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Sun-synchronous orbit spacecrafts


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Have you ever heard about Sun-synchronous orbit spacecrafts? According to the information I`ve found such name have weather satellites and some kind of scientific satellites. Also, I`ve heard that there are not only weather satellites and scientific satellites that are used in Sun-synchronous orbit. Do you know which kind of sats or maybe other spacecrafts are used for Sun-synchronous orbit?

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SSO is a special near-polar orbit which is primarily used for Earth observation, since you are able to pass over the same spot on Earth at the same local time each day. Imaging, weather, spy -- anything like that. But it's not particularly useful for anything else.

One exception is when you want a solar observation satellite. You can set up an SSO so that the satellite is never in the Earth's shadow and can observe the sun continuously.

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15 hours ago, sevenperforce said:

SSO is a special near-polar orbit which is primarily used for Earth observation, since you are able to pass over the same spot on Earth at the same local time each day.

To be clear, not any SSO has a property that it passes over the same point on Earth at the same time of day. But yeah, you can find Sun-synchronous orbits with periods that are integer fraction of solar day, letting it pass over the same point at exact same time once in several revolutions.

As for a good use, if you are making maps from satellite, having sun as high in the sky as possible is always good. So an SSO that always passes day side during noon can be a good choice for a mapping satellite.

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From how Scott Manley describes sun synchronous orbits, you won't be able to compute them under the normal assumptions of a spherical [bovine] Earth.  The oblateness [equatorial bulge, presumably due to rotation] is needed to pull the satellite into position.

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To add on a bit:

On 2/5/2021 at 12:21 PM, sevenperforce said:

SSO is a special near-polar orbit which is primarily used for Earth observation, since you are able to pass over the same spot on Earth at the same local time each day. Imaging, weather, spy -- anything like that. But it's not particularly useful for anything else.

One exception is when you want a solar observation satellite. You can set up an SSO so that the satellite is never in the Earth's shadow and can observe the sun continuously.

Some IR satellites (eg: IRAS, WISE) use(d) SSO so that the sun was in a consistent place so it could be shielded from.

 

1 hour ago, wumpus said:

From how Scott Manley describes sun synchronous orbits, you won't be able to compute them under the normal assumptions of a spherical [bovine] Earth.  The oblateness [equatorial bulge, presumably due to rotation] is needed to pull the satellite into position.

Yep, you need to torque the orbit around. Hence why you can't do true SSO orbits in stock KSP but need something like Principia.

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8 hours ago, wumpus said:

From how Scott Manley describes sun synchronous orbits, you won't be able to compute them under the normal assumptions of a spherical [bovine] Earth.  The oblateness [equatorial bulge, presumably due to rotation] is needed to pull the satellite into position.

It's a normal assumption that the Earth is a cow?

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8 hours ago, mikegarrison said:

It's a normal assumption that the Earth is a cow?

Yes. To high degree of precision, it is fair to assume that Earth is a spherical cow in vacuum.

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Interesting factoid:

You can also get SSOs around other planets, such as Mars, or Jupiter. But not Venus.

SSOs require oblateness to torque the orbit. Venus barely rotates, so it's a near-perfect sphere. It has barely any oblateness. So SSOs are functionally impossible around Venus.

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3 hours ago, RCgothic said:

Interesting factoid:

You can also get SSOs around other planets, such as Mars, or Jupiter. But not Venus.

SSOs require oblateness to torque the orbit. Venus barely rotates, so it's a near-perfect sphere. It has barely any oblateness. So SSOs are functionally impossible around Venus.

Given the extreme oblateness of Saturn, I wonder if that makes it more difficult or less difficult.

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1 hour ago, sevenperforce said:

Given the extreme oblateness of Saturn, I wonder if that makes it more difficult or less difficult.

Wikipedia suggests that angular precession per orbit is related to the oblateness and the radius of the planet squared, divided by the size of the orbit.

Assuming any low orbit around a planet would need to scale with planet size, this suggests precession is related to first power of planet radius and oblateness. Saturn would therefore cause a lot of precession!

Mercury is also in the "functionally impossible" column with Venus.

Edited by RCgothic
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1 hour ago, RCgothic said:

Wikipedia suggests that angular precession per orbit is related to the oblateness and the radius of the planet squared, divided by the size of the orbit.

Assuming any low orbit around a planet would need to scale with planet size, this suggests precession is related to first power of planet radius and oblateness. Saturn would therefore cause a lot of precession!

Mercury is also in the "functionally impossible" column with Venus.

You forgot the inclination. The reason SSO's work is because you never pass over the poles. This makes it so you are never equidistant from the increase in concentration of mass due to the oblateness, as you would be if passing directly over the poles. Thus, the satellite will always be pulled towards the equator directly north or south of its location as there is a "close" and "far" node. For Saturn, to reduce the increase in precession when compared to Earth, they would set the inclination closer to 90* to decrease the difference in the gravitational pull of the close and far nodes.

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2 hours ago, Meecrob said:

You forgot the inclination. The reason SSO's work is because you never pass over the poles. This makes it so you are never equidistant from the increase in concentration of mass due to the oblateness, as you would be if passing directly over the poles. Thus, the satellite will always be pulled towards the equator directly north or south of its location as there is a "close" and "far" node. For Saturn, to reduce the increase in precession when compared to Earth, they would set the inclination closer to 90* to decrease the difference in the gravitational pull of the close and far nodes.

I didn't forget it, it just wasn't relevant to whether a planet can have SSOs or not.

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