transfer orbit

The transfer orbit used by a spacecraft when travelling from Earth to Mars. The orbit is chosen to minimize the fuel needed by the spacecraft; the craft is in free fall for most of the journey.

A spacecraft in low Earth orbit (LEO) can change to a higher orbit in the same plane with minimum fuel expenditure by utilizing the Hohmann transfer orbit. Whilst in LEO the spacecraft fires its engines to increase velocity, causing the trajectory to become elliptic. This ellipse is carefully calculated to attain the desired final altitude. At this point the engines are again fired and the spacecraft transfers to the final 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.