Transfer orbits are a special kind of orbit used to get from one orbit to another. When satellites are launched from Earth and carried to space with launch vehicles such as Ariane 5, the satellites are not always placed directly on their final orbit. Often, the satellites are instead placed on a transfer orbit: an orbit where, by using relatively little energy from built-in motors, the satellite or spacecraft can move from one orbit to another.
This allows a satellite to reach, for example, a high-altitude orbit like GEO without actually needing the launch vehicle to go all the way to this altitude, which would require more effort – this is like taking a shortcut. Reaching GEO in this way is an example of one of the most common transfer orbits, called the geostationary transfer orbit (GTO).
Orbits have different eccentricities – a measure of how circular (round) or elliptical (squashed) an orbit is. In a perfectly round orbit, the satellite is always at the same distance from the Earth’s surface – but on a highly eccentric orbit, the path looks like an ellipse.
On a highly eccentric orbit like this, the satellite can quickly go from being very far to very near Earth’s surface depending on where the satellite is on the orbit. In transfer orbits, the payload uses engines to go from an orbit of one eccentricity to another, which puts it on track to higher or lower orbits.
After liftoff, a launch vehicle makes its way to space following a path shown by the yellow line, in the figure. At the target destination, the rocket releases the payload which sets it off on an elliptical orbit, following the blue line which sends the payload farther away from Earth. The point farthest away from the Earth on the blue elliptical orbit is called the apogee and the point closest is called the perigee.
When the payload reaches the apogee at the GEO altitude of 35 786 km, it fires its engines in such a way that it enters onto the circular GEO orbit and stays there, shown by the red line in the diagram. So, specifically, the GTO is the blue path from the yellow orbit to the red orbit.
