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rugbat t1_it46p5o wrote

That doesn't make sense. A geostationary orbit, by definition, cannot be retrograde.

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Tractorhash t1_it4bcrh wrote

It has the same velocity as geostationary objects, just in the opposite direction.

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OrganicGrownie t1_it4qh6g wrote

Geostationary means the same point on earth is in the same spot, or more specifically the satellite is in the same spot in the sky all the time relative to earth. Geosynchronous means the same spot is in the same position at the same time every day. Usually geosynchronous orbits require some amount of inclination and with inclination and retrograde orbit would make it really difficult to intercept anything as you'd be in a different plane from your "target" in every orbit. Further decreasing your odds of intercept.

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IAmBadAtInternet t1_it4gkkm wrote

It would be the same orbit but in reverse direction. So it would see everything in Geosync fly by at double velocity every 12 hours. Really good way to survey everything in geosync and possibly to attack them.

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_hester_ t1_it4n69l wrote

You're getting tied up in an earth orientation reference frame. Geostationary satellites orbit the earth with enough velocity relative to earth to sit over the same spot on the ground. Which means it has to orbit the planet once every 24ish hours. In an inertial frame, that's significant amounts of velocity. Going in the opposite direction means it will be traveling quite rapidly over the ground and still maintain a stable orbit at that altitude (or semimajor axis).

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GiraffeWithATophat t1_it4s2d3 wrote

Thank you! I was stuck on that too. I feel kind of dumb now.

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_hester_ t1_it4uflu wrote

I've been hanging out with aerospace engineers for a few years. But, even with basic physics you need to remember: all velocity is relative to a point of reference. When dealing with orbits it gets so complicated that we have to be mentally agile enough to accept that there are many useful reference frames that all make sense depending on what you're observing. Most humans default to thinking in terms of ECEF reference frame so it can be hard to think of other frames and how the relative numbers are affected.

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szpaceSZ t1_it68mfj wrote

But it won't be in geostationary orbit.

It will be at the same altitude, as geostationary/geosynchronous orbits, but it won't be either

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_hester_ t1_it7a92d wrote

Okay, I can understand where you are coming from. In some science communities, we end up with shorthand definitions for concepts that diverge from common definitions. There's even a term for this phenomenon: "jargon." When dealing with aerospace people, the rough properties of orbits are grouped into orbital planes. We don't really have a more generic term for the orbital plane where it requires little energy to maintain a stable progression that matches earth's rotation. In the 1960's we might have called it the "Clarke Belt." But I have never heard any aerospace person use that term. We all refer to it as the "geo belt" because that's the altitude where the biggest advantage is it is easier to remain stationary over the ground. You can also be stationary over the ground at any other altitude given enough energy. If someone is talking about the science or engineering of orbits, "geostationary" really only implies an altitude that exhibits some beneficial properties and not the more understood use of the word "stationary."

In this article, the author is using the term as in-community jargon. To differentiate this type of satellite motion, the author chose to qualify it with the term "retrograde" which implies an inverse motion.

In fact, I am currently working on a mission that will orbit the planet in the geostationary belt, but it will intentionally not match the rotation of the planet for most of its mission life.

Maybe as ASAT tech and other non-traditional mission profiles become more common, the community can eschew the "stationary" part of the term to better describe that orbital plane.

Also, don't let an aerospace person hear you refer to geostationary and geosynchronous as the same. I also made that mistake early on.

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szpaceSZ t1_it7pkcs wrote

> geostationary and geosynchronous

But exactly because even these are not considered the same you can't call a retrograde one with the same orbital distance either.

Those differ from each other by inclination and eccentricity.

And geostationary is the one specific geosynánc orbit with inclination 0, and e = 0. By definition.

Retrograde has inclination 180°.

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Bubbagumpredditor t1_it471yk wrote

No, but it could be the same height and going the other direction. But I'm not sure why that would be something you want to do?

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[deleted] t1_it49aqc wrote

It's for monitoring other satellites & debris in & near geostationary orbit. The US has satellites that do the same thing, but they are in prograde orbits so it takes a long time for them to catalog everything in GEO.

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UnoChance t1_it4ar1l wrote

Doing it in retro would make it near impossible to safely catalog anything. They just want to have the threat of it being there looming.

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[deleted] t1_it4c4yo wrote

They probably mean just above or below GEO. Such a satellite, equipped with radar, would catalog all satellites & debris in GEO every 12 hours.

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UnoChance t1_it4cl9o wrote

Well above or below wouldn't work. If you mean further out then that'd be a bad idea since that's where most graveyard orbits are and if you mean closer in then sure that's possible but positions of GEO satellites do not require space-based detection. There is really no point in doing this unless you were operating like gssap

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[deleted] t1_it4d9j6 wrote

Above as in higher altitude, not a different inclination. If space based detection isn't necessarily, why does the US do it?

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UnoChance t1_it4dvw9 wrote

"More accurate tracking and characterization of man-made orbiting objects." You can do a lot more at ~0 relative velocity versus ~6 kps though

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[deleted] t1_it4ex78 wrote

Not necessarily. Taking multiple measurements from different angles can be more useful than sitting still and staring at it all day.

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UnoChance t1_it4f5r7 wrote

RPO is not a static operation Edit: to expand, that article you sent says that gssap conducts RPO meaning it moves along the belt as you mentioned. Since it is so close it can actually look from more angles in a safer manner than a retro orbit

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phryan t1_it5fbwf wrote

The relative velocity at GEO altitude but in the opposite direction would be around 6km/s with very brief observation periods which would make determining anything about an object difficult. Going 'with the flow' so to speak would take longer to see everything but relative velocity is lower and longer observation periods, which makes determining an object's orbit much easier.

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