Earlier this year, Congress signed a new appropriations bill directing funding to multiple NASA projects, including funds for the James Webb Space Telescope, the 2023 Europa Clipper mission, and a follow-up 2025 Europa lander mission, among other initiatives.
Notably hidden among the budget items, however, was a $100 million allocation to develop a nuclear thermal rocket. Nuclear rocket propulsion has long been theorized and researched, but no fully-produced version exists yet today.
A nuclear thermal rocket, as the name suggests, is a rocket motor powered by a nuclear reactor. Aboard the spacecraft, a tank of hydrogen fuel is stored at supercool temperatures at -252 K (-423F). When the rocket is ready to be turned on, the hydrogen is pumped from its tank through a series of pipes into the nuclear reactor vessel where the hydrogen is heated into plasma and expelled through the rocket nozzle at high velocities.
The amount of thrust a rocket can produce depends on both the temperature and the molecular weight of the expelled molecule; in both regards, a nuclear rocket is superior to a chemical rocket because of its higher exhaust temperature and lower molecular weight of fuel. Nuclear rockets, depending on design, can achieve exhaust temperatures ranging from 2750 K (4490F) to as high as 21,000 K (37,340F).
For reference, most chemical rockets only go as high as 5,555 K (9540F). Because chemical rockets involve reactions between hydrogen and oxygen, the exhaust gas is mainly water vapor, while the exhaust gas of a nuclear rocket is pure hydrogen. This means that even the coolest form of a nuclear rocket can produce twice the power of a chemical rocket (~900 ISP for nuclear compared to ~450 ISP of the space shuttle).
NASA has experimented with nuclear propulsion in the past. The Nuclear Engine for Rocket Vehicle Application (NERVA) program was a nuclear experimentation program run jointly by NASA and the U.S. Atomic Energy Commission in the 1960s and 1970s. The program's goal was to establish a technological baseline for nuclear propulsion; several engines were test-fired in configurations that could be adapted to production-ready spaceflight.
However, the program was canceled in 1972 when public support waivered and plans to send humans to Mars were canceled by Congress. Any plans to use a nuclear rocket focused on utilizing it as a second stage once the craft had entered deep space; this would avoid contaminating Earth's atmosphere with the radioactive byproduct.
While we don't know exactly what NASA will produce with its $100 million allocation, a flight demonstration mission has been planned for 2024.
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