Nasa is developing ways to use nuclear power to send spacecraft to their destinations. Nuclear propulsion could greatly reduce the journey time to Mars, perhaps cutting a voyage of more than six months to three or four months.

The idea of nuclear propulsion in space goes back to the cold war. But Nasa has been pursuing it more aggressively since Jared Isaacman took over as the agency’s chief in December 2025. Isaacman is a well-known advocate of the technology and says it can “truly unlock humankind’s ability to explore among the stars”.

In March 2026, the agency even announced an uncrewed, nuclear-powered mission to the red planet, targeted for late 2028.

Every spacecraft begins its journey fighting Earth’s gravity by burning chemical fuel. Rockets mix fuel with an oxidiser, ignite them, and force the expanding gas through a nozzle. According to Isaac Newton’s third law, when gas pushes downward, the rocket gets an equal push upward.

Chemical propulsion is powerful, reliable, and quite simply the only practical way to leave Earth’s gravity. But it comes with a severe limitation. Rockets must carry both their fuel and, in most cases, the oxidiser needed to burn it.

That means much of a rocket’s mass at launch is propellant, not payload. The longer and more ambitious the journey, the more propellant is needed, and the heavier the rocket becomes.

Mars is far enough away that a long journey time, the threat to astronauts from cosmic radiation, the mass required to carry life-support systems and constraints on the return journey all pose serious problems for planning a mission.

This is why engineers keep looking for more sustainable alternatives to chemical rockets.

Two technologies

Nasa’s space nuclear propulsion programme distinguishes between two main approaches: thermal propulsion and electric propulsion.

Nuclear thermal propulsion follows a three-step process. First, the nuclear reactor inside the engine splits uranium atoms to generate massive amounts of heat. Second, liquid hydrogen is pumped through the reactor core, where it flash boils and expands into a high-pressure gas. Third, this super-heated gas is blasted out of a nozzle at high velocities to push the spacecraft forward.

According to the US Department of Energy, nuclear thermal propulsion can reduce travel times to Mars by up to 25% and, more importantly, limit a crew’s exposure to cosmic radiation. It would also widen the launch windows in which spacecraft can feasibly fly to Mars.

These depend on alignments of Earth and Mars that come along every couple of years. Greater flexibility with launch windows would allow astronauts to abort missions and return to Earth if necessary.

Nuclear electric propulsion, on the other hand, uses a nuclear reactor to generate electricity. This powers a type of engine called an ion thruster that accelerates charged atoms (like xenon) out of a nozzle. If nuclear thermal propulsion is the sprint approach, nuclear electric propulsion is the marathon option. Nuclear electric propulsion produces very low thrust, but it can run continuously for years.

This fuel efficient technology is perfect for sending robot explorers or heavy cargo (like habitats and food supplies) to Mars months before the humans arrive. In deep space, a small thrust applied for a long time can matter enormously.

A chemical rocket is like a powerful kick. Nuclear electric propulsion is more like a persistent hand on the shoulder.

It could make it easier to move heavy cargo through deep space, provide abundant onboard power, and remain effective far from the Sun, where the energy available to solar arrays is weaker.

This is the main idea behind Nasa’s Space Reactor-1 Freedom mission. SR-1 Freedom is a nuclear electric propulsion mission, which Nasa is currently targeting for launch in December 2028.

It would be the first nuclear-powered interplanetary spacecraft. It will journey to Mars to prove that nuclear energy can provide the sustained, high-efficiency power needed for deep space travel.

On arrival at Mars, roughly one year after its launch, SR-1 Freedom is expected to deploy the Skyfall payload. This is a set of small helicopter drones that will scout the Martian surface.

Nasa says the mission will establish nuclear hardware that can be used on other flights. It could also create a regulatory precedent and activate an industrial base for future systems based on nuclear fission.

For human exploration, the combination of both nuclear electric propulsion and nuclear thermal propulsion is very attractive. Because nuclear electric propulsion is incredibly fuel-efficient, it can move massive amounts of weight (habitats, years of food, rovers, and life-support machinery) using very little propellant.

It might not matter so much if cargo takes more than nine months to arrive on Mars. But our fragile human bodies mean that longer stays in space increase the risk of cancer from cosmic radiation and cause bone and muscle loss.

The second of these issues is because bones and muscles are not being exercised in microgravity. Nuclear thermal propulsion provides the high thrust needed to reach Mars in three to four months, drastically reducing these health risks.

Steep path

Despite the clear benefits, the path to the launch pad is steep, and the 2028 launch of SR-1 Freedom appears incredibly ambitious. A nuclear electric spacecraft needs a reactor, shielding, heat management, power conversion, radiators, electric thrusters, control systems and fault tolerance. Each of these components of the mission requires testing and careful integration for them to work together.

Reactor heat must be controlled without damaging other components. Thrusters must operate reliably for months. Other factors can interact in ways that only emerge when spacecraft subsystems are put together. If SR-1 Freedom is to make its December 2028 window, Nasa has very little time to assemble a mission that would normally require years of design, integration and review.

Nuclear propulsion has spent more than 60 years somewhere between engineering reality and technological myth – even though the physics has always been sound.

What has proved harder is making the technology safe, affordable, licensable (able to meet regulatory safety standards) and ready to fly on a real mission schedule. So far, the US has launched only one fission reactor into orbit, SNAP-10A, in 1965.

SR-1 Freedom could create the pathway for more capable systems to follow. Nuclear electric propulsion will not make Mars easy. But it might start to break down barriers to travelling to Mars, and that is a prospect we should be excited about.

Domenico Vicinanza does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.