If we are to expand the exploration of our solar system our next manned flight should be to Phobos, one of Mars's moons, rather than to Mars itself. The flight times to each are the same but the Phobos expedition would require less than half the fuel load of a Mars expedition and would, therefore, be much less costly. So, it is clear that Phobos should be our next step in space exploration.

Which one of the following, if true, would most help to explain the difference in fuel requirement?