Nuclear Power Behind Deep Space Missions and the Engines That Could Come Next

Nuclear sources provide heat and electricity independently of these conditions, giving mission planners a dependable option for destinations where solar power is limited or unavailable.

A spacecraft travelling far from the Sun needs an energy source it can depend on for years without maintenance or refuelling. On the Moon or Mars, keeping instruments warm through freezing darkness can be just as important as powering the equipment that collects scientific data.

Nuclear technologies have helped meet these needs for decades, supporting satellites, deep-space probes, planetary rovers, landers and missions involving astronauts. Their next applications could include electricity for lunar settlements and propulsion systems that change how spacecraft travel across the solar system.

Solar power supplies most spacecraft and satellites, but collecting enough sunlight becomes increasingly difficult as missions move towards the outer solar system. Larger solar panels require more surface area, creating practical problems for vehicles with strict limits on mass and size.

Sunlight can also disappear where missions need it most. Permanently shadowed lunar craters receive no direct sunlight, much of the Moon experiences nights lasting roughly two weeks, and dust storms on Mars can disrupt solar generation.

Nuclear sources provide heat and electricity independently of these conditions, giving mission planners a dependable option for destinations where solar power is limited or unavailable.

The energy density of nuclear material offers another advantage. Every additional kilogram makes launching a spacecraft more demanding, and chemical propulsion requires substantial quantities of propellant to extend a mission's reach. Nuclear propulsion could reduce the propellant needed for certain journeys, freeing capacity for scientific instruments, supplies and other equipment.

These benefits explain the interest in nuclear systems for both distant robotic missions and future human exploration, where a reliable energy supply would support longer stays and more demanding work.

Decades of Experience, Supported by Safety Frameworks

The first nuclear power sources used in space were developed in the early 1960s. The Voyager probes, launched in 1977, became especially striking examples of their endurance, with onboard nuclear batteries providing power across decades of exploration.

Several dozen low-power reactors were launched during the early decades of space exploration, generating a few kilowatts of electricity for missions lasting months or years and demonstrating that controlled nuclear fission could operate reliably in space.

Nuclear rocket research began even earlier, with engines developed and tested on the ground during the 1950s and 1960s. Those programmes provided experience that continues to inform interest in nuclear propulsion.

Using radioactive material in space also requires attention to safety. Concerns that became clear in the late 1970s helped drive the development of international guidance, including the 1992 Principles Relevant to the Use of Nuclear Power Sources in Outer Space.

A joint United Nations–IAEA safety framework adopted in 2009 added guidance for the responsible development and use of space nuclear power sources. These frameworks form an important part of the technology's history alongside its engineering achievements.

From Nuclear Batteries to Power Stations Beyond Earth

Some space nuclear systems use the natural decay of radioactive elements, which releases energy that can provide heat directly or be converted into electricity. Radioisotope heater units, known as RHUs, keep sensitive components from freezing in places such as the Martian surface and deep space. Radioisotope thermoelectric generators, or RTGs, convert decay heat into electricity and are commonly called nuclear batteries.

RTGs typically produce a few watts to several hundred watts, enough for suitable instruments and spacecraft systems. Their slowly decaying fuel allows them to supply steady power for decades, making them valuable for missions that cannot rely on sunlight.

These technologies have served deep-space probes, Mars landers and rovers, small satellites and lunar missions, including the Apollo programme. Different radioisotopes and newer designs are making smaller nuclear batteries possible, opening applications previously restricted by the size of the power source.

A future surface base would need considerably more electricity than a typical RTG supplies. Compact fission reactors, called microreactors, are being designed to generate anything from a few kilowatts to around 100 kilowatts for a decade or more. Early-stage concepts extend into several megawatts, although those remain under development.

Several microreactor concepts have undergone design and ground testing in recent years. Continuous electricity could allow astronauts to remain on the Moon or Mars for longer and give robotic outposts greater independence from sunlight and surface conditions.

How Nuclear Propulsion Could Change Space Travel

Conventional rocket engines burn fuel with an oxidizer to generate thrust. Nuclear thermal propulsion uses a fission reactor to heat propellant directly, potentially delivering more than twice the specific impulse of baseline chemical rockets. Specific impulse describes how efficiently an engine uses its propellant.

Nuclear electric propulsion takes a different approach: a nuclear source generates electricity for an ion or plasma thruster. It produces lower thrust over much longer periods, offering highly efficient propulsion for missions suited to gradual acceleration. Nuclear thermal propulsion provides stronger thrust for shorter engine burns.

The potential benefits depend on the propulsion system and mission design. Lower propellant requirements could leave more capacity for payloads, and some nuclear-powered mission designs could shorten travel times. Faster journeys would reduce astronauts' exposure to cosmic radiation during transit and leave more time for work at their destination.