34

Earth's rotation provides considerable horizontal thrust to all launched satellites, nearly 1700km/hour near the Equator. That means considerable fuel savings and generally lower costs, and so most satellites accelerate in the earth rotation direction, adding to that speed.

Of course, satellites at GEO will not travel against the Earth surface, and ones beyond GEO will "apparently" travel "backwards" as Earth's rotation overtakes them, but they still travel in the same direction as Earth rotates.

Are there any satellites currently that do travel in the opposite direction? Of course, their launch would have been significantly more expensive as they'd have to first brake against Earth rotation and then accelerate from zero in the other direction, but that doesn't mean it can't be done. Has it been done?

PearsonArtPhoto
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SF.
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3 Answers3

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As Undo mentioned, the type of orbit is called a retrograde. A pure retrograde orbit is quite rare, in fact, NORAD currently isn't tracking any (unclassified) objects with a 180 inclination (Pure Retrograde). However, there are a fair number of satellites with a retrograde inclination (Over 90 degrees). Of particular note is the Sun Synchronous orbits of about 98 degrees. The largest (Unclassified) inclination of any satellite currently in orbit that I can find any information about appears to be Minisat 01, with an inclination of about 150 degrees. The reasons for this unusual orbit are not explained.

In fact, there's a nice little graph of what inclination space collisions are expected in this presentation, page 5. It should give an idea of how common slight retrograde orbits are.

PearsonArtPhoto
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21

The correct term for this type of orbit is retrograde. Here's a shiny animated thing showing the orbit:

enter image description here

Due to the difficulty to achieve such an orbit, it is rarely used. Most commonly, artificial satellites using this orbit are commercial earth-observing satellites.

Some satellites which use this orbit are Israel's Shavit satellites and two US Future Imagery Architecture satellites.

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    Considering all sun-synchronous orbits are retrograde (as mentioned above), I would hardly say it's "rarely used". –  Jul 29 '13 at 18:54
  • It is rarely used in comparison to 'normal' orbits, right? –  Jul 29 '13 at 19:00
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    Define a "normal" orbit... sun-synchronous orbits are one of the most "popular" classes of low-Earth orbits. –  Jul 29 '13 at 19:27
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    This should not be the accepted answer because of the "rarely used" qualifier. The majority of the spacecraft in low Earth orbit are sun synchronous -- i.e., (slightly) retrograde. – David Hammen Mar 03 '14 at 04:34
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    I picked this answer primarily thanks to explaining the concept clearly. Also, Sun-synchronous satellites are pretty close to polar and only barely retrograde so while they are pretty "popular", they barely qualify. Ones travelling in clearly retrograde direction (inclination above 135 degrees; more of western direction component than northern-southern) are surely quite rare. – SF. Oct 20 '14 at 16:00
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There are satellites in polar orbit. Such an orbit can be effectively retrograde, depending on whether the launch is toward the North or South Pole.

Deer Hunter
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