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Excellent question, fascinating answer: http://www.orbitermars.co.uk/stdorbit.htm http://www.orbitermars.co.uk/stdorbit.htm Trivia: this was the first spot googling "energy cost change orbital plane". I almost didn't click the link "Standard Orbit, Mr Sulu!" but was very glad I did. Bing had Wikipedia as the first spot, not nearly as engaging.