Most spacecraft power themselves by absorbing the sun's energy with solar panels. Beyond Earth’s comfortable, brightly lit cradle, however, outer space assaults probes with deep cold, deeper darkness and intense bursts of radiation.
To keep a spacecraft safely aloft in the void seems to require magic. Thanks to the Cold War-era space race, U.S. engineers found some: Plutonium-238, a byproduct created during the production of weapons-grade plutonium-239.
Unsuitable for use in weapons but perfect in a spacecraft, plutonium-238′s gradual decay gives off warmth that safeguards fragile electronics. More importantly, wrapping plutonium with heat-to-electricity converting materials, called thermoelectrics, turns the radioactive metal into a nuclear battery.
“Plutonium-238 is perhaps the main reason why other nations haven’t gone as deep into space as we have,” said Jim Adams, NASA’s deputy chief technologist and former deputy director of the space agency’s planetary science division. “They don’t have access to material to make reliable, lightweight [nuclear batteries].”
Powering Cassini around Saturn, for example, would require solar panels the size of two tennis courts and weigh thousands of pounds. Barring a radical and unforeseen breakthrough in power systems, or heavy multiplying of NASA’s budget (launch costs range between $5,000 and $10,000 per pound), no other energy source is as practical as a nuclear battery.
As the world’s supply of plutonium-238 comes disastrously close to running out, WIRED reviews the greatest space missions of all time — some of which still phone home decades after launch and billions of miles away from Earth.
Image: A pellet of plutonium-238 glows under its own self-generated heat. (Department of Energy/Wikimedia Commons)