Do satellites use nuclear power?

The question of what powers a satellite, especially one venturing far from the Sun, often brings up visions of vast, gleaming solar panels. While photovoltaic cells dominate the low-Earth orbit and near-Earth environments, the reality for many long-duration and deep-space missions is quite different: some do indeed rely on nuclear power. ^[3][5] The choice between solar arrays and nuclear sources is not arbitrary; it hinges entirely on the mission's destination, required power output, and duration. ^[2][3]

# Typical Power Needs

For the majority of satellites operating in well-lit orbits, such as those providing global positioning or common communication services, solar power remains the default. ^[3] These systems are relatively straightforward, inexpensive, and proven reliable near Earth, where the intensity of sunlight is strong and predictable. ^[2][7] Even large solar arrays can efficiently generate the necessary kilowatts of electricity. ^[1]

The complication arises when a spacecraft travels away from the Sun. The power density of sunlight drops off rapidly with distance—a factor known as the inverse-square law. ^[3][8] By the time a probe reaches Jupiter, solar intensity is only about 4% of what it is at Earth. ^[8] To compensate, solar panels would need to become enormous, adding prohibitive mass and drag to the launch vehicle. For missions to the outer planets or beyond, a more consistent, dense energy source is essential. ^[1][3]

This necessity forces mission planners to consider the trade-off between the high initial complexity and regulatory burden of launching radioactive material versus the physical impossibility of operating a solar-powered craft at vast distances. For an Earth-orbiting communications relay, the added risk profile of launching nuclear components vastly outweighs the minimal gain, but for a probe destined for the Kuiper Belt, nuclear power shifts from an option to a fundamental requirement. ^[7]

# Nuclear Technologies

When engineers turn to nuclear energy for space applications, they are generally referring to one of two primary mechanisms, both exploiting the natural decay of radioactive isotopes to generate usable energy. ^[1]

# Isotope Generators

The most common form of nuclear power currently operating in space is the Radioisotope Thermoelectric Generator (RTG). ^[1][6] RTGs function by harnessing the heat produced as a radioactive material, typically plutonium-238 ($\text{Pu}^{238}$), naturally decays. ^[1][6] This heat is then converted directly into electrical energy using thermocouples—a process that relies on the temperature difference between the hot core and the cold exterior of the spacecraft. ^[1] RTGs provide continuous power, unaffected by shadows, planetary occultation, or distance from the Sun, making them ideal for decades-long missions like Voyager 1 and 2 or Cassini. ^[1][6][8]

A related, but distinct, device is the Radioisotope Heater Unit (RHU). ^[1] These units are much simpler, containing only a small amount of radioactive material used solely to generate a steady, low level of heat to prevent sensitive electronics from freezing in the harsh cold of space, rather than generating electricity. ^[1]

# Fission Power

While RTGs rely on decay heat, a far greater power output can be achieved through nuclear fission, the process used in terrestrial power plants. ^[1][4] These Space Nuclear Reactors are designed to generate much higher levels of electrical power, often in the range of multiple kilowatts to tens of kilowatts. ^[4][5] This level of output is necessary not just for advanced scientific instruments but also for high-speed electric propulsion systems or supporting permanent infrastructure, such as habitats on the Moon or Mars. ^[4][5][9] Although RTGs have a long flight heritage, fission reactor technology, while having been tested in earlier decades, is currently the focus of modern development for high-power applications. ^[6][5]

The power density of fission is significantly greater than that of RTGs. If one were to compare the mass required: an RTG might produce a few hundred watts of power, whereas a fission system could aim for several kilowatts using a relatively compact core. ^[4]

What was the first satellite to use nuclear power?

# Deep Space Missions

The history of space nuclear power is deeply intertwined with American exploration of the outer solar system. ^[6] The technology dates back to the 1960s, with early nuclear reactor experiments like the $\text{SNAP}$ series. ^[6] However, it is the RTG that has become the workhorse for deep-space exploration. ^[1]

Missions sent to the farthest reaches—the outer planets, their moons, and interstellar space—have depended on these generators. ^[1][8] For instance, the Pioneer, Viking, Galileo, and New Horizons missions all carried RTGs. ^[1][6] This reliance is non-negotiable because the time scales involved are vast. Missions planned to take decades to reach their targets cannot afford mission failure due to a gradual degradation of power capacity or an inability to charge batteries during eclipses. ^[8] The consistent output of a radioisotope system ensures that command signals can be received and critical systems remain operational throughout the extended mission lifetime. ^[3]

# Future Systems

Current research programs clearly indicate that the next generation of space exploration will continue to integrate nuclear technology, moving beyond basic electricity generation toward advanced propulsion and permanent off-world bases. ^[5][9]

NASA, for instance, is actively advancing the technology for Space Nuclear Propulsion. ^[5] Such a system would use a nuclear reactor to superheat propellant, creating exhaust with much higher efficiency (specific impulse) than traditional chemical rockets. This capability would dramatically cut travel times to Mars and beyond, addressing one of the biggest hurdles in human deep-space travel. ^[5]

On planetary and lunar surfaces, continuous power is crucial for operating bases through long periods of darkness, such as the two-week lunar night. ^[4] Solar power is inadequate for these scenarios unless paired with massive battery banks or complex energy storage systems. ^[4] Developing small, modular fission reactors, like those being explored by entities such as X-energy, aims to provide reliable, multi-kilowatt power directly on the lunar surface, enabling sustained human presence. ^[4][9]

The challenges associated with deploying these more complex fission systems—especially concerning launch safety and regulatory oversight—mean they are being developed primarily for destinations well away from Earth's immediate vicinity. ^[7] Regulations concerning the launch of fissile material near Earth are stringent, prioritizing public safety above mission expediency, which further entrenches solar power for near-Earth satellites where simpler, non-nuclear alternatives exist. ^[7] If a future satellite needs a city block's worth of power on Mars, however, the engineering calculus decisively shifts back in favor of onboard nuclear fission. ^[4]