A satellite orbiting a planet such as Venus that is almost spherical will need an outside push to maintain a Sun-synchronous orbit. Sun-synchronous orbits are possible around other oblate planets, such as Mars. This is slightly retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit, and 90° represents a polar orbit. Typical Sun-synchronous orbits around Earth are about 600–800 km in altitude, with periods in the 96–100- minute range, and inclinations of around 98°. The plane of the orbit is not fixed in space relative to the distant stars, but rotates slowly about the Earth's axis. 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 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.Ī 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. 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. 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. 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. The LST zones show how the local time beneath the satellite varies at different latitudes and different points on its orbit.
Diagram showing a Sun-synchronous orbit from a top view of the ecliptic plane with Local Solar Time (LST) zones for reference and a descending node of 10:30 am.
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