For decades, it has been understood in the space community that if humanity were to thrive for long periods of time in space, it would need to lean on a form of energy that we have historically been hesitant to embrace on Earth. That energy is nuclear.

Historically, nuclear has only been leveraged in limited ways in space applications. However, that may change in the coming years. Both the U.S. civilian government and the Department of Defense have expressed renewed vigor in exploring applications of nuclear systems for space applications. In fact, there are now multiple projects that have received tens to hundreds of millions in funding to explore the use of nuclear systems for enabling power and propulsion in space.

This newsletter will discuss the existing applications in which nuclear has been used in space before covering future nuclear space technologies and ultimately addressing the government programs that are aiming to turn them into reality.

Here's a quick summary of what will be discussed:

• Types of Space-Based Nuclear applications

  • Radioisotope Thermoelectric Generators

  • Nuclear Reactors

• Current US Government Programs for Nuclear in Space

  • Civilian Government Programs (NASA)

  • Defense Department Programs (DARPA, DIU)

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Types of Space-Based Nuclear Systems

Radioisotope Thermoelectric Generators

The first nuclear mission in space involved the launch of a Radioisotope Thermoelectric Generator (RTG) named Snap-3 aboard the US Navy's Transit 4A navigation satellite in 1961. RTG's use thermocouples to convert the heat from a small sample of decaying radioactive material into an electric potential. That potential is then used to provide reliable electric power. RTG's are typically used in locations with remote access making them appealing for applications of satellite electrical power.

For satellites operating in proximity to Earth, solar panels have become the prevailing means of electric power generation. As a result, RTG's have in most recent decades been used almost exclusively for powering deep space probes and rovers, including Voyager, Galileo, and Curiosity. Despite their reliability, RTG's are only capable of generating a few hundred watts of power, making them insufficient for the electrical demands of more substantial space applications, such as for large spacecraft or eventually powering crewed habitats or spacecraft.

Nuclear Fission Reactors in Space

Historical Use

For more significant power or heat requirements than can be met by RTG's, nuclear reactors leveraging nuclear fission are necessary. The process of nuclear fission involves a neutron colliding with the nucleus of a heavy atom such as Uranium or Plutonium. The collision results in the emission of other high-energy neutrons, leading to a chain reaction and the release of significantly more energy which can be harnessed for power generation.

In 1965, the Air Force launched the nuclear-powered SNAP-10 satellite aboard an Atlas rocket from Vandenberg Air Force Base, marking the world's first operation of a nuclear reactor and ion thruster in space. After 43 days, the spacecraft failed for reasons unrelated to its reactor and it remains in a 1,300km polar orbit. While the US never launched any additional fission reactors after SNAP-10, the Soviet Union sent to orbit a total of 31 reactors to enable nuclear electric propulsion applications.

Why Nuclear Now?

As the US looks to return to the moon, this time with a permanent presence, nuclear power has emerged as a clear enabling technology. NASA recently released potential landing zones for the Artemis III mission, which will be the first Artemis mission to place a crew on the lunar surface. The landing zones are all near the Moon's poles because of interest in accessing potential ice sources in craters at the Moon's polar regions.

Colder temperatures, due to shadows and reduced sunlight, in the polar regions increase the likelihood of finding ice. However, reduced sunlight also means solar panels would be less reliable. Furthermore, for long-duration missions, it would be difficult for crews to carry from Earth enough oxygen and hydrogen to generate electrical energy using fuel cells, which were used during the Apollo missions. As a result, nuclear power becomes an attractive means of reliable and long-term power generation.

In addition, nuclear provides benefits as a reliable heat source for propulsion systems on missions to further planetary bodies like Mars or beyond. Two forms of nuclear propulsion, Nuclear Thermal Propulsion (NTP) and Nuclear Electric Propulsion (NEP) offer capabilities that enable high-energy missions requiring large delta-v maneuvers. NTP uses a fission reactor to heat a propellant, typically considered to be hydrogen, to high temperatures before discharging the propellant to generate thrust. NEP uses a nuclear reactor to first generate electricity which is then used to create an electric field that accelerates an ionized propellant particle. That particle is then emitted to create thrust.

NTP affords a theoretical specific impulse as high as 900s, approximately three times that of a chemical rocket engine, with thrust levels on the order of around 50,000N, much higher than for electric propulsion systems. For comparison, NEP is theoretically capable of Isp levels as high as 10,000s, but with thrust of less than 1N. This means NTP can accelerate spacecraft at a much higher rate, with long-distance NEP missions taking months or years longer.

The unique capabilities of nuclear fission reactors towards powering crewed habitats and novel propulsion systems have led the US government to recently place emphasis on maturing these technologies.

Current U.S. Government Programs for Maturing Nuclear in Space

U.S. Civilian Government Programs

Surface Power

One of the key programs that NASA has prioritized for enabling an extended crewed presence on the Moon or Mars is the Fission Surface Power project. The goal of the program is to develop a system that can generate 40kWe of electric capacity, can be delivered by a large lunar lander, and can be transported on a lunar rover.

In June 2022, a joint effort between NASA and the Department of Energy announced that it will award 12-month Phase 1 contracts to three companies to develop preliminary designs for these reactors.

• Lockheed Martin of Bethesda, Maryland – The company will partner with BWXT and Creare.

• Westinghouse of Cranberry Township, Pennsylvania – The company will partner with Aerojet Rocketdyne.

• IX of Houston, Texas, a joint venture of Intuitive Machines and X-Energy – The company will partner with Maxar and Boeing.

Propulsion

NASA and the Department of Energy's near-term focus on nuclear propulsion primarily concentrates on nuclear thermal propulsion. In July 2021, that partnership announced a 12-month contract, similar to that for the fission surface power program, that will be awarded to the following companies to produce a conceptual design reactor that could support future missions

• BWX Technologies, Inc. of Lynchburg, Virginia – The company will partner with Lockheed Martin.

• General Atomics Electromagnetic Systems of San Diego – The company will partner with X-energy LLC and Aerojet Rocketdyne.

• Ultra Safe Nuclear Technologies of Seattle – The company will partner with Ultra Safe Nuclear Corporation, Blue Origin, General Electric Hitachi Nuclear Energy, General Electric Research, Framatome, and Materion.

Military Government Programs

In addition to NASA and the U.S. civilian government's expressed interest in nuclear application to space, the U.S. Department of Defense has its own focus in this arena as well.

Defense Innovation Unit Programs

Based in Mountain View, California the Defense Innovation Unit (DIU) is a US Department of Defense organization founded to help the U.S. military make faster use of emerging commercial technologies. In May of 2022, the DIU announced awards of two Prototype Other Transaction contracts for space nuclear projects, with the purpose of accelerating ground and flight testing for nuclear-powered prototypes.

One of these was awarded to Ultra Safe Nuclear, for a demonstration of a nuclear radioisotope batter called EmberCore, which Ultra Safe is developing for both propulsion and space power applications. The EmberCore radioisotope product aims to offer 10x higher power levels compared to previous plutonium radioisotopes systems. The stated ambition is to offer "more than 1 million kilowatt hours (kWh) of energy in just a few kilograms of fuel."

The second contract from DIU was awarded to Avalanche Energy to develop a small fusion reactor, approximately the size of a lunch pail. This device, which Avalanche calls Orbitron, uses electrostatic fields to contain high-temperature, high-energy ions in overlapping orbits, offering opportunities for collisions and fusion events within the small fusion reactor. Orbitron is intended for use either as a propulsion system, presumably as an electric propulsion system by emitting the high-energy ions, or as a power supply for spacecraft electrical systems.

DIU has stated that these two awards are specifically for supporting highly maneuverable, smaller spacecraft. This is in contrast to NASA systems which are intended for larger spacecraft or powering crewed habitats on the Moon or Mars. When the awards were made, DIU's Program Manager for the Nuclear Advanced Propulsion and Power (NAPP) program emphasized the bridge between the government and commercial capabilities when it comes to nuclear, stating "Nuclear tech has traditionally been government-developed and operated, but we have discovered a thriving ecosystem of commercial companies, including start-ups, innovating in space nuclear."

DARPA DRACO

The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military.

In April 2021, DARPA had announced three contracts awarded as part of its Demonstration Rocket for Agile Cislunar Operations (DRACO) program. The goal of the program is to ultimately demonstrate a nuclear thermal propulsion system on orbit.

There are two tracks to the DARPA DRACO program. Track A involves development of the nuclear fission reactor while Track B focuses more on the conceptual design of the spacecraft system and mission objectives. The 2021 contract awarded Phase 1 funding for both tracks.

For Phase 1 of Track A, General Atomics was awarded $22M to develop a study of the nuclear reactor design. For Phase 1 of Track B, Lockheed Martin and Blue Origin received $2.9M and $2.5M to each develop their own plan for a spacecraft design to leverage the nuclear reactor and implement an NTP system. DARPA plans to select only one spacecraft design to carry into later phases of the program.

Conclusion

After decades of limited interest and modest investment in applying nuclear reactor systems to space, there is a sudden growing momentum. While it is easy to maintain skepticism regarding near-term timelines for projects hoping to place nuclear material in space, the level of funding behind these civilian and military government programs provides reason to be optimistic. Given that existing propulsion and power systems offer little realistic path for a sustained economy beyond the moon, there seems to be an inevitability around the need for nuclear in space. We'll see how soon that future becomes reality.