NASA And ESA Experimenting With Americium-241 For What Purpose?

Americium-241 (Am-241) could become NASA and ESA’s nuclear fuel of choice for Deep Space missions in the future. Am-241 is produced from Plutonium-241 when it decays. It can be harvested from nuclear waste. Today, its primary use is in smoke detectors.

Am-241 is a silver-white metal (see picture above) and in small quantities poses no radiation threat. So, how can this nuclear waste product be NASA’s fuel choice of the future? Work is being done for ESA at the UK’s University of Leicester, and the NASA Glenn Research Center, where testing Am-241 using Stirling generators is converting heat into electricity that in turn is being used to heat gases that can create propulsion. The advantages of Stirling generators are that they have few movable parts and are sealed containers in which an engine with pistons produces oscillations under pressure without a crankshaft. Stirling generators are low-maintenance and durable, making them highly reliable for long-duration missions.

The significant advantage of using Am-241 as a fuel is that it provides a half-life of 432 years compared to Plutonium-238 (Pu-238) at 88 years. Additionally, Am-241, as a byproduct of nuclear waste from plutonium-fuelled reactors, is relatively cheap and easy to source. A final advantage is Am-241 produces less radiation than Pu-238, meaning it is less damaging to onboard electronics on long-duration space flights and less of a threat to humans and other onboard life.

The disadvantage of Am-241, versus Pu-238, is that it needs two to four times the mass to produce the same amount of power. Currently, NASA uses Pu-238 for missions to the outer planets. The furthest current solar technology can be used to power spacecraft is Jupiter. Pu-238 is also being used to generate power and heat on Mars for the Perseverance and Curiosity rovers.

NASA and ESA, however, are accelerating plans to use Am-241 in future missions. An Am-241 powered prototype is already being tested by both space agencies. If all goes well, the plan for both agencies is to use Am-241 to replace Pu-238 for robotic spacecraft and surface missions to the Moon and Mars. It will become the fuel of choice to power and heat infrastructure for future human missions to both.

Will Am-241 take us to Alpha and Proxima Centauri, our nearest stellar neighbours?

No.

Alpha and Proxima Centauri are a triple star system. Alpha Centauri A (the one that most resembles our Sun) and B (a smaller orange star) are a binary system, while Proxima Centauri (a red dwarf) orbits the two from a considerable distance. This star system is 4.25 to 4.4 light-years (40 trillion kilometres) from Earth.

Am-241’s half-life of 432 years would run out well before we would reach these destinations and wouldn’t provide the horsepower needed to achieve even 1% of the speed of light.

None of the radioisotope-powered technologies in use today can achieve the propulsion speeds needed, and with them, a voyage to our nearest stellar neighbours would take several thousand years.

To make interstellar travel possible, we need to achieve speeds between 10 and 50% of the speed of light. Today, this type of propulsion is the province of science fiction.

What are likely future candidates to get us even closer?

Nuclear Fission Propulsion – NASA has experimented with fission rockets going back to Project Rover in 1955, the NERVA project in 1961, and Project Orion, with work stopping on the latter after the signing of the Partial Nuclear Test Ban Treaty in 1963.

NERVA work ended in 1973, largely a victim of federal budget cuts because of spending on the Vietnam War.

Today, Project Orion, a nuclear pulse technology that uses repeated nuclear explosions to drive a massive rocket, is garnering renewed interest.

Another concept, a nuclear thermal rocket, would use uranium or plutonium salts dissolved in water for fuel and propellant. At best, these propulsion systems could get us to between 7 and 10% of the speed of light.

Fusion-Based Propulsion– a controlled fusion drive rocket could reach speeds of 10 to 15% of the speed of light. Theoretically, a multi-stage fusion-powered rocket could achieve those speed parameters.

Laser Array Power – would involve building large ground-based laser arrays that would push ultra-light spacecraft to speeds close to 20% of the speed of light. This is the proposed technology for Breakthrough Starshot.

Laser-Pushed Sails – would also use lasers to push sail-equipped spacecraft to achieve speeds of nearly 20% of the speed of light. Currently, a number of spacecraft have been built to test the practicality of using sail technology in space powered by the solar wind.

Antimatter Propulsion – matter-antimatter annihilation could provide thrust to achieve 50% of the speed of light. This is technology that has yet to be invented. See Warp Drives below for another take on using fuel made of matter and antimatter.

Warp Drives – Star Trek used this type of technology to achieve faster-than-light travel. The idea involves manipulating space-time.  A physicist, Miguel Alcubierre, in 1994, proposed a propulsion system that would contract space in front of a spacecraft and expand space behind it, creating a warp bubble and achieving faster-than-light speeds without conflicting with Einstein’s speed-of-light upper limit. On Star Trek, the fuel for the warp drive is deuterium and its antimatter opposite, antideuterium, regulated by dilithium crystals (a fictional element). It is hard to imagine engineering a warp drive, knowing our current understanding of physics.