milestone - ORNL
- RPS Timeline - NASA Science
- RTG Overview - Stanford University
