transfer orbit

Elliptical path followed by a spacecraft moving from one orbit to another, designed to save fuel although at the expense of a longer journey time.

Space probes have often travelled to other planets on transfer orbits. A probe aimed at Venus has to be ‘slowed down’ relative to the Earth, so that it enters an elliptical transfer orbit with its perigee (point of closest approach to the Sun) at the same distance as the orbit of Venus; towards Mars, the vehicle has to be ‘speeded up’ relative to the Earth, so that it reaches its apogee (furthest point from the Sun) at the same distance as the orbit of Mars. Geostationary transfer orbit is the highly elliptical path followed by satellites to be placed in geostationary orbit around the Earth (an orbit coincident with Earth's rotation). A small rocket is fired at the transfer orbit's apogee to place the satellite in geostationary orbit.

Such transfer orbits are too costly in energy to be used to visit the outer planets. For such journeys a probe gets a ‘gravity assist’ from other planets. For example, after the Cassini probe was launched it travelled inwards towards the Sun to fly by Venus twice. The ‘slingshot’ effect of the planet's gravity boosted the probe outwards, so that it flew by the Earth and later by Jupiter, getting a gravity assist each time. Cassini arrived at Saturn after a journey of over six years.