Nuclear technology has been used in space since the beginning of the space program. The first unmanned space vehicle was Sputnik, launched by the Soviet Union in 1957. The United Statesfollowed three months later with its first unmanned flight, Explorer 1. The number of US and Soviet space missions increased dramatically in the next few years. By the end of 1969, more than 1,000 spacecraft were orbiting Earth. Five years later the number of these satellites had climbed to nearly 1,700.
In 1961, the first radioisotope thermo-electric generator (RTG) used in a space mission was launched aboard a US Navy transit navigation satellite. The electrical power output of this RTG, which was called Space Nuclear Auxiliary Power (SNAP-3), was a mere 2.7 watts. But the important story was that it continued to perform for 15 years after launch.
RTGs are electricity generators which convert the heat generated by the decay of plutonium-238, a radioactive isotope, into electricity through the use of thermocouples. Thermocouples convert heat energy directly into electricity which can be used to power electrical systems. To date, NASA has launched over 25 missions equipped with RTGs into space and the former Soviet Union has launched over 40 satellites and probes using similar technologies.
RTGs are used in spacecraft for several reasons. First, they are rather simple to construct and very reliable, often providing electricity for years, and in some cases decades, after launch. Second, they are able to provide power for deep space missions to the outer planets and beyond the solar system where there is not enough sunlight to power solar panels. Third, they can provide much more electrical energy than is possible with solar panels, with the result that more equipment and experiments can be powered. RTGs also provide heat energy which helps keep spacecraft from freezing in the depths of space where temperatures can reach minus 200 degrees Celsius.
In 1997, NASA and ESA (European Space Agency) launched the Cassini-Huygens obiter/probe to investigate Saturn and its moons. The Cassini obiter is powered by three RTG units and took seven years to reach Saturn, where it still orbits today, radar mapping and photographing the Saurian system. On January 15, 2005, the Huygens probe successfully landed on Saturn’s largest moon, Titan, and sent back data on atmospheric composition and photographic images of the surface for several hours. Some of the discoveries of the mission include: Titan has hydrocarbon lakes, Saturn has hurricanes, and the rings of Saturn are much older than what was once thought. Without the use of RTGs, missions like Cassini-Huygens would have been impossible and much of what we now know about the outer solar system would still be a mystery.
The launch of the Cassini-Huygens probe did however, cause some controversy. Scientists and environmental groups raised concerns that if a launch failure of the rocket carrying the Cassini-Huygens probe were to take place, it might result in the release of radioactive plutonium-238 into the atmosphere. This possibility does exist but the chances are very remote. RTGs are designed to withstand re-entry and remain intact even after a high speed impact from space.
There was an RTG to be used to power a lunar experiment package inside the lunar module which was jettisoned after helping bring the astronauts home at the end of the aborted Apollo 13 moon mission in 1970. The lunar module re-entered Earth’s atmosphere and disintegrated. The RTG crashed and sank in the South Pacific Ocean, but remained intact and did not release any radiation. Because of its construction, it is expected to remain intact for over 800 years, long enough for the radioactivity to decay to a safe level.
In the future, RTGs will play an even larger role in space exploration. RTGs power the next generation of larger, more sophisticated Mars rover, the Mars Science Laboratory, scheduled for launch in 2011. Presently the New Horizons probe is on its way to give us our first close-up look at Pluto. Pluto is so far away, the New Horizons probe will not arrive there until July 2015. Such a mission is only possible with RTG technology.
If the planned manned mission to Mars takes place in the coming decades, the astronauts will be in space for 30 months and will need to be almost completely self sufficient. RTGs will be used to provide a reliable means of both heat and electricity needed for survival on the Mars mission.