Editors Note: This blog was originally posted during Energy.gov's #SpaceWeek in 2015.
You may not associate space travel with the Energy Department. But you should -- because nuclear power systems developed here have made dozens of truly amazing interplanetary research missions possible.
The Energy Department’s Office of Space and Defense Power Systems and its predecessors, in tandem with the National Labs and private industry partners, have developed and provided radioisotope power systems to NASA for use in numerous long-term missions, from Voyagers 1 and 2 to the Mars rovers. These compact, reliable systems provide basic mission fuel and keep critical spacecraft components warm enough to function in the cold, dark reaches of deep space.
Although relatively simple, these systems have powered some of the most successful and inspiring missions in U.S. space program history. Explore our interactive timeline of these missions, above, and read on to learn more about the “space batteries” that made them a reality.
POWER FROM PLUTONIUM
Despite what you see in movies and TV shows, there are only two practical ways to supply electrical power for multi-year space missions: the sun’s rays or heat generated by natural radioactive decay. Radioisotope power systems -- which directly convert heat generated by the decay of plutonium-238 into electric power -- use the latter, and are essential for long missions to distant parts of the solar system, where solar-powered space travel may be impractical or impossible.
Plutonium-238 works well as a space power source for several reasons. It has a half-life of 88 years, meaning it takes that long for its heat output to be reduced by half. It’s stable at high temperatures; can generate substantial heat in small amounts; and emits relatively low levels of radiation that is easily shielded, so mission-critical instruments and equipment are not affected. This type of plutonium is different than those used for nuclear weapons or nuclear power plant reactors.
In a radioisotope power system, commonly called a “space battery,” the plutonium is processed into a ceramic form -- similar to the material in your morning coffee mug. Just like a shattered mug, it breaks into large chunks instead of being vaporized and dispersed, preventing harm to people and the environment in the unlikely event of a launch or reentry accident. For more than 50 years, every radioisotope power system launched into space has worked safely and exactly as designed.
EARLY NUCLEAR SPACE POWER SYSTEMS
In 1961, the U.S. Navy’s Transit 4A navigation satellite became the first U.S. spacecraft to be powered by nuclear energy. Transit 4A was powered by a radioisotope thermoelectric generator, or RTG, developed by the Atomic Energy Commission, the predecessor to the Energy Department. Since then, eight more generations of radioisotope power systems were developed by the Energy Department for use in space by NASA, the U.S. Navy and the U.S. Air Force.
With no moving parts, RTGs convert heat from plutonium-238 decay into electricity using devices called thermocouples. The RTG on the Navy's Transit 4A satellite produced 2.7 watts of electrical power. Transit 4A held the record for oldest broadcasting spacecraft for its first decade in orbit, during which time it traveled nearly 2 billion miles and circled the Earth more than 55,000 times.
In 1969, NASA launched the RTG-powered Nimbus III, the first U.S. weather satellite to measure air pressure, solar ultraviolet radiation, the ozone layer and sea ice during both day and night. Nimbus also included on-board infrared sensors that took early satellite photographs of the Earth. Aside from its RTGs, Nimbus also drew power from 10,500 built-in solar cells.
SPACE BATTERIES ON THE MOON
The Apollo missions to the moon included experimental packages known as ALSEP -- for Apollo Lunar Surface Experiment Package -- containing scientific instruments that were left on the moon by U.S. astronauts to send data back to Earth. The first package was solar-powered but relied on two 15-watt radioisotope heater units (RHUs) to keep its instruments warm enough to function.
The subsequent packages were each powered by 70-watt SNAP-27 radioisotope thermoelectric generators. The ALSEPs contributed to a significant amount of what we now know about the moon -- including data on solar wind and radiation, and the observation that the moon is geologically active. The five ALSEP stations were shut down in 1977.
EXPLORING THE SOLAR SYSTEM
RTGs have also powered missions to explore other planets. In 1989, Galileo became the first spacecraft to orbit Jupiter. Galileo showed evidence of an ocean of liquid water on Europa, one of Jupiter’s moons, and volcanoes on Io, another moon; and took the first close-up pictures of an asteroid and the first photos of a comet colliding with a planet, when Shoemaker-Levy 9 struck Jupiter. The Galileo orbiter was powered by two RTGs and included 120 RHUs to ensure its scientific instruments functioned properly.
The Ulysses mission to study the heliosphere -- the part of space that is affected by the Sun’s magnetic field -- launched in 1990, powered by a general purpose heat source radioisotope thermoelectric generator, or GPHS-RTG. It operated for about two decades before being powered down, during which time the Ulysses spacecraft flew past Jupiter and made three full polar orbits of the sun. Ulysses gathered previously unknown data about solar storms, solar wind, interstellar dust particles and cosmic radiation. It also discovered 30 times more dust coming into the solar system from deep space than scientists had originally expected.
Cassini, an ongoing international mission to explore Saturn and its moons, is powered by three RTGs and kept warm by 117 small, strategically placed RHUs -- 82 on the Cassini orbiter and 35 on the Huygens probe, which Cassini carried to and released over Saturn’s moon Titan. On January 14, 2005, Huygens successfully landed on Titan’s surface, the first-ever landing of a craft from Earth in the outer solar system. Cassini is also responsible for the first comprehensive study of the Saturn system from orbit -- including discoveries of active, icy geysers on Enceladus, another moon. Data collected from the Cassini mission is helping scientists understand more about what the Earth may have been like before life evolved.
Launched in 2006, the New Horizons spacecraft was designed to study Pluto and to explore other little-known, ice-cold places in the Kuiper Belt. New Horizons passed Jupiter and captured photos of that planet’s rings and lightning at its poles before completing its flyby of Pluto in July 2015, returning the highest-resolution images ever captured of the dwarf planet and its moons. It later reached Kuiper Belt object Ultima Thule on January 1, 2019, completing the farthest planetary flyby ever in exploration history at more than 4 billion miles away from us, here on Earth. The spacecraft is powered by a GPHS-RTG, similar to the one used on Ulysses.
REACHING FOR INTERSTELLAR SPACE
Pioneer 10 and Pioneer 11, launched in the early 1970s, were precursors to the Voyager missions that followed. The spacecraft were designed to travel far -- each powered by four RTGs and kept warm by 12 RHUs -- and to withstand intense radiation from planets further out in the solar system.
Pioneer 10’s power systems were designed to last at least five years but operated for more than three decades before communications ceased. During that time it was the first spacecraft to fly past Mars, visit (and photograph) Jupiter, cross the asteroid belt and transmit data about interplanetary particles.
Pioneer 11 took the first up-close pictures of Saturn, discovered two new moons and an additional ring around the planet, and found that Saturn emits more than twice as much heat as it receives from the Sun. The mission lasted 22 years before communications ceased; now, Pioneer 10 and 11 are headed toward the edge of the solar system, bearing plaques with a message for intelligent beings they may encounter
Voyager 1 and 2 built on Pioneer’s legacy in the late 1970s. Taken together, these two missions have yielded some of the most important discoveries in U.S. space exploration history. Each spacecraft uses nine RHUs to stay warm and draws power from three multi-hundred watt radioisotope thermoelectric generators, or MHW-RTGs -- a type of power system specific to these two missions. The power systems are still operating today, more than 35 years after they were deployed. As the Voyager spacecraft slowly lose power, mission controllers back on Earth may turn off instruments one by one to conserve energy as long as possible.
Voyager 1 flew by Jupiter and Saturn and recently entered interstellar space. Along the way, it photographed the Earth -- appearing as a pale blue dot -- from 4 billion miles away. Voyager 2 is the only spacecraft to study all four of the giant planets -- Jupiter, Saturn, Uranus and Neptune -- at close range. It discovered previously unknown moons around Neptune and Uranus, as well as liquid nitrogen geysers on Triton, one of Neptune’s moons, and transmitted back hundreds of never-before-seen images.
MISSIONS TO MARS
Viking 1 and 2, launched separately in 1975, were NASA’s first effort to harvest data directly from the surface of the red planet. Each mission had two parts: an orbiter and a lander. Both Viking missions sent back photographs of the surface of the red planet and helped scientists back on Earth learn more about elements present there (carbon, nitrogen, hydrogen, oxygen and phosphorus -- all essential to life on our own home planet). The two 42.6-watt RTGs on Viking 1 and 2 were designed to last at least 90 days but lasted for six and four years, respectively.
Interestingly, Viking 1 was not the first spacecraft to land on Mars -- although it was the first successful one. A failed Soviet mission touched down on the Martian surface in 1971 but only survived for seconds before losing communication. Between Viking 1 and 2, more than 55,000 images of Mars were transmitted back to Earth -- including the first space "selfie" on Mars, taken by Viking 2 itself. The image is one of the most famous pictures in the history of the U.S. space program.
NASA took Mars exploration one step further in 1996, launching the microwave oven-sized Mars Pathfinder rover. Designed to last seven days, the mission endured 12 times longer -- demonstrating a cost-effective way to send a scientific mission to the red planet. Pathfinder used solar panels for electric power and relied on three RHUs to keep its scientific instruments warm.
In 2003, NASA separately launched twin rovers Spirit and Opportunity, designed to search Mars for evidence of water, climate changes and other clues that the planet may have once supported life. Both rovers used solar panels for power and RHUs to support on-board scientific instruments. Spirit explored the red planet for six years -- finding strong evidence that at one time Mars was much wetter than it is now -- before it got stuck in sand and went no further. Opportunity has lived on, studying rock layers and returning stunning photographs of the Martian landscape.
The most recent nuclear-powered space mission to launch was 2011’s Curiosity, which tweets from space. For both heat and power, Curiosity relies on a single multi-mission radioisotope thermoelectric generator that was constructed, assembled and tested by the Energy Department and the Idaho, Oak Ridge, Los Alamos and Sandia National Laboratories. This SUV-sized rover contains numerous scientific instruments and was sent to Mars to study rock layers and climate, determine if favorable conditions for life ever existed there, and to pave the way for future human exploration. Curiosity is far more powerful than its red planet predecessors and is expected to last for at least two years -- presumably drilling and analyzing rock samples across an unprecedented range.
NASA plans to send a similar rover back to Mars in 2020. The rover is based on Curiosity’s design but this time will carry a drill for coring samples from Martian rocks and soil. The mission is expected to launch in July 2020 and land on Mars in February 2021. The RTG is currently being constructed, assembled and tested by Idaho, Oak Ridge, Los Alamos and Sandia National Laboratories.