Also found in: Acronyms.
transfer orbitThe trajectory followed by a spacecraft moving from one orbit to another, usually to the orbit of another body, such as another planet. The trajectory is generally part of an elongated ellipse – a transfer ellipse – that intersects the new orbit. The spacecraft would have to maneuver into an orbit around the planet by firing its rocket motors.
The transfer orbit requiring the minimum expenditure of energy is an ellipse that just touches the original (circular) orbit and the new coplanar orbit. This is called a Hohmann transfer, after the German engineer Walter Hohmann, who described it in 1925. Enough velocity is put in at the perigee (or equivalent point) of the Hohmann transfer for the craft to reach the new orbit at apogee; at apogee an additional velocity input injects the craft into the desired orbit. Although requiring the lowest possible energy, Hohmann transfers between planets involve a long flight time. The flight time can be much reduced by using a somewhat greater velocity at perigee than that needed for a Hohmann transfer. See also gravity assist.