SOLAR power is used extensively by satellites and space probes. But there are places where the sun’s rays do not penetrate or are merely a distant twinkle. For those a different power source is required. One of the favourites used in space missions is a radioisotope thermoelectric generator (RTG).

RTGs were developed by America in the 1950s and work by converting heat produced by the decay of a radioactive material into electricity directly. This is not the same as nuclear fission, a more complex process used in power plants to split radioactive material and release a much larger amount of energy. The former Soviet Union also used RTGs to run hundreds of lighthouses and navigation beacons in remote areas. Many of these have since been abandoned and some have been dismantled by scrap-metal dealers. While the isotopes used are not much use in bombs, they can still make people ill, even when partially depleted.

America’s RTGs use plutonium-238 (238Pu). The American plant that produced it closed in 1988 and the isotope was then imported from Russia. That stopped in 2009, leaving NASA with 35kg in stock, although only about 17kg of that is estimated to be still suitable for RTGs. After years of hand-wringing about being cut off from space without the material to make an RTG, a deal was reached in 2013 for NASA to pay the Department of Energy to resume production.

Work has begun refurbishing some elderly production kit. At the current rate of progress about 1.1kg of 238Pu will be made annually by 2021. That may not seem very much considering that Cassini, a Saturn probe launched in 1997, carried 33kg of 238Pu. But NASA has learned to be more parsimonious with the stuff. Despite being as big as a large car, the Curiosity rover which landed on Mars in 2012 manages with just 4.8kg. When it is not driving around, excess energy from its RTG is stored in batteries.

America has used RTGs in 27 space missions since 1961. Despite continuing improvements in collecting solar energy, NASA says it still needs RTGs—and not just to reach destinations beyond Saturn. The space agency’s planetary-science division has a list of places where solar power cannot be relied upon, including the dark side of Mercury, craters on the Moon and the poles of Mars, which are partly obscured from the sun.

NASA has been working on a system called the Advanced Stirling Radioisotope Generator, which offers four times the efficiency of a current RTG. This uses a process known as the Stirling cycle, which uses heat to drive a piston that in turn moves a magnet back and forth through a coil of wire to generate electricity. The project was suspended in 2013 because of budget cuts, but NASA officials are determined to find the money to complete testing of the generator and, if that proves successful, adding it to their power sources.