The distances a spacecraft has to travel for deep space exploration are enormous. The engines used to power spacecraft currently simply aren't up to the task of exploring deep space or even our own solar system in a reasonable timeframe. Another problem with spacecraft currently is that they rely mostly on solar power to operate sensors and scientific equipment.
The further spacecraft get from the sun, the less power they're able to harvest and the less efficient and functional they become. Researchers are currently testing a small prototype engine that could be the future of deep space exploration. The prototype engine is based on a Stirling engine that was invented in the 19th century.
A Stirling engine uses hot pressurized gas to push a piston. The engine that the scientists envision would use a 50-pound nuclear uranium battery to generate heat that would then be carried to 8 Stirling engines able to produce about 500 W of power. Scientists at NASA's Glenn Research Center and Los Alamos National Laboratory are currently testing a smaller prototype of the proposed design.
The prototype uses a small nuclear source and a single Stirling engine that is able to produce 24 W of power. According to the scientists, most deep space probes need 600 to 700 W of power to operate. This project is also notable because it is the first test of a nuclear reactor system for powering spacecraft the US has conducted since 1965. The scientists say that nuclear power sources for deep space probes are important because once a spacecraft passes Mars, sunlight is so weak that it would require solar panels about the size of a football field to produce enough power to transmit data back to Earth.