Flying in space requires reliable, uninterrupted, stable electrical power, not only for engines to maneuver and navigate but for systems on spacecraft performing a range of functions. During the first two decades of the space age in the 1950s and 1960s both the United States and the Soviet Union developed capabilities to power spacecraft using nuclear power, in the form of radioisotope thermoelectric generators (RTG).
This is one of the critical components of any satellite either in Earth orbit or dispatched elsewhere is the power system that allows the operation of its many systems. There are only four methods of providing the electrical power needed for spacecraft, all of them with positives and negatives. The first method, and the one used on the first spacecraft launched into orbit, was batteries. Their wattage was limited, but even more limited was their longevity. Within a few weeks they always ran down and the spacecraft’s systems no longer operated.
Second, to help resolve that problem NASA pioneered in the 1960s fuel cell technology, which generated more electricity for the size of the cell and had a longer effective life. Even so, fuel cells have an effective life of less than two months. Of course, this may change in the future as NASA pursues more efficient fuel cells for its Constellation program that could have remarkably long lives.
Third, photovoltaic solar cells emerged in the 1960s as a useful alternative to batteries and fuel cells. They have a long life measured in years rather than weeks or months, and with additional refinement they have become the critical power generation technology for most spacecraft. They have one important drawback; they require the Sun’s powerful light source to be effective. For spacecraft traveling into deep space beyond Mars, where the Sun becomes much less intense, photovoltaic systems up to this point have proven insufficient. This may change in the future as new technologies increase the efficiency of energy collection and power management but past and present capabilities have not allowed their use. Accordingly, when requirements are for short mission times or do not require high power, chemical and/or solar energy may be used effectively to make electricity.
But for the generation of high power levels over longer periods of time, especially farther away from the Sun, nuclear energy has thus far been the only way to satisfy mission requirements. For this reason, as well as others of a more sublime nature, many spacecraft designers have adopted nuclear power technology as a means of powering spacecraft on long deep space missions. As NASA’s chief of its nuclear electric power programs remarked in 1962:
Basically, radioisotopes are of interest because they represent a compact source of power. The energy available in radioisotopes is many orders of magnitude larger than that available in batteries, and thus they constitute a unique, concentrated energy source that may be used for space purposes if design requirements are met. Radioisotope power is inherently reliable. It cannot be turned on or off. There are no moving parts of oriented arrays. It will provide heat energy in accordance with the fixed laws of radioactive decay. This heat is absorbed in a device that converts the heat directly into electricity.
There are several types of nuclear power that could be employed, everything from small reactors to nuclear heaters to the dominant technology of radioisotope thermoelectric generators (RTG). In those small space nuclear reactors, energy could be generated through controlled fission of uranium. Creating heat through this process, it is then used to power either a thermoelectric or a dynamic turbine or alternator conversion system. While excess heat would be dissipated through a radiator, electricity generated through this process served to power the spacecraft. These reactors had the capability to generate more than 100 kilowatts (kWe) of electricity, making them much more powerful than other forms of energy generation in space, including RTGs.
In addition to its longevity, space nuclear power offers a significant saving in terms of mass associated with an individual mission compared to the other possibilities. As policy analyst Steven Aftergood reported in 1989: “for all practical purposes, nuclear reactors are required when moderate to high levels of continuous power are required for an extended period.” Another observer wrote in 2005:
Nuclear power has been used for deep space vehicles for over 40 years. RTGs have been used for spacecraft electrical power since 1961. All RTGs have operated as designed, both in normal operations and accident conditions. RTGs were designed carefully with consideration for the accident environments that might be experienced during every phase of the launch. The design requirement is to protect public and worker health and safety during all phases of operations during launch and accident conditions.
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, an Atomic Energy Commission (AEC)developed power source on which so many involved in the space program pinned their hopes for exploration of the solar system. 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.”
In the context of the post-Sputnik high technology competition with the Soviet Union in the latter 1950s, Eisenhower undoubtedly viewed this showing of the first RTG as a useful propaganda device, graphically demonstrating American technological verisimilitude. He emphasized that this nuclear device was not destructive; rather it was a means of supporting peaceful scientific expeditions for ramifications for the positive development of humanity. Accordingly, the SNAP-3 served as a proof-of-concept for Eisenhower’s “Atoms for Peace” initiative, a positive use of nuclear technology around the globe. Its small size, inconspicuousness, and non-threatening nature served Eisenhower well in helping to defuse the caustic international confrontations between the U.S. and the Soviet Union.
The first American nuclear powered satellite, Transit-4A, launched on June 29, 1961, from Launch Complex 17 and operated for fifteen years until the satellite finally shut down. The Transit series of spacecraft were navigation satellites built by the Naval Research Laboratory. 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.” Concerns voiced by officials from the State Department withered with the success of this flight. By October 1961 AEC head Glenn Seaborg was promoting the use of atomic power as the logical technology to power spacecraft. He asserted:
The presence of the “atomic battery” in the satellite is a symbol of a “marriage” that was bound to occur—between Space and the Atom. We have known for some time that the two were made for each other. No one would be tempted, at the present time, to abandon other sources of energy for space. However, the atom has made greater strides toward coming of age for space application in the past few years than many of us could have hoped. The day is not far off when atomic energy will be available in many different packages for practical use in space vehicles.
At the same time Seaborg lobbied with Vice President Lyndon B. Johnson, the chair of the Space Council, for greater use of space nuclear power. He argued that the success of the first mission could be replicated over and over, providing efficient power systems for spacecraft. it represented a major point of transition for space exploration technologies.