Following are the important characteristics and uses of the mentioned orbits –
Molniya orbit –
- From the Russian word for ‘lightning,’ it is a 12-hour highly elliptical and inclined orbit
- Because of the Earth's size, geostationary satellites are not visible north of 55 degrees, which includes much of Russia, so Molniya orbits fill the gap
- Their great eccentricity means they travel slowly at apogee, allowing long periods of contact
- The 12-hour period means there is one apogee period over Russia, and another over North America, making them also useful as spy satellites for both sides. The orbital inclination is 63.4 degrees, which results in zero precession of the major axis of the orbit, so the apogee remains fixed above the northern hemisphere.
- Conclusively, a Molniya orbit is used primarily by Russia for satellite communication.
A Molniya orbit
(L-R) A Sun-synchronous orbit and a Ggeostationary orbit
- Sun-synchronous orbit –
- It is a circular, near polar, and slightly retrograde orbit with inclinations of 90+ degrees
- Orbits are chosen so precession of the orbital plane matches the Earth's motion around the Sun. When a sun-synchronous satellite passes southbound over the daylight side of the Earth, it is always late morning, which is an optimum time for photography. (Early morning haze has typically burned off and afternoon cumulus has not built up)
- Orbits are also chosen to pass over the same tracks at regular intervals, generally 2-3 weeks. Sun-Synchronous orbits are used by most Earth observation systems like Landsat.
- Geostationary orbit –
- It is an equatorial circular orbit. There is only one geostationary orbit
- A satellite in this orbit keeps pace with Earth's rotation and appears to remain stationary in the sky.
- An interesting factoid is that at the equinoxes, these satellites have brief blackout periods when they are in line with the Sun
- It is used in weather satellites and flight monitoring.
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