Geostationary transfer orbit

A geostationary transfer orbit, often called a GTO, is a special path that satellites take around Earth. It is a stretched-out, oval-shaped route called an elliptical orbit. One end of this path, called the perigee, is quite low—about the same height as low Earth orbit. The other end, called the apogee, is very high—about the same height as a geostationary orbit, which is a special orbit that moves at the same speed as Earth’s rotation.

Satellites that are going to stay in a geostationary orbit often start in a GTO. This allows them to use less fuel to reach their final position. The GTO is like a stepping stone that helps satellites get to where they need to go.

The amount of weight that a rocket can carry into a GTO is an important number that rocket makers often share. It helps people know how much the rocket can deliver into this special path. For example, the satellite EchoStar XVII used this kind of orbit to reach its final place in space.

Background

Geostationary and geosynchronous orbits are great for many communication and Earth observation satellites. But sending a spacecraft there costs a lot because these orbits are very far from Earth. A geostationary transfer orbit (GTO) helps make this easier. Satellite teams use a powerful but less efficient launch vehicle to place their satellite into a GTO. After the launch vehicle leaves, the satellite uses its own smaller engines to move into its final orbit. This way, the satellite uses less fuel, and the launch vehicle can fall back to Earth safely.

Technical description

A geostationary transfer orbit (GTO) is a special path in space that satellites use to reach their final positions. It has one point very close to Earth and another point much farther away, at about 35,786 kilometres above the ground. This far point matches the height of geostationary orbits, where satellites stay fixed over one spot on Earth.

Satellites often start in a GTO before moving to their final orbits. Some use special engines to make this change slowly, which takes longer but uses less fuel. The path a satellite takes depends on where it was launched from and the direction it was sent. Changing the angle of the orbit and making it circular usually happen together at the farthest point, which uses less fuel than doing these changes separately.

Other considerations

Launch sites near the equator help when sending satellites into space. For example, Russia’s Baikonur Cosmodrome is far from the equator, while Guiana Space Centre is very close.

Some rockets can send satellites straight to their final orbit. But many put them into a middle step called a geostationary transfer orbit (GTO). The satellite then uses its own engines to reach its final spot. This is often done when launching from the Space Shuttle.

Because many rockets carry several satellites at once, their capacity is usually measured for reaching GTO, not the final orbit. This allows more weight to be carried.

For example, the Delta IV Heavy can carry 14,200 kg to GTO, but only 6,750 kg directly to the final orbit.

Sometimes, more complex paths are used. The Proton-M rocket, launching from Baikonur Cosmodrome in Kazakhstan, uses several steps to place satellites into their final orbits. This helps save fuel because of the launch site’s high latitude.