Supersynchronous transfer orbit (SSTO)

Telecommunications satellites typically fly in geosynchronous orbit (GEO), a circular path around the equator at an altitude of nearly 36 000 km. At the GEO altitude, a satellite takes 24 hours to orbit, and so from the perspective of ground antennae appears stationary in the sky.

Reaching GEO is a two-step journey. Satellites are often launched into a geostationary transfer orbit, or GTO – an elliptical path with its highest point, or apogee, at the GEO altitude. As a launch site, Europe’s Spaceport in French Guiana has the geographical advantage of being close to the equator, so inclination in GTO may be about 6° or less. From there, a satellite uses its own thrusters, and fuel, to gradually circularise its orbit and reduce its inclination to 0°.

However, when payload mass allows, a satellite may be launched to an elliptical orbit with an apogee even higher than the ultimate GEO destination. The so-called supersynchronous transfer orbit (SSTO) is attractive because the satellite gets more energy from the rocket and uses less of its own fuel to circularise its orbit and reduce its inclination to reach its operating position in GEO.