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Packing for Mars? Don’t forget the nuclear reactor.

Did you see the movie The Martian? The hero, Mark Watney, an astronaut given up for dead by NASA, uses a radioisotope thermoelectric generator (RTG), a sort-of "space battery," to keep warm during his trek across Mars.

The movie is science fiction but these devices are real- NASA has been using RTGs to power satellites for nearly forty years, and they've been used on major trips to the moon and other planets. But NASA recently announced plans to use nuclear power in a different way- one that hasn't been fully attempted in fifty years. 

The RTGs like Mark Watney’s harness the heat from passive radioactive decay and produce a few hundred watts of electricity, which on Earth would be enough to run a handful of household appliances. But a mission to Mars would require far more power. Now, NASA is working on a reactor that splits atoms, as reactors on Earth do, to make 100 times more electricity than an RTG. The initial plan calls for 40 kilowatts, which on Earth would meet the needs of a small apartment building.

For three years, NASA has been working on the project, which it calls Kilopower (Kilo is the Latin prefix for thousand.) According to the development team, which is based at Los Alamos National Laboratory, "The Kilopower technology demonstration is the practical and affordable first step to getting a reactor power system in space. We're seizing the opportunity to demonstrate system-level technology readiness of space fission power."

Artist's conception of the Kilopower reactor.

Surface exploration requires readily available power, but electricity-generating reactors make up only a small slice of nuclear's role in space. So far, RTGs have been much more common than reactors, beginning in the 1950s.

On December 8, 1953, President Eisenhower delivered his "Atoms-for-Peace" address in which he described the promise and potential of nuclear energy for humanity. Six years later, the first radioisotope thermoelectric generator, SNAP 3-A (for Space Nuclear Auxiliary Program), sat on his desk in the Oval Office. This new technology created by the Atomic Energy Commission (AEC) inspired wonder and awe not only from the President, but from the public as well. This new technology made it possible for nuclear energy to power the instruments aboard a satellite.

RTGs convert heat into electric power. They sometimes also function as simple heat generators to keep satellite components from freezing in the deep cold of space. The heat comes from the natural decay of a radioactive isotope, typically plutonium 238.

As the country entered the space race and the tremendous pressure to advance technologically, NASA realized the usefulness of RTGs. They were lightweight, compact, had no moving parts, and only depended on readily available plutonium, rather than the sometimes-unavailable sun. Plutonium 238, a different type of plutonium than used in weapons or power reactors, also seemed to be the most fitting fuel for the job. It has a half-life of 88 years, meaning it puts out high heat and can power the spacecraft for decades.

The first U.S. spacecraft powered by an RTG was the Navy's Transit 4A navigation satellite, which was launched in 1961. That day, the front-page headline of the New York Journal American read: "U.S. ORBITS ATOMIC BATTERY." The government and the public enthusiastically embraced nuclear energy as a means to lead the world in space exploration and technology.

RTGs progressed in the sixties. Like the 3A, the 1964 SNAP-9A used plutonium 238 and was expected to operate for five to ten years. It generated 25 watts of electrical power. RTGs went on to power many more missions throughout the seventies and eighties, including the Cassini space probe, which is completing its mission exploring Saturn in the next few months. Pioneer and Voyager missions used RTGs, as does the Curiosity robotic rover currently exploring Mars.          

However, project Kilopower marks the first time in fifty years that NASA is looking to use nuclear energy in a different form. Back in the 1960s, the agency began working on a way to put a nuclear reactor into space. While RTGs generate heat through the simple decay of a radioactive isotope like plutonium, reactors generate far greater quantities of heat by the splitting of uranium atoms in a controlled nuclear reaction. In 1965, the SNAP-10A launched from Vandenberg Air Force Base with the goal of producing 500 Watts of electricity for at least one year. With such technology, NASA could start the nuclear reaction remotely when the satellite entered orbit.

The project ultimately succeeded, as the reactor produced more than 600 watts of power. After forty-three days, however, an unrelated failure in the Agena spacecraft caused the reactor to shut off. While Russia has launched over thirty reactors into space, the United States has only launched one: SNAP-10A. Now, NASA plans to start testing a new reactor for space at the end of this year.

The new reactor is about 6.5 feet tall and produces 1 kilowatt of electric power, about as much as is consumed by a window air conditioner. The project managers envision 40 kilowatts and multiple small reactors on Mars in the future. Nuclear energy remains the best option for power, as Mars receives less sunlight than earth, making solar power an unrealistic option.

Kilopower begs the question- if reactors here on Earth require refueling and maintenance, how will reactors on space comply with this requirement? Dr. Bhavya Lal of the Science and Technology Policy Institute in Washington, D.C. says that ideally, the reactor would be designed so it would need to be minimally refueled. The Kilopower design uses high-enriched uranium, which allows a greater operational lifetime. Many of these logistical questions can be explored after the testing that begins at the end of this year.

Both the space community and the nuclear energy industry eagerly wait to see how the Kilopower reactor will perform. The potential of electricity on Mars means further exploration and discovery, and nuclear energy is the key to operating there. Although fifty years has passed since the last reactor left Earth, AEC Chairman Glenn Seaborg said in the 1960s what still holds true today:

"The presence of the ‘atomic battery’ in the satellite is a symbol of a ‘marriage’ that was bound to occur—between Space and the Atom. We have known for some time that the two were made for each other."

Before we can even think about making Mars habitable, we must figure out a way to support astronauts exploring the surface. Sunlight is only one-third as strong on Mars as on Earth, and as happens here, the sun is only up for half the day. So solar energy can't provide the immense amount of electricity needed to power a Mars base. NASA explains that it will need nuclear power in many places, including craters in shadow, and has several research efforts now under way.

Nuclear reactors don't just have applications on the actual surface of a planet, either: they can also be a key to getting there. NASA just awarded BWXT Nuclear Energy a contract to design a nuclear thermal propulsion reactor. On a manned trip to Mars, this reactor would create an electric current that shoots ions out of the back of a rocket, propelling the spaceship forward.

A lot has changed since Eisenhower first marveled at the SNAP 3-A in the Oval Office. But as new projects like Kilopower and thermal propulsion reactors take shape, nuclear energy's many applications echo what Seaborg said fifty years ago. There is an eternal, unmatched link between nuclear energy and space.

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