POWER SYSTEM IN SPACE EXPLORATION
The' Atomic Energy Commission's isotopic space power program dates back to several years before Sputnik I, but the program suffered a severe setback in 1959 when the Snap-1A
generator development program was cancelled.' This pioneer program was
not completed because it may have been ambitious for its day. The need for isotopic power had .not yet become apparent to space program planners; its place and full significance in the nuclear space power program were not clearly established; its applicable thermoelectric energy conversion technology was still very new large quantities of isotopic fuel materials were not readily available.
Because of this sound technical basis, the Commission's space-oriented isotopic power development program has made a steady, although sometimes slow, comeback through a series of events since 1959, so that today a program technically comparable to Snap-1A could once more be undertaken with a high probability of successful completion. This series of events can help demonstrate the status of today's space isotopic power program. Details of the various systems have been described many times and will not be repeated here. For reference purposes, the characteristics of several. space isotopic power systems.
Snap 11, a 25-w RTG being developed for use on NASA's Surveyor soft lunar-landing missions, has also contributed significantly to isotopic space power systems technology After a design study and a preliminary safety analysis had been completed, NASA established a requirement for the Snap-11 generator development program late in 1961. During the past year, a detailed design was completed that would meet all the interface requirements of the Surveyor spacecraft. These included the electrical, physical, nuclear radiation, and thermal interface specifications." The electrical output can be easily matched to the payload through a DC-to-DC voltage converter similar to that used with conventional power supplies. The physical limitations of the vehicle naturally dictate the size, weight, and shape of an RTG. For the Surveyor program, it was decided to extend Snap-11 out from the spacecraft (because of overriding thermal considerations) so that an optimized RTG configuration could be used. The separation distance and provisions for shielding in the design of the curium-242 fuel capsule will allow Snap-11 to meet the extremely stringent background radiation levels specified for the sensitive radiation detectors aboard the spacecraft. Thermal integration problems were most severe and caused abandonment, for the present of a design for conducting heat to the sensitive payload instruments during the cold lunar night. A thermal mock up of the Snap-11 has been fabricated and is undergoing tests. Electrically heated prototype generators will be available for integration tests later this year. Because of launch vehicle problems, Snap-11 is not scheduled to fly before 1965, unless the results of earlier solar powered Surveyor spacecraft dictate otherwise.
Creating robotic spacecraft that could thrive in these extreme environments demands technical innovation. One of the most important components for such missions is their electrical power supply. For most space exploration missions where sunlight is abundant, solar power has been the preferred choice. But as the environments at chosen destinations grow harsher, and missions evolve to be more demanding, it becomes more likely that effective power and heating for a spacecraft would require a Radioisotope Power System (RPS). An RPS converts the heat generated by the natural decay of the radioactive isotope plutonium-238 into
electricity; this material is not used in weapons and cannot explode like a bomb. A portion of this decay heat often has an important secondary use in helping to keep spacecraft subsystems warm in cold environments. An RPS offers the key advantage of operating continuously, independent of unavoidable variations in sunlight. Such systems could provide power for long periods of time (significantly longer than chemical batteries), an little sensitivity to temperature, radiation or other space environmental effects. They are ideally suited for missions involving autonomous, long-duration operations in the most extreme environments in space and on planetary surfaces.
The power system,
if adopted by NASA, will include two
RTGs placed on opposite sides of the
spacecraft to maintain proper weight
and balance for stabilization. The IMP
generators will each produce appr oximately
25 wand will be fueled with
Pu-238 because of the longer than
one-year mission lifetime. These RTGs
incorporate design improvements over
Snap-9A which provide for easier fabrication.
and lower system weights.
The MMRTG is based on the proven RPS design used to provide
electrical power for NASA’s two earlier Viking landers, which operated
on the surface of Mars for 40 months and more than six years, respectively. Other missions in NASA’s heritage of safe and successful use of such generators for solar system exploration over the past 40 years include Voyager 1 and 2. The Voyagers continue to operate more than three decades after their launches, seeking the boundary of true interstellar space more than nine billion miles from the Sun.
Any NASA mission that proposes to use an RPS undergoes a
comprehensive multi-agency environmental review, including public
meetings and open comment periods during the mission planning
and decision-making process, as part of NASA’s compliance with
the National Environmental Policy Act. Additionally, any such mission
proposed by NASA would not launch until formal approval for
the mission’s nuclear launch safety is received from the Office of the
President.
Radioisotope power systems are used when they enable or significantly
enhance missions to destinations where inadequate sunlight,
harsh environmental conditions, or operational requirements make
other electrical power systems infeasible.
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