The application of nuclear propulsion and nuclear energy systems to space exploration is a natural development of the work that has gone on before.
Space exploration has had an attraction for mankind throughout history: first to the extremes of space on earth, then in earth-confined orbits, journeys to the moon and the nearest planets, and even outside the galaxy.
The early pioneers in rocketry were Russia's Konstantin Eduardovich Tsiolkovski (1857-1935) and America's Robert Hutchings Goddard (1882-1945.) Their work led to the first satellite launch in October 4, 1957 (of the Soviet Union's Sputnik I), the first man in earth orbit in April 12, 1961 (Russian Yuri Gagarin), and a first lunar landing on July 20, 1969 (by Neil A. Armstrong and Edwin E. Aldrin Jr. of the U.S.). Since then the International Space Station has built upon the experience of Russia's Soyuz and Mir, motorized vehicles have landed on Mars and Venus, and missions have been sent beyond this galaxy.
What was first a keen curiosity-and still is primarily scientific and astronomical research-has now been supplemented by the possibility of making use of mineral resources contained in distant bodies and even the possibility of human expansion beyond the earth.
This exploration has also brought about great benefits in knowledge. Experiments in zero gravity have contributed to our understanding of biological and physiological behavior on earth. The Hubble telescope has demonstrated enormous strides in scientific understanding of space by making its investigations beyond the limits of earthly atmospheric distortion. Lunar exploration provided the first steps in expanding human boundaries and the first retrieval of material samples. Furthermore, for the first time we have been able to view planet Earth from a distance and to gain a new perspective of its human custodians.
Space exploration has been taken in successive steps. Each step of this progress has taken with it the technology and the lessons learned in prior steps. For example, what was learned in exploring the Polar Regions and climbing the highest peaks on Earth has contributed to man's survival in space. Further, the development of remote applications of radioisotope power on earth has contributed to far distant space travel.
Throughout the years, space exploration has been marked by an increasing need for power, first to escape the earth's gravity and, secondly, to power the satellites and space ships and their instruments through missions of greater and greater duration. This is where parallel discoveries of the 20th century-radioisotopes and nuclear power-can and do contribute.
The use of nuclear power in space was first proposed at Peenemunde, Germany in 1943. But after early rocket experiments used solid chemical fuel, the propellants of choice for launching first rockets and then space vehicles became principally liquid hydrogen and liquid oxygen.
However chemical fuels are characterized by massive initial launch loadings and rapid burn-up. Instead nuclear reactor-powered thrusters can be used as initial propellants for launching and for later course adjustments with far smaller launchto-flight mass ratios. Extensive US testing was done on reactor-powered thrusters from 1955 to 1973 to prove the concept.
Chemical fuels are also unsuitable for missions of great duration, which need light weight and high reliability. In these circumstances, just as in earthly applications in remote areas, radioisotope fuels perform admirably and have been used in space since 1961. As further development is made in nuclear space applications, it is probable that these developments will in turn improve applications used on earth.
Forty years have passed since the practical use of nuclear power in space missions was started in the USA and former Soviet Union. They testify to the fact that nuclear power systems built around isotope generators and nuclear reactors have found a vital niche in space programs along with other conventional space power sources. The prospects of their future use will increase with the increasing power requirements of space missions and the emerging demand for thrust.
Between 1961 and 1988, over 30 radioisotope thermoelectric generators were used for space missions in the USA and the former USSR; 32 Russian BUK thermoelectric space nuclear reactor systems were successfully employed in spacecraft for marine observations; one US space nuclear thermoelectric system SNAP-10a was tested on the Snapshot experimental spacecraft in 1965 amd in 1987 two Russian TOPAZ space nuclear thermionic systems successfully underwent tests on spacecraft of Cosmos-1818 and 1876 series.
Furthermore, extensive R&D activities in nuclear thermal propulsion was carried out in USA and Russia, including tests of ground prototypes of reactors and nuclear propulsion units.
The work on space nuclear power systems and nuclear thermal propulsion that has already been performed in the US and Russia provides a basis for future development of the use of nuclear power in space. There are prospects for a sound technological base in the early 2000's, which will be followed by the development of powerful power/propulsion units. These will be capable of transferring 2 to 3 times greater payload masses, compared to conventional chemical propellant orbital boosters, from radiation safe near-earth orbits (H 800 km) to high-power-demand interplanetary orbits. They would be capable of supplying 50 to 100 kW of electrical power, and more, to on-board special-purpose instrumentation over a long period (up to 10 years).
The main constraint in these activities is compliance with safety requirements. In space conditions, a radical solution to the radiation safety problem consists of putting spacecraft that carry operating nuclear power systems into orbits wherein the spacecraft life exceeds the time it takes radioactivity to decay significantly. Such a consideration is clearly subject to international review.