Space Shuttle / ISS orientation during orbit - how is it maintained? (orbital mechanics question)

[JoeSchmuckatelli]

I've read that as the shuttle orbited the earth it kept the bay doors toward the surface and the bottom of the shuttle toward the sun.  I believe the reason for this was to keep the thicker shielded bottom of the shuttle between the crew and the giant radiation hazard we call the sun. 

Similarly, I've read that ISS cupola faces the earth to give the crew a view of home, and I've seen night and day pictures of the earth taken by crew of the ISS. 

The question I'm curious about is this: are the bay doors of the shuttle and the cupola of the ISS oriented on the surface during a full orbit? (i.e. Once in orbit, does the craft maintain its orientation to the surface like a plane in flight, and/or does it require gyros to keep the window /bay facing the surface?) 

The problem I have is that part about keeping the bottom of the shuttle between the crew and the sun.  I'm wondering if the shuttle bay doors were open to the day side during the sunward part of the orbit, but open to the darkness of space while on the night side of the planet. 

Ultimately, what I would like to know is whether keeping one side of a spacecraft facing the surface requires gyros (or whatever), or whether, once in orbit a craft would 'naturally' fly 1/4 of the orbit backwards, then 1/4 nose to the earth, then 1/4 forward and the final 1/4 with the tail pointed to the earth. 

Thanks in advance for any info! 

[DerekL1963]

1 hour ago, JoeSchmuckatelli said:

I've read that as the shuttle orbited the earth it kept the bay doors toward the surface and the bottom of the shuttle toward the sun.  I believe the reason for this was to keep the thicker shielded bottom of the shuttle between the crew and the giant radiation hazard we call the sun. 

The bottom of the Shuttle was not noticeably better at protecting the crew from ionizing radiation than the top...  (And except during a flare the main problem is cosmic rays, not from the Sun.)  The Shuttle relied mostly on being beneath the Van Allen Belts and inside the Earth's magnetosphere to protect the crew from radiation.

The Shuttle's orientation at any given time (it varied) was chosen around three factors:  Maximizing the efficiency of the radiators mounted to the back of the bay doors.  Thermal control and field of view for cargo in the bay.  Protecting the radiators and the equipment behind the bay walls and floor (which were made of Beta cloth) from debris strikes.

 

1 hour ago, JoeSchmuckatelli said:

The question I'm curious about is this: are the bay doors of the shuttle and the cupola of the ISS oriented on the surface during a full orbit?

That depends on the orientation mode chosen.

[JoeSchmuckatelli]

Thanks - that answers some of my questions. 

I guess what I want to know is how an orbiting object behaves differently from a plane.

I know that a plane in suborbital flight is kept aloft by lift, and as it travels around the world its bottom will always be pointed 'down' / parallel to the surface (level flight).  But a space craft is in a ballistic free fall, going fast enough to keep missing the planet.  As such, I'm trying to figure out whether it would circle the earth like a plane (nose forward the whole time) or if it would maintain the orientation it had when it entered orbit - ie if it entered orbit with the belly towards the sun - would it keep that orientation throughout the orbit? 

Where you write 'depends on the orientation mode chosen' - I can infer that mission control could do whatever they wanted - but what is the 'natural state'?  Or what would a defunct satellite or even a refrigerator do once in orbit? Would it be tidally locked or show the same face only when in the same part of the orbit? 

 

Edited October 23, 2018 by JoeSchmuckatelli

[DerekL1963]

1 hour ago, JoeSchmuckatelli said:

As such, I'm trying to figure out whether it would circle the earth like a plane (nose forward the whole time) or if it would maintain the orientation it had when it entered orbit - ie if it entered orbit with the belly towards the sun - would it keep that orientation throughout the orbit? 

Until it was disturbed by an external force it will retain it's existing (inertial) orientation (presuming it has no pre-existing rates).  That is, if the nose is pointed towards Polaris and belly towards Betelgeuse it will maintain that orientation at least for a little while.  But having no rates is rare and external forces abound; gravity, Earth's magnetic field, the solar wind, drag (even though it's a near vacuum)...

To stay nose forward, the vehicle has to rotate at a rate equivalent to it's orbital period.  That requires active control to establish and maintain over any length of time.
 

1 hour ago, JoeSchmuckatelli said:

Where you write 'depends on the orientation mode chosen' - I can infer that mission control could do whatever they wanted - but what is the 'natural state'?  Or what would a defunct satellite or even a refrigerator do once in orbit? Would it be tidally locked or show the same face only when in the same part of the orbit? 

There is no "natural state" as there is no airflow or anything to force it towards a 'natural' state.  Generally, absent active stabilization, AFAIK anything in orbit will eventually tumble.

[JoeSchmuckatelli]

Thanks! 

This part is what I was looking for.  It makes sense. 

 

... Although, when I started reading about Minkowski space my head twisted up and I began to wonder whether curved space would make an object in orbit follow the curve... Ah heck I can't describe what I was thinking! 

 

Gracias! 

 

[Dafni]

Just to add the terminology for completeness sake:

NASA calls the attitude control mode where the nose is always pointing the same way relative to the ground Orbital Rate Attitude. Examples are most orbiting things actually: Comm sats, spy sats, geo science sats, the Apollo spacecraft etc.

The mode that has the nose always pointing in the same direction relative to the stars is called Inertial Attitude. Famous example is the Hubble telescope.

[YNM]

14 hours ago, JoeSchmuckatelli said:

The problem I have is that part about keeping the bottom of the shuttle between the crew and the sun. 

If you've played KSP with PersistentRotation you know full well it is possible to keep a spinning object spinning. You just choose the right spinning rate.

Rationale :

Imagine a spacecraft in geostationary orbit : with respect to a distant star, the prograde direction is always moving. However, with respect to the point on Earth's surface, the prograde direction of the spacecraft is stationary. Hence, if you've made your spacecraft rotation to be zero wrt the prograde direction, it'll have zero rotation rate wrt the Earth's surface; however, it is rotating wrt the star's direction. The reverse apply if you have fixed it with the star's direction.

Replace the earth's rotation rate with your orbital revolution rate (angular speed), and you can instantly manage to stay perpendicular/parallel/whatever wrt the Earth's surface beneath you (case with ISS). Or you can choose to stay pointed to one distant star (Hubble).

In fact, you can maintain a rotation rate such that :

- Your solar panels keep pointed to the Sun;

- Your camera keep pointed to Earth's surface beneath you perpendicularly.

You'll see them on spacecrafts in SSO orbits.

Stationkeeping is required not to make sure it doesn't go into another mode, but it's done to make sure it isn't perturbed off by either atmospheric drag or gravity gradients or other perturbations.

Edited October 24, 2018 by YNM

[JoeSchmuckatelli]

7 hours ago, Dafni said:

Just to add the terminology for completeness sake:

NASA calls the attitude control mode where the nose is always pointing the same way relative to the ground Orbital Rate Attitude. Examples are most orbiting things actually: Comm sats, spy sats, geo science sats, the Apollo spacecraft etc.

The mode that has the nose always pointing in the same direction relative to the stars is called Inertial Attitude. Famous example is the Hubble telescope.

Thanks!  Getting the terminology correct is always a better way to phrase a question (hell even finding out what the terminology is is crazy hard sometimes!) 

[JoeSchmuckatelli]

3 hours ago, YNM said:

If you've played KSP with PersistentRotation you know full well it is possible to keep a spinning object spinning.... .

So... Yes.  And yes! 

 

While I haven't played KSP for a while, I've known that it's absolutely possible to keep a space object oriented however we want it. I've also presumed that orbiting objects maintain their inertial attitude. 

What I'm struggling with is more abstract. 

I saw the full eclipse this year. It got me to thinking about the Eddington experiment that confirms Einstein - Minkowski space time for most folks. Under this framework gravity isn't a force like magnetism but a curvature of spacetime where objects in space (including light) move along 4 dimensional geodesics.  In other words, orbits around massive objects are defined /caused by the the curvature of spacetime. 

So... Okay. 

But. 

If this is truly correct... Why is it that things in orbit maintain the inertial attitude?

The earth maintains its internal attitude around the sun (hence the seasons) just as a refrigerator would around the earth. 

If spacetime is truly curved - how is inertial attitude maintained at all?  Why doesn't an orbiting object move more like a car on a track? (or orbital rate attitude?) 

[Nibb31]

24 minutes ago, JoeSchmuckatelli said:

If this is truly correct... Why is it that things in orbit maintain the inertial attitude?

The earth maintains its internal attitude around the sun (hence the seasons) just as a refrigerator would around the earth.

It doesn't. It rotates (hence the day/night cycle).

So does Mercury, or the Moon. They rotate, but their rotation period is the same rate as their orbital period. This can be due to tidal lock and mass irregularities, which seems to be a fairly common occurrence, since we have multiple examples of it in our solar system.

Quote

If spacetime is truly curved - how is inertial attitude maintained at all?  Why doesn't an orbiting object move more like a car on a track? (or orbital rate attitude?) 

I don't see how space-time curvature comes into play here. I don't think there are any objects that do not have some kind of rotation in them.

Edited October 24, 2018 by Nibb31

[DerekL1963]

46 minutes ago, JoeSchmuckatelli said:

If this is truly correct... Why is it that things in orbit maintain the inertial attitude?

The earth maintains its internal attitude around the sun (hence the seasons) just as a refrigerator would around the earth. 

Inertial attitude is defined with respect to the stars  (which is why I used them in my example).  The Earth's inertial attitude is not fixed...  As to why they maintain it (at least over short period)?  Newton's first Law of Motion - "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it".  (It's sometimes formulated as "every object at rest", and that's the case here.)  They don't stay fixed very long because, as I said, there are plenty of sources of external forces.

[JoeSchmuckatelli]

3 hours ago, Nibb31 said:

It doesn't. It rotates (hence the day/night cycle).

So does Mercury, or the Moon. They rotate, but their rotation period is the same rate as their orbital period. This can be due to tidal lock and mass irregularities, which seems to be a fairly common occurrence, since we have multiple examples of it in our solar system.

I don't see how space-time curvature comes into play here. I don't think there are any objects that do not have some kind of rotation in them.

You are correct and I misspoke... With the earth's spin it retains the inclination angle through it's orbit (hence the seasons). 

But as I try to imagine curved space I would guess that the inclination angle might behave differently - rather than the northern hemisphere angled to the sun in summer and away in winter... That following the curve of spacetime would keep one hemisphere angled towards the sun year round. 

Edited October 24, 2018 by JoeSchmuckatelli

[JoeSchmuckatelli]

3 hours ago, DerekL1963 said:

Inertial attitude is defined with respect to the stars  (which is why I used them in my example).  The Earth's inertial attitude is not fixed...  As to why they maintain it (at least over short period)?  Newton's first Law of Motion - "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it".  (It's sometimes formulated as "every object at rest", and that's the case here.)  They don't stay fixed very long because, as I said, there are plenty of sources of external forces.

Yes - which is why earth's inclination angle keeps us oriented to Polaris regardless of where we are in the orbit. 

I can envision this with gravity resulting from a particle (like a boson), but if what we perceive as gravity is merely an effect of the curvature of spacetime... Why doesn't the earth's axial tilt precess throughout the year? 

If the earth is traveling in a straight line - but the space it is traveling through is curved, wouldn't the same hemisphere be angled towards the sun the whole time? 

[JoeSchmuckatelli]

3 hours ago, Nibb31 said:

It doesn't. It rotates (hence the day/night cycle).

So does Mercury, or the Moon. They rotate, but their rotation period is the same rate as their orbital period. This can be due to tidal lock and mass irregularities, which seems to be a fairly common occurrence, since we have multiple examples of it in our solar system.

I don't see how space-time curvature comes into play here. I don't think there are any objects that do not have some kind of rotation in them.

It's defining gravity as a warping of spacetime that I'm struggling with. 

Tidal locking makes sense to me given mass irregularities if gravity is a particle effect, as well as if it's an effect of warped spacetime.  But once you add spin to an object the examples (ie planets) seem to conform more to the particle idea than the curved space idea. 

I'm certain that I'm not the first to see this... And I hope someone can help me understand why the apparent discrepancy is not a discrepancy at all 

[JoeSchmuckatelli]

FYI - as I understand the Eddington observation as a confirmation of Einstein / Minkowski (curved spacetime)... Photons (aka light) are massless and travel in a straight line and could not be affected by gravity if gravity is created by a particle. 

But if gravity is a curvature of spacetime, a massive object (like a sun, black hole or galaxy) will warp spacetime and while the photon travels in a straight line - the space through which it travels is curved, giving rise to observed phenomenon such as gravitational lensing. 

It's trying to correlate the gravitational lensing idea with what we observe with planetary orbits that's driving me nuts 

[DerekL1963]

1 hour ago, JoeSchmuckatelli said:

Yes - which is why earth's inclination angle keeps us oriented to Polaris regardless of where we are in the orbit. 

No - because the Earth isn't stable in the inertial reference frame and as a result, Polaris being the North Star is only a temporary thing.  A couple of millennia ago it was Kochab.  A couple of millennia hence it will be Gamma Cephei.

[YNM]

7 hours ago, JoeSchmuckatelli said:

If this is truly correct... Why is it that things in orbit maintain the inertial attitude?

Because of Newton's Law. And no it's not necessarily inertial attitude, it can be non-inertial as well (though you can transform it into the inertial frame).

As I said, there is an effect due to gravity gradients (you can count this as being under relativity... or just tidal forces) but they're very small (but still enough to be worried of and to keep some propellants to ensure you don't go into the mode). In fact, if you want to expend no effort in maintaining the attitude at all, just orient yourself such that the gravity gradient will stabilize your attitude wrt. Earth's surface directly beneath you. (gravity gradient stabilization)

[JoeSchmuckatelli]

58 minutes ago, DerekL1963 said:

No - because the Earth isn't stable in the inertial reference frame and as a result, Polaris being the North Star is only a temporary thing.  A couple of millennia ago it was Kochab.  A couple of millennia hence it will be Gamma Cephei.

Quibble - the precession is about 26,000 years, so while I grant you are correct it does not resolve my issue. 

If you think about those coin donation funnels; if you were able to get a spinning top to drop out of the chute it would generally follow the same path as the coin, and the axial tilt would precess with every orbit.  But the earth's axial tilt does not precess with every orbit.  

Wouldn't it, though if it's orbit around the sun were a result of curved spacetime? 

[DerekL1963]

6 minutes ago, JoeSchmuckatelli said:

the earth's axial tilt does not precess with every orbit. 

Yes, the Earth's axial tilt does precess every orbit - if it didn't, then the pole star wouldn't change.  What it doesn't do is precess at the same rate as it's orbit.  (And neither will the top - that high a rate or angular change will cause it to tumble.)

And none of this has anything to do with curved space-time.

Edited October 24, 2018 by DerekL1963

[JoeSchmuckatelli]

5 minutes ago, DerekL1963 said:

Yes, the Earth's axial tilt does precess every orbit - if it didn't, then the pole star wouldn't change.  What it doesn't do is precess at the same rate as it's orbit.  (And neither will the top - that high a rate or angular change will cause it to tumble.)

And none of this has anything to do with curved space-time.

Hmmmm. Okay... I can accept that. 

 

The bold part of your response has my attention:

 

Why does orbital mechanics have nothing to do with curved spacetime? 

[DerekL1963]

8 minutes ago, JoeSchmuckatelli said:

Why does orbital mechanics have nothing to do with curved spacetime? 

Because it doesn't, in the same way that the flavor of tomatoes has nothing to do with the color of begonias.  Orbital mechanics is Newtonian mechanics, space-time is Einsteinian physics - different mathematical constructs covering (and describing) different phenomena.  They only overlap when the curvature is high (such as deep in the Sun's gravitation well - I.E. Mercury's orbit) or other special circumstances.  At the level we're discussing things (attitudes in orbit, etc...), only Newtonian mechanics are applicable.

[JoeSchmuckatelli]

Hmmmm.  Gonna have to think about this for a bit. 

So... Are we using Newtonian physics for even far away missions like New Horizons? 

I remember reading a Scientific American article years ago about utilizing the geodesic for ultra low energy transfers between planets... I figured that meant we were using Einstein's theories to figure out orbital mechanics - but what you write also jibes with something else I've read about Newton's work being accurate enough for everyday physics problems. (by which I thought the writer meant terrestrial ballistics - but not interplanetary space flight). 

 

So what makes Mercury a special circumstance? 

EDIT: wait... I remember reading something about the transit of mercury being a big issue: I'll start there. 

 

Thanks for the discussion /answers this far! 

Edited October 25, 2018 by JoeSchmuckatelli

[DerekL1963]

55 minutes ago, JoeSchmuckatelli said:

So... Are we using Newtonian physics for even far away missions like New Horizons? 

Yes.
 

56 minutes ago, JoeSchmuckatelli said:

So what makes Mercury a special circumstance? 

I explain it in the dozen or so words right before Mercury.

[JoeSchmuckatelli]

Heh my edit crossed your post.  I'll read more about this and return better informed. 

 

Again, thank you - finding this very informative 

[kerbiloid]

1 hour ago, JoeSchmuckatelli said:

Are we using Newtonian physics for even far away missions like New Horizons? 

Especially for them.
The farther from gravity sources - the newtonianer is mechanics.