Monday, January 17, 2011

Nuclear Reactors and Space Propulsion

The new NASA administrator, Charles Bolden, made a speech a year ago about the future of the agency. He spoke of game-changing technologies that would allow astronauts to travel to Mars in "days not months". Bolden is referring to a new kind of space propulsion, one that has not yet been used to push astronauts around in space. These rockets don't derive their energy from the chemical combustion of propellant, but rather from an external thermal or electrical source. As a consequence, they can accelerate exhaust gases to far greater velocities than chemical rockets can—so long as they have enough power available from the external source.

VASIMR concept with nuclear reactors
One concept for a nuclear-powered
VASIMR rocket
What Bolden didn't make clear was the amount of power needed to get to Mars so quickly. His remarks are informed by a study on the VASIMR plasma rocket, which concluded an expedition could reach Mars in 39 days if it had an available power of 200 megawatts. Two hundred megawatts of electrical power is an enormous amount. The whole International Space Station generates less than 250 kilowatts from its solar arrays, so it would take about 1000 times as many solar arrays for VASIMR to get humans to Mars in 39 days. To make things worse, the intensity of sunlight drops off as one travels away from the Sun. Solar panels alone will not work for a human mission to Mars.

The only realistic sources of power for these rockets are nuclear fission reactors. One nuclear reactor was flown in space by the United States in the 1960s, and over 30 were flown by the Soviet Union. Nonetheless, these reactors produced far less than 200 megawatts. A Nimitz-class aircraft carrier uses two reactors, each capable of producing 100 megawatts of electrical power. So it would take the power plant of one of these carriers for the VASIMR to push a crew to Mars in 39 days. Someone will have to get started on designing these reactors if NASA is going to live up to Bolden's words.

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