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The US government is taking a serious step toward space-based nuclear propulsion

https://arstechnica.com/space/2023/07/nasa-seeks-to-launch-a-nuclear-powered-rocket-engine-in-four-years/#p3

"NASA is looking to go to Mars with this system."

The US government is taking a serious step toward space-based nuclear propulsion

Four years from now, if all goes well, a nuclear-powered rocket engine will launch into space for the first time. The rocket itself will be conventional, but the payload boosted into orbit will be a different matter.

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freeskier

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Some satellites and rovers have used Radioisotope Thermoelectric Generators (RTGs), which are very different from a nuclear reactor. They use polonium-210, which generates heat, and that heat is converted to electricity with thermocouples. They are low power and inefficient.

To my knowledge no satellite, with an RTG, has ever used ion propulsion. Few interplanetary satellites have ever even used ion thrusters. Dawn, Hayabusa, and Deep Space 1 are the only I can think of, and they all used solar arrays.

Ion thrusters are super efficient, but produce extremely small amounts of thrust. They aren't practical for getting large spacecraft to Mars. These proposed nuclear engines produce large thrust while have efficiency somewhere between regular chemical propulsion and ion propulsion.