The Transit Program and the Origins of Nuclear Power Systems for Spaceflight


Transit 4A being prepared for flight.

Transit 4A being prepared for flight.

Since the dawn of the space age more than 50 years ago, the United States has pursued a variety of methods for delivering electrical power to spacecraft in flight. Nuclear power systems are the only ones that have been found acceptable for deep space missions. Even so, there have been many satellites in Earth orbit that have employed nuclear power sources for the generation of electricity. The first of these for the United States was the Transit navigation satellite program pursued by the U.S. Navy in the early 1960s.

It’s story really began in the latter part of the 1940s when several engineers began to consider the possibility of using nuclear power sources to power spacecraft. By 1949 a full-scale analysis by the RAND Corp. had sketched out the possibilities for uclear power systems for satellites in Earth Orbit. In May 1953, USAF Headquarters took the next step by authorizing development work on a nuclear power source for satellites. This research effort led directly to the nuclear power systems used on spacecraft in the early 1960s.

Pursuing two related avenues, researchs developed both a small nuclear reactor and an RTG, or radioisotope thermoelectric generator. Codenamed SNAP for “Systems for Nuclear Auxiliary Power,” these power sources were numbered with the odd numbers designating RTGs and even numbers for the small nuclear reactors.

For the RTGs, SNAP-1 was built at the Mound Laboratory under the Atomic Energy Commission’s (AEC) supervision in 1954. It used a thermocouple heated by polonium (Po)-210 for fuel. Those first RTG’s capabilities were modest to be sure, and power management was always a consideration in these systems, but they lasted for years and could power a spacecraft on extended missions. In the reactor arena, the SNAP-2 system used a 50-kw(t) reactor system weighing about 600 pounds employing liquid NaK—a sodium (Na) and potassium (K) alloy—as a coolant to transfer heat through a mercury loop. This reaction, basic chemistry really, produced 3 kw of electricity. This led to the research on two additional space power units, SNAP-8 and SNAP-10, emphasizing a metal hydride reactor technology first used in SNAP-2.

These efforts led to a longstanding record of success in meeting the electrical needs of space vehicles. As historian Richard Engler has concluded:

The history of the radioisotope power program is basically a success sto[r]y, although it is certainly not one of linear success. The program was initiated by the AEC under impetus from the Department of Defense but first went public late in that decade as part of the “atoms for peace” movement, with President Eisenhower showing an atomic battery to the world and extolling its peaceful potential uses. Subsequently, while the Defense Department supported mostly test applications of the radioisotopic power devices in space, the program reached its pinnacle of success through uses by the civilian space agency, NASA.

The possibilities of space nuclear power first entered the public sphere in January 1959 when President Dwight D. Eisenhower posed for a photo op with an RTG in the Oval Office of the White house. It was SNAP-3, the AEC-developed power source on which so many engineers pinned their hopes for spacecraft power. AEC officials hailed this RTG as a “significant breakthrough,” one that was reliable, simple, flexible, safe, and just as importantly, they said, “We can tailor the product to fit the customer.”

EisenhowerThe application of nuclear power to spaceflight really began in the 1950s, when the Navy through its contractor, the Applied Physics Laboratory (APL) of the Johns Hopkins University, developed RTGs for space navigational satellites. Intended as a method of ensuring the capability of the inertial navigation systems of the U.S. Navy’s Polaris ballistic missile submarines, the Transit system promised 80–100 meter accuracy. Accordingly, it supported one-third of the nation’s strategic triad in enabling targeting and ensuring the deterrent threat posed to the Soviet Union was real.

It originated on March 18, 1958, when the APL’s Frank T. McClure wrote two memoranda to APL Director Ralph E. Gibson: “Yesterday I spent an hour with Dr. [William H.] Guier and Dr. [George C.] Weiffenbach discussing the work they and their colleagues have been doing on Doppler tracking of satellites. The principal problem facing them was the determination of the direction which this work should take in the future. During this discussion it occurred to me that their work provided a basis for a relatively simple and perhaps quite accurate navigation system.” Most important, McClure noted, it offered the solution to a vexing problem of genuine military significance during the Cold War.

The first Transit satellite, Transit 1A, took off from the Space Operations Center at Cape Canaveral, Florida, on September 17, 1959, but failed during launch. A second satellite, Transit 1B, was launched on April 13, 1960, and operated for 89 days. There followed a succession of Transit satellites, with a general development of greater capability and longevity interspersed with failures. A vexing issue was how to maximize the spacecraft’s useful service life on orbit—the best that the Navy could achieve seemed to be about a year with batteries and solar arrays. RTGs offered a ready alternative. As John Dassoulas of APL recalled: “I had been looking into the possibilities of isotopic power since we first began the Transit program. We had a five-year goal for the life of the operational Transit, and we weren’t confident that the hermetic seals on batteries would hold up for five years.”

The AEC’s Glenn Seaborg proved a persistent advocate for the use of RTGs. He officially asked the president on May 6, 1961, to approve the first launch, citing the findings of a hazards study that “any danger to the public is extremely unlikely.” He added: “I call this to your attention since this first application of a nuclear auxiliary power source in space is likely to have a wide public impact.” The Department of State resisted this launch, in no small part because of its international implications, but the DoD and the AEC persisted and eventually succeeded in obtaining approval. 

As this took place, the public learned of the impending launch of a nuclear power plant and organized a protest. Picking up on the high-level discussions inside the Kennedy administration, on May 16, 1961, the New York Times broke the story, suggesting that the “problem confronting the Administration…is not so much a technical decision as one of diplomatic, political and psychological considerations.”

Three days later, the New York Times pressed the issue, highlighting concerns from State Department officials “that in event of an unsuccessful launching, the satellite, with its radioactive parcel, could fall on Cuba or some other Latin-American country.” They feared, in the politically charged environment after the failed Bay of Pigs invasion of Cuba, that this would add fuel to any international incident that might result. Some even expressed concern that other nations might “take offense about having radioactive materials flown over their territory.”

Accordingly, the DOD reconfigured Transit 4A to fly without the RTG, reluctantly accepting a lesser capability on orbit. The story differs on how the approval finally came down to fly the RTG on Transit 4A. Some believed that it was the culmination of a monthlong set of internal negotiations between the DoD and the State Department to proceed with the June 1961 launch of Transit 4A. Others claimed that it only contained the RTG because of the intervention of President Kennedy, who personally gave approval to proceed during a small dinner party in which Glenn Seaborg pled the case for the mission.

Figure 1-RTG CasingRegardless, about two days before the scheduled liftoff, a military team flew the RTG from Baltimore to Patrick Air Force Base in Florida, where the launch team destacked the payload and inserted the SNAP-3 system. The vehicle then launched on June 29, 1961, from Launch Complex 17 and operated for fifteen years until the satellite was finally shut down. Transit 4B followed on November 15, 1961, and operated until June 1962 when a thermoelectric converter in the power unit failed. The satellite ceased communications on August 2, 1962, but there were some reports of picking up telemetry from it as late as 1971.

The launch of Transit 4A made headlines. The New York Journal American offered a positive story. It reported: “The successful orbiting of the nuclear device…gives American scientists a significant lead over Russia in the race to harness atomic power for space exploration.” Previous concerns voiced by officials from the State Department withered with the success of this flight, and serious intergovernmental opposition never found traction thereafter.

The initial successes prompted the development of the Transit 5B series of satellites containing nuclear power sources. Launched atop Thor Able-Star rockets, Transit 5BN-1 reached orbit on September 28, 1963, but it achieved gravity-gradient stabilization upside down, which limited its signal output to the ground. Transit 5BN-2 was launched on December 5, 1963, with an RTG power source and operated for approximately one year. The last RTG-powered navigation satellite, Transit 5BN-3, was launched on April 12, 1964, but failed to achieve orbit, and its failure prompted widespread concern. As a U.S. GAO report noted in the latter 1990s: “In 1964, a TRANSIT 5BN-3 navigational satellite malfunctioned. Its single RTG, which contained 2.2 pounds of plutonium fuel, burned up during reentry into Earth’s atmosphere. This RTG was intended to burn up in the atmosphere in the event of a reentry.”

It did, and this sent shock waves through the world community. The Atomic Energy Commission tried to assuage the public’s fears, reporting that “from previous safety analysis and tests it had been concluded the re-entry will cause the plutonium-238 fuel to burn up into particles of about one millionth of an inch in diameter. These particles will be widely dispersed…and would not constitute a health hazard.” This analysis proved too optimistic, leading many to question the use of nuclear power sources for orbital missions into space.

Those questions have abounded ever since. Despite concerns the use of radioisotope thermoelectric generators to power spacecraft to the outer planets has proven a boon to the space program in the first fifty years of its history. Use of this technology invites opposition because of the danger inherent in any launch or reentry to the atmosphere. There have been failures in the past, duly taken notice of by the public, and in each instance refinements and additional requirements to ensure future safety have resulted. This is as it should be.

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One Response to The Transit Program and the Origins of Nuclear Power Systems for Spaceflight

  1. Pingback: Space-for-All at HobbySpace » Space History: Early US space nuclear power projects + The Saturn V F-1 engine

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