A major shift in the use of RPSs came with NASA’s decision to pursue outer-planet exploration. This initiative was driven by the discovery of “grand tour” trajectories that could enable relatively short missions to the planets of the outer solar system by using multiple planetary gravity assists.3 This planetary configuration is rare, occurring only about every 176 years, but it was due to occur in the late 1970s and led to one of the most significant space exploration efforts undertaken by the United States (Dethloff and Schorn, 2003).

The nearly identical Pioneer 10 and 11 spacecraft were launched in 1972 and 1973, respectively, to make the first trips through the asteroid belt to Jupiter and beyond. Both relied on RPSs to provide power far from the Sun. Pioneer 10 flew past Jupiter in late 1973. It transmitted data about the planet and continued on its way out of the solar system. Pioneer 11 provided scientists with an even closer view of Jupiter, whose gravity was used to send Pioneer 11 to Saturn before it, too, departed the solar system. Pioneer 11 ended its mission in 1995, when the last transmission from the spacecraft was received. NASA continued to receive signals from Pioneer 10 until 2003, when the spacecraft was 7.6 billion miles from Earth. The success of the Pioneer missions would not have been possible without the four SNAP-19 RTGs that each spacecraft carried as their sole source of power. Each Pioneer spacecraft also had a dozen radioisotope heater units (RHUs), each generating 1 W of thermal energy, to heat selected components (Wolverton, 2004). A third spacecraft, the flight spare Pioneer H, is displayed in the National Air and Space Museum.

After the success of the Pioneer missions, two Voyager spacecraft were built to conduct intensive flyby studies of Jupiter and Saturn, in effect repeating on a more elaborate scale the flights of the two Pioneers. These spacecraft were scaled back versions of the proposed Grand-Tour spacecraft, which was rejected at the time for budgetary reasons. Voyager 1 and 2 were launched in 1977, each with three Multi-Hundred Watt (MHW) RTGs. With the successful flyby of Saturn’s moon Titan by Voyager 1 in November 1980, Voyager 2 was targeted for one of the grand-tour trajectories.4 Voyager 2 subsequently had close flybys of Saturn (August 1981), Uranus (January 1986), and Neptune (August 1989), providing the bulk of all human knowledge about the latter two “ice giant” planets (Dethloff and Schorn, 2003).

Voyager 1, which is traveling faster than Voyager 2, is now farther from Earth than any other human-made object. Now traveling out of the solar system, both Voyager 1 and Voyager 2 have passed the “termination shock” of the solar wind and continue to send back the first information ever received from the outer boundary of our solar neighborhood. The Voyagers are expected to return scientific data until the RPSs can no longer supply enough electrical energy to power critical systems. With the adoption of power sharing among the still-operating instruments, the final transmission is expected to occur in about 2020. Whether Voyager 1 will reach the heliopause, the “boundary” between the shocked solar wind and interstellar plasma, by then is unknown.

NASA has continued to use RPSs on missions to the outer planets and on selected long-term missions closer to the Sun when necessary to enable the mission. In 1989, NASA deployed the Galileo spacecraft from a space shuttle and sent it on a 6-year, gravity-assisted journey to Jupiter, where it became the first spacecraft to orbit the giant planet (Launius and Johnston, 2009). The flight team for Galileo ceased operations in 2003, and the spacecraft was deorbited by command into Jupiter’s atmosphere to guard against any potential future contamination of Jupiter’s moon Europa by an uncontrolled spacecraft impact.

Galileo carried two newly developed general purpose heat source (GPHS) RTGs. These units produced 300 W of electricity at beginning of life and had a total mass of 55.9 kg, giving these devices the highest specific power of any RPS the United States had ever flown.

The Ulysses spacecraft was also launched from a space shuttle in 1990 with one GPHS RTG to undertake a sustained exploration of the Sun. To enable a trajectory nearly over the Sun’s poles, the spacecraft was sent to Jupiter to use a gravity assist to rotate the heliocentric orbital plane of the spacecraft by almost 90°. Ulysses made the first and only observations of fields and particles in interplanetary space out of the ecliptic plane. It recently fell silent because of problems with its telecommunications system.

Cassini became the first mission to orbit Saturn. It is an international program involving the United States, the Italian Space Agency, and the European Space Agency. Conceived in 1982, Cassini was launched in October 1997 with three modified GPHS RTGs and multiple RHUs. Cassini arrived at Saturn and began orbiting the planet in July 2004. It also sent a probe (Huygens) to the surface of Saturn’s moon Titan early in 2005. Huygens is the first outer-planet mission built by the European Space Agency. Now in extended mission, Cassini continues to make fundamental discoveries in the Saturn system (Launius and Johnston, 2009).

New Horizons is the most recent mission to employ RPS generators. It will be the first spacecraft to visit Pluto and the Kuiper Belt. Launched in January 2006, New Horizons conducted a Jupiter flyby 13 months later to increase speed. New Horizons will make its closest approach to Pluto on July 14, 2015. The half-ton spacecraft contains scientific


A gravity assist is used to speed up or slow down the speed of a spacecraft by a close flyby of a planet that exchanges momentum between the spacecraft and the planet. Prograde approaches to planets in the outer solar system increase spacecraft speed, enabling them to reach planets farther from the Sun faster than they could otherwise.


As the backup for Voyager 1, Voyager 2 would have been targeted to Titan if Voyager 1 had failed.

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