The transit ship should remain in space instead of slowing for every planetary landing. Gravity assists save fuel, and the best route follows the available energy instead of drawing a straight line between two points. Age-of-sail navigators faced a similar planning problem: a longer course could be faster or more practicable because it worked with natural forces. Earth and Mars also keep moving while the ship is in transit, so a route that looks short at departure may end with the planets far apart. Navigation, fleet scheduling, and fuel economy become parts of the same operational problem.
Notable Point · 16:02-18:51
Gravity-assist routes resemble age-of-sail navigation
Interplanetary ships must exploit gravity and planetary motion as sailing ships used favorable natural forces, even when the efficient route is longer than a straight line.