Geostationary transfer orbit
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A geostationary transfer orbit (GTO) is a Hohmann transfer orbit around the Earth between a low Earth orbit (LEO) and a geostationary orbit (GEO). It is an ellipse where the perigee is a point on a LEO and the apogee has the same distance from the Earth as the GEO.
More generally, a geostationary transfer orbit is an intermediate orbit between a LEO and a geostationary orbit.
After a typical launch the inclination of the LEO (the angle between the plane of the orbit and the plane of the equator) is determined by the latitude of the launch site and the direction of launch. The GTO inherits the same inclination. The inclination must be reduced to zero to obtain a geostationary orbit. This is done at the GEO distance because that requires less energy than at LEO. This is because the required delta-v (Δv) for a certain inclination change Δi is directly proportional to orbit velocity v which is lowest in its apogee. The required delta-v for an inclination change in either the ascending or descending node of the orbit is calculated as follows:
Assuming a typical Ariane 5 GTO with a semimajor axis of 24,582 km, the perigee velocity of a GTO is 9.88 km/s while the apogee velocity is at 1.64 km/s.
A launch vehicle can move from LEO to GTO by firing a rocket at a tangent to the LEO to increase its velocity. Typically the upper stage of the vehicle has this function. Once in the GTO, it is usually the satellite itself that performs the conversion to geostationary orbit by firing a rocket at a tangent to the GTO at the apogee. Therefore the capacity of a rocket which can launch various satellites is often quoted in terms of separated spacecraft mass to GTO rather than ditto to GEO. Alternatively the rocket may have the option to perform the boost for insertion into GEO itself. This saves the satellite's fuel, but considerably reduces the separated spacecraft mass capacity.
For example, the capacity (separated spacecraft mass) of the Delta IV Heavy:
- GTO 12 757 kg (185 km x 35,786 km at 27.0 deg inclination), theoretically more than any other currently available launch vehicle (has not flown with such a payload yet)
- GEO 6 276 kg
Usually, insertion into geostationary orbit is performed at the ascending node. This is because most launch sites from which launches into a GTO are performed are located on the northern hemisphere.
In most cases, the spent upper stages of launch vehicles are left behind in the GTO (some are occasionally left in GEO, like the Proton Block DM). If the perigee of the GTO is chosen to be low enough to make atmospheric drag quickly decrease apogee altitude, the upper stage will be no collision threat to the satellites in the geostationary ring. Eventually, it will reenter the atmosphere of the Earth. Most upper stages that are used to bring payloads to a GTO are designed to meet this requirement.
Heavy Lift Launch Vehicles are the only rockets capable of moving heavier satellites into geostationary or geosynchronous orbits. The capability of achieving geostationary transfer orbit is critical to the placement of modern satellites, as well as to the success of space programs going to the Moon, Mars, and the outer parts of the solar system. The reason for this is that the GTO is an orbit cycling between a perigee tangent to LEO and an apogee tangent to a geostationary orbit. At the point where the orbit is tangent to the geostationary orbit, the payload can conduct a controlled burn and insert itself into the geostationary orbit, where it will hold its position 22,240 miles (35,792 kilometres) over a specific spot on the equator. By contrast, geosynchronous orbits have the same period of orbit as the Earth has of rotation (24 hours), but the orbits themselves may be elliptical, and can also be outside of an equatorial orbit.
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