In astrodynamics, orbital station-keeping is keeping a spacecraft at a fixed distance from another spacecraft or celestial body. It requires a series of orbital maneuvers (reboosts) made with thruster burns to keep the active craft in the same orbit as its target. For many low Earth orbit satellites, the effects of non-Keplerian forces, i.e. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, gravitational forces from Sun/Moon, solar radiation pressure and air drag, must be counteracted.

For spacecraft in a halo orbit around a Lagrange point, station-keeping is even more fundamental, as such an orbit is unstable; without an active control with thruster burns, the smallest deviation in position or velocity would result in the spacecraft leaving orbit completely.

Earth observation spacecraft

For Earth observation spacecraft typically operated in an altitude above the Earth surface of about 700 – 800 km the air-drag is very faint and a re-entry due to air-drag is not a concern. But if the orbital period should be synchronous with the Earth's rotation to maintain a fixed ground track, the faint air-drag at this high altitude must also be counter-acted by orbit raising maneuvers in the form of thruster burns tangential to the orbit. These maneuvers will be very small, typically in the order of a few mm/s of delta-v. If a frozen orbit design is used these very small orbit raising maneuvers are sufficient to also control the eccentricity vector.

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. These are executed as thruster burns orthogonal to the orbital plane. For Sun-synchronous spacecraft having a constant geometry relative to the Sun, the inclination change due to the solar gravitation is particularly large; a delta-v in the order of 1–2 m/s per year can be needed to keep the inclination constant.

Geostationary orbit

thumb|upright=1.4|Inclined orbital planes

For geostationary spacecraft, thruster burns orthogonal to the orbital plane must be executed to compensate for the effect of the lunar/solar gravitation that perturbs the orbit pole with typically 0.85 degrees per year. The delta-v required to compensate for this perturbation and maintain the desired inclination relative to the equatorial plane is approximately 50 m/s per year. This part of the GEO station-keeping is called North-South control.

The East-West control is the control of the orbital period and the eccentricity vector performed by making thruster burns tangential to the orbit. These burns are then designed to keep the orbital period perfectly synchronous with the Earth rotation and to keep the eccentricity sufficiently small. Perturbation of the orbital period results from the imperfect rotational symmetry of the Earth relative the North/South axis, sometimes called the ellipticity of the Earth equator. The eccentricity (i.e. the eccentricity vector) is perturbed by the solar radiation pressure. The fuel needed for this East-West control is much less than what is needed for the North-South control.

To extend the life-time of geostationary spacecraft with little fuel left one sometimes discontinues the North-South control only continuing with the East-West control. As seen from an observer on the rotating Earth the spacecraft will then move North-South with a period of 24 hours. When this North-South movement gets too large a steerable antenna is needed to track the spacecraft. An example of this<!-- when did this condition apply to the Artemis sat? does it still, in 2016? --> is Artemis.

To save weight, it is crucial for GEO satellites to have the most fuel-efficient propulsion system. Almost all modern satellites are therefore employing a high specific impulse system like plasma or ion thrusters.

Lagrange points

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. For example, there are five of these points in the Sun-Earth system, five in the Earth-Moon system, and so on. Spacecraft may orbit around these points with a minimum of propellant required for station-keeping purposes. Two orbits that have been used for such purposes include halo and Lissajous orbits.

Earth-Sun —approximately 1.5 million kilometers from Earth in the anti-sun direction—is another important Lagrange point, and the ESA Herschel space observatory operated there in a Lissajous orbit during 2009–2013, at which time it ran out of coolant for the space telescope. Small station-keeping orbital maneuvers were executed approximately monthly to maintain the spacecraft in the station-keeping orbit.

The James Webb Space Telescope will use propellant to maintain its halo orbit around the Earth-Sun L2, which provides an upper limit to its designed lifetime: it is being designed to carry enough for ten years. 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.

The CAPSTONE orbiter and the planned Lunar Gateway is stationed along a 9:2 synodically resonant Near Rectilinear Halo Orbit (NRHO) around the Earth-Moon L2 Lagrange point.

See also

  • Delta-v budget
  • Orbital perturbation analysis
  • Reboost
  • Teleoperator Retrieval System (robotic device for attaching to another spacecraft and boosting or changing its orbit)

References

  • Station-keeping at the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
  • XIPS Xenon Ion Propulsion Systems
  • Jules Verne boosts ISS orbit Jules Verne boosts ISS orbit (report from the European Space Agency)