The U.S. military is planning to launch a nuclear reactor into space for the first time since 1965.  The reactor will be started up once it reaches space. It will generate heat, which will be used to produce electricity. This electricity can power spacecraft payloads or electric thrusters for propulsion. 

The U.S. plans to test a nuclear-powered spacecraft engine by 2027. This is part of a NASA effort to develop more efficient ways to transport astronauts to Mars. 

Nuclear thermal propulsion (NTP) engines are more efficient, can shorten travel times, and can carry bigger payloads. 

The U.S.’ first and only known space nuclear reactor is the SNAP-10A. It was the result of the government-sponsored System for Nuclear Auxiliary Power (SNAP) program.

This will be the first time that the U.S. military has launched a nuclear reactor into space since 1965, when the U.S. launched the SNAP-10A experimental nuclear-powered satellite (which was also the first ion thruster ever in space).

Yes, there have been other nuclear reactors in space: 

• SNAP-10A: The world’s first operational nuclear reactor in space. It was launched in 1965 and was operational for 43 days. 
• Radioisotope thermoelectric generators: Used on many space probes and crewed lunar missions. 
• TOPAZ nuclear reactor: A small fission reactor used for Earth observation satellites. 
• Nuclear fission reactors: Mainly used by Russia. 

Other nuclear power sources in space include: Plutonium-238, Radioisotope batteries. 

Nuclear reactors can be used in space for: 

• Heat 
• Electricity 
• Propulsion 
• Powering a Mars or Moon base 
• Nuclear-powered rockets

NASA has been working on nuclear power and propulsion systems for space exploration since 2003.  In 1961, NASA and the Atomic Energy Commission started the Nuclear Engine for Rocket Vehicle Application (NERVA) program. The program involved designing, building, and testing rocket engines and reactors.  NASA tested reactors and engines from 1959 to 1972. 

In 2008, NASA announced plans to use a small fission power system on Mars and the Moon. They started testing key technologies for this system. 

Nuclear technology is important for space exploration. It can: 

• Power spacecraft over long distances 
• Generate electricity for spacecraft on long journeys 
• Provide propulsion for spacecraft to travel great distances at high speeds 
• Power rovers on Mars 
• Study radiation and its impact on living organisms

Nuclear energy has several advantages for space exploration: 

• Durability: Nuclear energy can withstand the harsh conditions of space. 
• Power: Nuclear energy can provide long-lasting power for spacecraft. 
• Mass: Nuclear systems can be less massive than solar cells of the same power. 
• Reliability: Nuclear systems are safe, reliable, and widely used. 
• Fuel: Nuclear propulsion systems don’t need to be refueled during a vessel’s lifetime. 
• Emissions: Nuclear propulsion systems don’t produce greenhouse emissions. 
• Thrust: Nuclear propulsion systems can provide high thrust. 
• Efficiency: Nuclear propulsion systems can double the propellant efficiency of chemical rockets. 

Nuclear power is used for missions that are too far away from the sun to receive enough sunlight. For example, nuclear reactors might be needed during long dust storms on Mars

Yes, nuclear fusion could power spacecraft. Fusion is the only energy source that could make rocket travel to Mars, the outer planets, and nearby stars efficient and rapid. Fusion could also cut travel time to Mars by more than half, which would reduce exposure to radiation and weightlessness. 

Here are some advantages of fusion energy: 

• Power: Fusion offers 1,000 times the power of conventional ion thrusters. 
• Speed: Fusion rockets could travel at 500,000 mph, which is 20 times faster than the fastest crewed rocket. 
• Efficiency: Fusion could reduce travel time to Mars by more than 50%. 

Here are some fusion propulsion concepts: 

• Steady-state fusion propulsion: This concept could send a 2,200 lb spacecraft to Pluto in four years. 
• Direct Fusion Drive: This concept is based on a compact fusion reactor.(full article source google)

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