We have sent spacecraft to other planets using traditional chemical rocketry (technique briefly explained in our article here), however this requires an amount of time which is not feasible for human space flight. In order to advance humans to other planets we need to reduce the travel time so that we minimize the exposure to radiation, bone density loss and other adverse effects of space flight. A number of solutions to chemical rocketry have been proposed including solar sails, warp drives, and matter-antimatter engines. Based on current technology it appears that nuclear is the most feasible for reducing spaceflight times within the solar system to allow humans to travel further faster.
Nuclear Thermal Rockets
Nuclear energy is not a new concept to the space industry and has been around longer than you may expect. The Viking missions to Mars in the 1970s used landers which were powered by Radioisotope Thermoelectric Generators (RTGs). Nuclear Thermal Rockets (NTRs) could be a solution to the transport of humans to other celestial bodies in reasonable time frames. Liquid hydrogen is a common type of NTR which has the potential to have two times the specific impulse (efficiency) of chemical rockets.
In NTRs usually liquid hydrogen is heated through a fission reaction to expand through a nozzle, producing thrust to propel the rocket, and the astronauts on board. Due to the thrust capability of NTRs they take up less space on the rocket and therefore are capable of carrying a large payload. There are three main types of NTRs; solid core, liquid core, and gas core. The solid core is the simplest of the three and is the only one that has ever been built. Testing of NTRs has only occurred on the ground and no tests in space have been conducted. Since some of the cryogenic hydrogen may not be used for days or weeks in a long mission they are subject to cryogenic boil-off in space which may reduce the restart capability of the rocket. Although a promising technology issues such as cryogenic storage may prevent us from seeing NTRs get off the ground for a couple of years.
Nuclear Fusion Rockets
Perhaps a more distant source of propulsion for spaceflight may be a nuclear fusion rocket. A nuclear fusion rocket would use the energy generated from nuclei of two or more atoms combining to generate thrust and propel the spacecraft. Nuclear fusion is the same thing that powers our sun and other stars. The sun converts the energy generated from fusion to produce light. Although the nuclear fusion rocket technology has not been completely demonstrated on the ground, scientists at the University of Washington (funded by NASA’s Innovative Advanced Concepts Program) have demonstrated fusion using plasma, compression and a magnetic field which seems to be a step in the right direction.
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Theoretically using nuclear fusion rockets you could make it to Saturn in a matter of months in comparison to the almost 7 years it took the Cassini spacecraft to enter Saturn’s orbit. If this technology succeeds the future of human spaceflight would look very different!
Because we are all Born For Space!