Orbital station-keeping

[1] The deviation of Earth's gravity field from that of a homogeneous sphere and gravitational forces from the Sun and Moon will in general perturb the orbital plane.

A large part of this compensation can be done by using a frozen orbit design, but often thrusters are needed for fine control maneuvers.

An example of this is the International Space Station (ISS), which has an operational altitude above Earth's surface of between 400 and 430 km (250-270 mi).

To maintain a fixed ground track it is also necessary to make out-of-plane maneuvers to compensate for the inclination change caused by Sun/Moon gravitation.

[3] The delta-v needed to compensate for this perturbation keeping the inclination to the equatorial plane amounts to in the order 45 m/s per year.

Almost all modern satellites are therefore employing a high specific impulse system like plasma or ion thrusters.

Orbits of spacecraft are also possible around Lagrange points—also referred to as libration points—five equilibrium points that exist in relation to two larger solar system bodies.

Station-keeping propellant use can be quite low, facilitating missions that can potentially last decades should other spacecraft systems remain operational.

The three spacecraft—Advanced Composition Explorer (ACE), Solar Heliospheric Observatory (SOHO), and the Global Geoscience WIND satellite—each have annual station-keeping propellant requirements of approximately 1 m/s or less.

[6] However, the precision of trajectory following launch by an Ariane 5 is credited with potentially doubling the lifetime of the telescope by leaving more hydrazine propellant on-board than expected.