Does Low earth orbit pass through the same point every time?

Submitted by Passeride t3_xw9p7n in askscience

Hello

I'm writing a paper where I touch upon Starlink and their satellites, i know they are in low earth orbit, at 550km heigh and goes around the earth about 15 times a day, so not exactly geostationary orbit.

My question is: does those satellites pass through the same "point" 15 times a day, or is it possible that they "drift" X km east/west with every passing, and what's the word I'm looking for to search for this?

Thanks for all help

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Weed_O_Whirler t1_ir68hk2 wrote

The only orbits which pass over the same part of the Earth each time around are geosynchronous orbits (geostationary orbits are geosynchronous orbits which have a 0 degree inclination- aka, they are directly above the equator). All other orbits will "track across" the globe. The path they take across the globe is referred to as their ground track. There are special satellite orbits called "Earth-Repeat orbits which will, after every so many revolutions, repeat the same ground track. However, these are a special case and take special planning to achieve.

To understand why, you can think of how the satellite orbits- it doesn't care the Earth is rotating. Essentially, it's orbit would not change if the Earth was rotating once every 24 hours (like it does now), or once every 12 or 36. If you were observing the satellite from a "fixed point" relative to the Earth- aka, a point orbiting the Sun with the Earth, but not orbiting the Earth, you would see each satellite repeating it's orbit with the Earth rotating underneath. To see some good pictures of how all of this works, I like this write-up.

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Passeride OP t1_ir6ptng wrote

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Thank you kind and gifted stranger

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karantza t1_ir6aub0 wrote

The answer is a little complicated.

  • Any object in orbit will pass through the same points in its orbit every time, approximately.
  • The Earth is rotating under this path, so except for the special case when the length of the orbit is exactly 24 hours, a different part of the Earth will be under the object each pass.
  • The orbit is not perfect. Orbits precess (probably the word you're looking for) for various reasons, because the Earth pulls on them a little differently due to it not being a perfect sphere, because of the sun and moon's gravity, etc. So while it'll pass through just about the same point (in space, not over the ground) each orbit, that can drift over time.
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Passeride OP t1_ir6pkda wrote

OMG, lightbulb moment. OFC the orbit would only orbit the same place "on earth" if they sync up.

I would assume if the satellites orbit time is divisible by earths rotation time, it syncs up.

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Thank you kind and gifted stranger

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jurc11 t1_ir7gtdb wrote

>I would assume if the satellites orbit time is divisible by earths rotation time, it syncs up.

Right, that's how you get to SSO - sun-synchronous orbits. In terms of Starlink, that's Group 3 at 97.6° inclination. If the orbit is inclined just right for its altitude, it will be sun-synchronous and will appear over the same area at the same time every day. It will precess just right to keep up with the planet below. This is used by imaging sats to have consistent shadows and will be used by Starlink Group 3 to ensure additional peak time coverage.

Here's a plot of altitude vs. inclination, you can see Group 3 at 560km should be inclined to 97.6° to be SSO.

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[deleted] t1_ir6auna wrote

[deleted]

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jurc11 t1_ir7h8fo wrote

Precession, not procession. You made the same mistake at least four times, maybe you haven't noticed it's not procession.

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9998000 t1_ir6bqig wrote

The satellites stay in the same orbit and earth rotates under them. So they appear to drift a bit on each orbit.

They also move with the orbit of earth around the sun, so they don't return to same place in space.

The solar system also cruises through the universe, so it is never in the same spot twice.

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jurc11 t1_ir7igki wrote

One thing I don't see mentioned yet is SpaceX are using precession to separate their launches into separate orbits. They launch 60 sats (numbers slightly simplified) to around 260 km (or whatever it is now), start raising the altitude of 20 of them, whilst keeping the other 40 at insertion altitude. Doing this for a month separates the two groups by a couple of degrees, then they repeat it with the next 20. This way they end up with 3 groups in separated orbits without having to actively do the separation with rocket fuel (and a regular shell has 72 such separate planes/orbits).

The reason this works is that the effect of nodal precession depends on altitude. Sats at different altitudes drift slower/faster.

Precession alone is enough to ensure sats won't visit the same "points" as you have imagined, unless perfectly synced (and that's then related to SSO orbits, as I've mentioned in another comment).

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