[Thatcherite]

I imagine that you'd get to Mars in the end if you course corrected on the way and used more fuel than the most efficient trajectory. If you aim from earth using Newton you'll miss Mars orbit. I have this on authority from Radio Astronomer, whose job it actually is to calculate the orbits.

[js1138]

For your amusement.

http://scienceworld.wolfram.com/physics/HohmannTransferOrbit.html

I had a lunar lander game for my Trash-80 that used this equation to tell you how you were doing with your burn times and fuel. You could play with parameters before committing to a burn, but the game continued in real time, so the time you spent testing your estimates also changed the situation.

[RadioAstronomer]

http://public.ccsds.org/publications/archive/500x0g1.pdf

http://hpiers.obspm.fr/webiers/general/syframes/convent/CONV.html

http://www.agi.com/resources/help/stk613/helpSystem/whatsnew.htm

"High Precision Orbit Propagator (HPOP) has been updated with relativistic accelerations to model the effects of general relativity in accordance with IERS, Technical Note 21, IERS Conventions (1996)."

Side Note: A non-relativistic orbit propagator (Newtonian only) should get you there, however, for VLBI, GPS, timing, high accuracy, etc. relativity is taken into account. :-)