Nuclear ion propulsion uses an energy source – in this case a nuclea rreactor – to ionize a noble gas. The charge differential between the positive ions created and a negative electrode grid causes the ions to accelerate toward the electrode. Once there, the positive ions pick up electrons. The neutral atoms continue out the booster nozzle, producing the thrust that propels the vessel toward its destination.
Nuclear ion propulsion has been deployed by both NASA and Roscosmos since the late 1950s. Many satellites use small units for orbital maneuvering, and to de-orbit at the end of their useful life. A proposed NASA lunar orbital space station called Lunar Gateway (unfunded) utilizes a 50 kW solar ion propulsion system called Advanced Electric Propulsion System (AEPS) to provide orbital maneuvering thrust. AEPS has been designed and tested and is awaiting deployment.
On the nuclear side, NASA has developed relatively lightweight Kilopower nuclear reactors. These units weigh in at a sprightly 1500 kg and can generate up to 10 kW of electric power. November 2017 through March 2018 a Kilopower reactor wedded to a Stirling electrical generator successfully generated 5.5 kW of fission power. NASA acknowledges these Kilopower reactors could one day power nuclear ion propulsion.
Nuclear ion propulsion generates just a fraction of the thrust of the chemical boosters used in today’s boosters. So why use it? Two reasons. First, its way more efficient. A trip to Mars would require something like 100,000 kg of Argon verses nearly 3.5 million kg of conventional chemical propellant. Second, it’s estimated that a nuclear ion propulsion unit could operate continuously for up to five years.
That longevity is important. Getting to Mars using a chemical booster entails a brief minutes-long burst of enormous thrust to achieve the speed necessary to coast there. Its like a bullet being shot from a rifle. Quick acceleration, then momentum takes you to the target. Nuclear ion propulsion on the other hand, uses continuous gentle acceleration to leave Earth (or trans-lunar) orbit and reach Mars.
I worked out the math on the back of a napkin. (OK, it was a piece of scratch paper.) A 100 kW reactor (or 10 Kilopower reactors) using 10,000 kg of Argon propellant stock on board a 15,000 kg vessel (comparable to the SpaceX Starship) would take nine months to reach Mars. With a little optimization and weight trimming - like using induced hibernation to cut down on food consumption and life support requirements and pre-positioning a base and supplies ahead of the mission – a six-month transit is entirely feasible.
Will this technology be used to propel humanity to Mars within the next fifteen years? It’s a definite maybe. My money is on private enterprise, or possibly a public-private venture. SpaceX is developing Starship and is under contract with NASA for several lunar missions. SpaceX’s Starlink satellite constellation utilizes ion thrusters to maneuver and de-orbit. NASA has developed the nuclear reactor(s) and the AEPS ion propulsion unit. The next step will combine them for an upcoming mission either to the moon, or to a near-Earth asteroid to prove the technology.
So, will the first mission to Mars in 2035 use nuclear ion propulsion ala the DeepStar booster? If you’re risk-averse, you may want to hold your money. But by 2040? It’s a sure bet.
Want a deeper dive? There are a number of excellent Wikipedia articles that cover the material I discussed here. In the order I discussed:
Kilopower
https://en.wikipedia.org/wiki/KilopowerIon Thruster
https://en.wikipedia.org/wiki/Ion_thrusterAEPS
https://en.wikipedia.org/wiki/Advanced_Electric_Propulsion_SystemSpaceX Starship
https://en.wikipedia.org/wiki/SpaceX_Starship
