|Fig. 1: Nuclear Thermal Rocket engine. (Source: Wikimedia Commons)|
The cosmos has long captured human imagination as a last, incomprehensibly vast, frontier for exploration. Since the first V2 rockets of WWII, to the surprise launch of Sputnik that kicked off the space race, mankind and it's machines continue to push further into space, relying on nuclear energy to power them for decades after they have left the surface of the Earth.
Developments in nuclear power enabled the space age and are vital to future missions that look to travel far beyond the solar energy of the Sun. Two nuclear technologies in particular, radioisotope thermoelectric generators and fission reactors, have played important roles in space exploration thus far and are instrumental to interstellar aspirations.
Nuclear energy has played an essential role in space development and exploration since the first nuclear powered spacecraft, the navigational satellite Transit 4A, was launched in 1961. It used a radioisotope thermoelectric generator (RTG) to convert heat from the decay of Pu-238 to heat energy using the Seebeck effect.  Essentially the RTGs work by converting the natural decay of radioactive elements to electricity. Since the launch of Transit 4A, RTGs have powered numerous satellites as well the Spirit and Opportunity Mars rovers. In 2012, the probe Voyager 1 became the first human-made artifact to leave the solar system, powered by a set of radioisotope thermoelectric generators to provide electricity to accelerate hydrazine for propulsion.  With no moving parts, RTGs require no maintenance and only minimal shielding, making them ubiquitous in satellites, rovers, and probes, as well as human missions including the Apollo lunar landing program.
In addition to RTGs, fission reactors have been studied and developed for applications that require more power than is feasible from RTGs. Development of fission reactors for space started with a simplified fission reactor that would be used to provide electrical power for satellites; in 1965, the United States launched the first space reactor, called SNAP-10A, which functioned for 43 days before a voltage regulator (not part of the reactor) failed. Though the United States abandoned the concept after SNAP-10A, the Soviet Union launched over 30 reactors to power surveillance satellites. 
In addition to providing electricity for electronic satellite systems, space fission reactors have also been developed to provide energy to replace chemical combustion propulsion systems. In the nuclear systems, the fission reaction heats a liquid hydrogen propellent to a gaseous state which is then ejected through a cone to give thrust, as seen in Fig. 1. The reactors can also be configured to as a dual mode system to heat propellent as well as generate electricity that can be used to power spacecraft electrical systems for crew habitation. 
The United States built and tested 23 nuclear thermal fission systems for testing as part of the NERVA program in the 1960s.  Interest in the technology has renewed as it is seen as the best way to power a human mission to mars. NASA uses a nuclear thermal rocket in its mars reference mission to heat liquid hydrogen for propulsion, yielding a thrust capability that is twice that of conventional liquid oxygen chemical rockets. 
Nuclear energy has been instrumental to humankind's space exploration since its beginning and will play an even larger role in future missions that look to venture beyond our solar system to new stars and worlds. RTGs will continue to be important components for low power needs that need long-lasting, durable stores of energy. Nuclear Thermal Rockets are the best available engine systems for interplanetary travel at higher speeds.
Investment in nuclear technology development is essential to the next generation of spacecraft and the key to unlocking the potential for interstellar travel. For the foreseeable future, missions will limited by the amount of propellent they can carry, but investments in nuclear could lead to new discoveries that enable more ambitious spacecraft that have been proposed to explore deep space and beyond.
© Travis Lanham. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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