HomeNews & MediaFeatured StoriesFrom Transit 4A to New Horizons: Fifty Years of Nuclear-Powered Spacecraft 

June 29, 2011

From Transit 4A to New Horizons: Fifty Years of Nuclear-Powered Spacecraft

Transit spacecraft
The Transit 4A satellite, launched on June 29, 1961, and the first satellite to use a nuclear power source, is shown here at the bottom of a triple-decker payload.

APL's 1961 Transit 4A satellite and its 2006 New Horizons spacecraft don't seem like they would have very much in common. The former was an experimental navigation satellite; the latter is the first spacecraft sent to Pluto and the Kuiper Belt. But both share a similar "heart": Transit 4A, launched on June 29, 1961, used a nuclear power source called a radioisotope thermoelectric generator (RTG) to operate its instruments, just as the New Horizons spacecraft does today. Now, with New Horizons more than halfway to Pluto, we mark the 50th anniversary of nuclear power in space.

"New Horizons and other deep space exploration missions to the outer solar system would not be possible without the development of RTGs pioneered by Transit 4A," says Alan Stern of the Southwest Research Institute, principal investigator for New Horizons. "This is a key area of U.S. space leadership that's tremendously benefited both unmanned and manned exploration of the Moon and planets."

Transit 4A was part of a triple-decker payload of satellites launched from Cape Canaveral Air Force Station, Florida. Aboard the satellite was a grapefruit-sized device known as SNAP-3 (System for Nuclear Auxiliary Power)—the first RTG ever launched into space. It provided direct electrical current to instrumentation and two of Transit 4A's four transmitters.

NH spacecraft
Artist's concept of the New Horizons spacecraft as it approaches Pluto and its three moons in summer 2015.

In 1966, after 5 years in orbit, Transit 4A became the oldest broadcasting spacecraft to date, and it continued to hold that title through its 10th anniversary in 1971. By that time, the satellite had traveled approximately 1.7 billion miles and had circled the Earth more than 55,000 times.

APL invented and developed the Transit navigation system for the Navy's Trident submarines. The first Transit satellite, 1A, was launched in 1959. Although the satellite plunged into the sea after the booster's third stage failed to fire, enough data were gleaned to prove that such a system was viable. In April 1960, Transit 1B became the first to attain orbit, followed by 2A, 3B, and 4A. Transit became a fully operational navigation system in 1964.

Forty-five years after Transit 4A was launched from Cape Canaveral, New Horizons took to the stars from the same complex. The spacecraft is headed for a rendezvous with Pluto and its three moons in July 2015 and, soon after, possible encounters with smaller bodies in the distant Kuiper Belt. The fastest spacecraft ever launched, New Horizons has already covered a massive amount of space since lifting off in January 2006—traversing more than 20 times the distance between Earth and the Sun. New Horizons crossed the orbit of Uranus in March. To learn more about New Horizons, visit http://pluto.jhuapl.edu.