60 Years Of Nuclear In Space Is Just The Beginning

This summer NASA marked the 60th anniversary of a nuclear-powered spaceflight. The Transit IV-A was an experimental navigational satellite with a radioisotope-powered generator, launched by Johns Hopkins University Applied Physics Laboratory from Cape Canaveral on 29 June 1961.

Transit IV-A's SNAP-3B radioisotope generator produced 2.7 W of electrical power, which is enough to power an LED lightbulb. Nevertheless, the satellite broke mission-duration records, having travelled over 25,000 times around the Earth by the time it became the oldest operating US satellite in May 1964. The satellite confirmed that the Earth's equator is elliptical.

Since that time, NASA has flown more than 25 missions carrying a nuclear-powered system.

Radioisotope power systems use thermocouples to convert heat from the decay of plutonium-238 into electricity and are one of only two practical ways to provide long-term electrical power in space. Solar panels are another option, but solar power becomes less efficient as spacecraft travel farther from the Sun – beyond Mars, they are worthless. Chemical sources don’t work for very long as their energy density is too low and their weight is prohibitive on long missions.

Radioisotope power systems are reliable and efficient, according to June Zakrajsek, manager for NASA's Radioisotope Power Systems Program office at the Glenn Research Center.

"They operate continuously over long-duration space missions regardless of sunlight, temperature, charged particle radiation, or surface conditions like thick clouds or dust. They’ve allowed us to explore from the Sun to Pluto and beyond," she said.

The plutonium-238 fuel used in NASA's radioactive power systems is provided through a partnership with the US Department of Energy (DOE). The isotope is made by irradiating neptunium-237, and is currently produced by Oak Ridge National Laboratory in partnership with Idaho and Los Alamos National Laboratories.

Radioactive power systems powered the Lunar Surface Experiment Packages of each Apollo landing site, as well as the Pioneer, Viking, Voyager, Galileo, Ulysses, Cassini and New Horizons space missions.

New Horizon’s spacecraft flew by the most distant object ever observed up close - Ultima Thule, far beyond Pluto, in the region called the Kuiper Belt, outside the Solar System proper. It will continue on into the Oort Cloud, the outermost region of the Solar System that remains from the original nebula from which the Sun and planets formed.

Then it will exit our Solar System completely.

The spacecraft could not have done so without energy supplied by the nucleus.

So far, NASA has done well with small nuclear systems that power our unmanned spacecraft to distant planets. But for human flights to Mars and beyond, or for setting up colonies on the Moon or Mars, larger systems are needed, even actual nuclear reactors, as concluded by a National Academies report.

And NASA is moving in that direction at a good pace. The Energy Department and NASA have awarded three teams one-year, $5 million contracts to carry out nuclear thermal space propulsion concept studies. Even the Department of Defense is looking into nuclear thermal propulsion rocket engines for sending maneuverable satellites further outside Earth's orbit.

NASA believes that a nuclear reactor that would be used for space travel could be ready in a few years from the time it is approved, according to Patrick McClure, head of the Kilopower project, which is an Energy Department and NASA collaboration. The reactor has successfully completed ground tests and is set for flight tests.

We are even making radiation protection suits for individual astronauts to use on long missions, a radiation-shielding vest called StemRad.

So as we gear up for more government and private commercial space exploration and development, we will continue to explore radiological and nuclear power to fuel our spacecraft and colonies, and perhaps take to places beyond our own Solar System.