New Delhi is experimenting with radioisotopes to charge its robotic missions to the Moon, Mars and beyond

In the Hollywood sci-fi movie ‘The Martian’, astronaut Mark Watney, played by Matt Damon, is presumed dead and finds ways to stay alive on the red planet, mainly thanks to a big box of Plutonium known as a Radioisotope Thermoelectric Generator (RTG).

In the film, Watney uses it to travel in his rover to the ‘Pathfinder’, a robotic spacecraft which launched decades ago, to use its antenna to communicate with his NASA colleagues and tell them he's still alive. Additionally, the astronaut dips this box into a container of water to thaw it.

In real life, the RTG generates electricity from the heat of a decaying radioactive substance, in this case, Plutonium-238. This unique material emits steady heat due to its natural radioactive decay. Its continuous radiation of heat, often lasting decades, made it the material of choice for producing electrical power onboard several deep-space missions of the erstwhile USSR and the US.

For short-duration voyages, Soviet missions used other isotopes, such as a Polonium-210 heat source in the Lunokhod lunar rovers, two of which landed on the Moon, in 1970 and 1973.

NASA has employed Plutonium-238 to produce electricity for a wide variety of spacecraft and hardware, from the science experiments deployed on the Moon by the Apollo astronauts to durable robotic explorers, such as the Curiosity Mars rover and the Voyager 1 and 2 spacecraft, which are now at the edge of the solar system.

The ISRO first used nuclear fuel to keep the instruments and sensors going amid frigid conditions during a successful lunar mission, Chandrayaan-3. A pellet or two of Plutonium-238 inside a scaled-down version of the RTG known as the radioisotope heating unit (RHU) made its way to space aboard the rocket. It was provided by India’s atomic energy experts.

RHUs are similar to RTGs but smaller. They weigh 40 grams and provide about one watt of heat each. Their ability to do so is derived from the decay of a few grams of Plutonium-238. However, other radioactive isotopes could be used by today's space explorers. RHUs and RTGs feature rugged, heat-resistant casings to safely contain the radioisotope in case of a launch or re-entry vehicle failure.

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