thumb|upright=1.0|right|Diagram showing the orientation of a Sun-synchronous orbit (green) at four points in the year. A non-Sun-synchronous orbit (magenta) is also shown for reference. Dates are shown in white: day/month.

A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is an orbit arranged so that, for each revolution of the planet around the Sun, its orbital plane (specifically the longitude of the ascending node) precesses through one complete revolution around the planet.

Applications

A Sun-synchronous orbit is useful for imaging, reconnaissance, and weather satellites, because every time that the satellite is overhead, the surface illumination angle on the planet underneath it is nearly the same. This consistent lighting is a useful characteristic for satellites that image the Earth's surface in visible or infrared wavelengths, such as weather and spy satellites, and for other remote-sensing satellites, such as those carrying ocean and atmospheric remote-sensing instruments that require sunlight. For example, a satellite in Sun-synchronous orbit might ascend across the equator twelve times a day, each time at approximately 15:00 mean local time.

thumb|upright=1.5|right|Sun-synchronous orbit from a top view of the [[Ecliptic|ecliptic plane with local solar time (LST) zones for reference and a descending node of 10:30. The LST zones show how the local time beneath the satellite varies at different latitudes and different points on its orbit.]]

Special cases of the Sun-synchronous orbit are the noon/midnight orbit, where the local mean solar time of passage for equatorial latitudes is around noon or midnight, and the dawn/dusk orbit, where the local mean solar time of passage for equatorial latitudes is around sunrise or sunset, so that the satellite rides the terminator between day and night. Riding the terminator is useful for active radar satellites, as the satellites' solar panels can always see the Sun, without being shadowed by the Earth. It is also useful for some satellites with passive instruments that need to limit the Sun's influence on the measurements, as it is possible to always point the instruments towards the night side of the Earth. The dawn/dusk orbit has been used for solar-observing scientific satellites such as TRACE, Hinode and PROBA-2, affording them a nearly continuous view of the Sun.

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File:Orbit path - Aqua ascending - day.png|As of 2021 Aqua's ascending orbital path crosses equator at 13:30 local time

File:Orbit path - Aqua descending - night.png|As of 2021 Aqua's descending orbital path crosses equator at 01:30 local time

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Orbital precession

A Sun-synchronous orbit is achieved by having the osculating orbital plane precess (rotate) approximately one degree eastward each day with respect to the celestial sphere to keep pace with the Earth's movement around the Sun. This precession is achieved by tuning the inclination to the altitude of the orbit (see Technical details) such that Earth's equatorial bulge, which perturbs inclined orbits, causes the orbital plane of the spacecraft to precess with the desired rate. The plane of the orbit is not fixed in space relative to the distant stars, but rotates slowly about the Earth's axis.

Typical Sun-synchronous orbits around Earth are about in altitude, with periods in the 96–100-minute range, and inclinations of around 98°. This is slightly retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit, and 90° represents a polar orbit. (It can be alternatively said that the rotation of the apse line is minimized.)

Sun-synchronous frozen orbits have also been used around other Solar System bodies, including asteroids. It is possible to design a Sun-synchronous orbit that also naturally tracks the Sun (orients the solar panels towards the Sun).

There is also a concept of "artificial frozen orbits", orbits that take a low amount of continuous thrust to maintain. These allow some effects of a frozen SSO to be gained around a planetary body that does not allow SSOs by its own gravitational parameters.

Other synchronous orbits can also be made frozen. For example, there is a design for a Ganymede-synchronous frozen orbit around Europa.

Earth orbit allocation

The SSO is a limited resource. A "slot" system has been proposed to standardize the orbits in order to minimize the risks of conjunction.

See also

  • Orbital perturbation analysis (spacecraft)
  • Analemma
  • Dyson ring
  • Geosynchronous orbit
  • Geostationary orbit
  • List of orbits
  • Polar orbit
  • Space-based data center
  • World Geodetic System

References

Further reading

  • Sandwell, David T., The Gravity Field of the Earth - Part 1 (2002) (p. 8)
  • Sun-Synchronous Orbit dictionary entry, from U.S. Centennial of Flight Commission
  • NASA Q&A
  • List of satellites in Sun-synchronous orbit