SR-1 Freedom Mission Could Fulfill NASA’s 70-Year Dream of Putting Nuclear Reactors in Space
It looks like something out of science fiction, but its basic structure is probably older than you.
Credit: NASA
A few days ago, NASA announced a solid plan to send a brand new vehicle to Mars in 2028. It will carry the planned Skyfall payload, full of Ingenuity-class spacecraft, which should greatly increase scientists’ understanding of the Red Planet’s composition—but the ship’s power and propulsion system are truly exciting.
Named SR-1 Freedom, this latest spacecraft will use an electric system intended to be used to reposition asteroids, then relaunch the canceled space station called Gateway, and now it has been relaunched for Mars.
In terms of propulsion, it’s basically just a very basic version of an ion thruster. In classical ion thrusters, the type found in many current satellites, solar panels that generate small amounts of electricity, are used to create a weak electromagnetic field inside the open reaction chamber. A solid puck of fuel slowly diffuses ions into the chamber, which are then shot into the magnetic field, creating an equal thrust in the opposite direction.
This official NASA rendering shows a lunar base (stage 3 no less) powered by nuclear reactors on the lunar surface.
Credit: NASA
The SR-1 propulsion system, called the Power and Propulsion Element (PPE), is a collection of several large thrusters of this type. But there are two important ways in which this idea is not easily achieved. One is that a solid puck of fuel cannot supply ions fast enough for meaningful implosion, so it has been replaced with tanks of compressed Xenon gas, which can fill the reaction chamber very quickly.
Another problem was difficult to solve: too much power source. Not only does creating thousands of times more power require thousands of times more surface area for the generating panels, but these panels can deliver less power than they would from the Sun. For long-distance Mars missions, fully electric propulsion will require a new system that delivers more power wherever it’s needed.
Enter, nuclear power. The use of nuclear power in space dates back 70 years or more to NASA alone; it was originally incorporated into the Systems for Nuclear Auxiliary Power (SNAP) program, dating back to the 1950s. The basic finding was that space nuclear didn’t make sense for missions like the ones they were planning at the time, like the Apollo missions to the Moon.
The SNAP 10-A experiment, originally intended to produce 500 or more Watts of electricity for a year or more.
Credit: NASA
Given the launch weights and difficulties involved, this idea made perfect sense for advancing the core of a mission to another planet—like, for example, Mars.
This is different from existing nuclear technologies such as Radioisotope Thermoelectric Generators (RTGs) for internal rovers such as Curiosity and Perseverance; these do not actually initiate a fission reaction but instead take advantage of the natural decay of plutonium to produce very small amounts of electricity.
This has the effect of being small, light, and very durable, but it generates 110 watts of current. That compares to the 20,000 watts (20kW) the proposed generator in Freedom would produce.
This reaction center will be held securely in place in the spacecraft’s core by a long rod, which helps prevent radiation leaks from interfering with the computers inside the capsule. Of course, this will also help prevent radiation leaks in the future thinking space of the workers.
The reactor is also designed to reach large sizes to power large ship engines and even permanent installations, such as the proposed Lunar Reactor 1. This will be launched in 2030, and produce up to megawatts of power for a permanent Moon base.
It looks like an ambitious timeline for both the spacecraft and the installation, but such progress will be necessary if NASA’s equally ambitious goals for the Moon and Mars are to be realized.
