Hohmann Transfer

Named after Walter Hohmann, who described the maneuver in his 1925 book The Attainability of Celestial Bodies, the Hohmann transfer remains the most fundamental concept in orbital rendezvous and interplanetary travel. The maneuver uses an elliptical transfer orbit whose perigee touches the lower circular orbit and whose apogee touches the higher one. A spacecraft begins by firing its engines prograde at perigee, which raises the opposite side of the orbit. When it reaches apogee, it fires again to circularize at the new altitude. The same logic works in reverse for descending to a lower orbit. The Hohmann transfer is optimal in terms of delta-v when the ratio between the two orbital radii is less than about 12:1. For larger ratios, a bi-elliptic transfer can be more efficient, though it takes longer. Time of flight for a Hohmann transfer equals half the period of the elliptical transfer orbit — roughly 5.5 hours from LEO to GEO, or about 260 days from Earth to Mars under ideal alignment. The maneuver assumes impulsive burns and no external perturbations, so real missions must account for gravity from other bodies, solar pressure, and timing constraints.