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BlackFive #1 Posted Mar 24 2020 - 17:11

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Just a random question about physics... and not game physics.

 

What happens to a 'dead' satellite in orbit?

 

See the figure below:

 

Orbit Question.jpg

 

i.e. WITHOUT any active correction, would a satellite in orbit follow the path of Fig 1, or Fig 2?  (Or... neither, and if so, why?)

 

-- Presuming no atmospheric drag: I'm struggling with the behavior of an inertial mass in orbit... like how would a 'dead' satellite behave.

 

Thanks to anyone willing to comment!

 

EDIT!  Please note: this is not a simple "How do spaceships / satellites work" question!  


Edited by BlackFive, Mar 24 2020 - 17:25.


GeorgePreddy #2 Posted Mar 24 2020 - 17:20

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Spin stabilization and three-axis stabilization are two methods that are used to orient satellites. With spin stabilization, the entire spacecraft rotates around its own vertical axis, spinning like a top. This keeps the spacecraft's orientation in space under control. The advantage of spin stabilization is that it is a very simple way to keep the spacecraft pointed in a certain direction. The spinning spacecraft resists perturbing forces, which tend to be small in space, just like a gyroscope or a top. Designers of early satellites used spin-stabilization for their satellites, which most often have a cylinder shape and rotate at one revolution every second. A disadvantage to this type of stabilization is that the satellite cannot use large solar arrays to obtain power from the Sun. Thus, it requires large amounts of battery power. Another disadvantage of spin stabilization is that the instruments or antennas also must perform “despin” maneuvers so that antennas or optical instruments point at their desired targets. Spin stabilization was used for NASA's Pioneer 10 and 11 spacecraft, the Lunar Prospector, and the Galileo Jupiter orbiter.

 

With three-axis stabilization, satellites have small spinning wheels, called reaction wheels or momentum wheels, that rotate so as to keep the satellite in the desired orientation in relation to the Earth and the Sun. If satellite sensors detect that the satellite is moving away from the proper orientation, the spinning wheels speed up or slow down to return the satellite to its correct position. Some spacecraft may also use small propulsion-system thrusters to continually nudge the spacecraft back and forth to keep it within a range of allowed positions. Voyagers 1 and 2 stay in position using 3-axis stabilization. An advantage of 3-axis stabilization is that optical instruments and antennas can point at desired targets without having to perform “despin” maneuvers.

 

 

 

 

 

 

 

 

 



MagillaGuerilla #3 Posted Mar 24 2020 - 17:22

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Google is your friend :  https://youtu.be/IC1JQu9xGHQ

BlackFive #4 Posted Mar 24 2020 - 17:22

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View PostGeorgePreddy, on Mar 24 2020 - 17:20, said:

Spin stabilization and three-axis stabilization are two methods that are used to orient satellites. With spin stabilization, the entire spacecraft rotates around its own vertical axis, spinning like a top. This keeps the spacecraft's orientation in space under control. The advantage of spin stabilization is that it is a very simple way to keep the spacecraft pointed in a certain direction. The spinning spacecraft resists perturbing forces, which tend to be small in space, just like a gyroscope or a top. Designers of early satellites used spin-stabilization for their satellites, which most often have a cylinder shape and rotate at one revolution every second. A disadvantage to this type of stabilization is that the satellite cannot use large solar arrays to obtain power from the Sun. Thus, it requires large amounts of battery power. Another disadvantage of spin stabilization is that the instruments or antennas also must perform “despin” maneuvers so that antennas or optical instruments point at their desired targets. Spin stabilization was used for NASA's Pioneer 10 and 11 spacecraft, the Lunar Prospector, and the Galileo Jupiter orbiter.

 

With three-axis stabilization, satellites have small spinning wheels, called reaction wheels or momentum wheels, that rotate so as to keep the satellite in the desired orientation in relation to the Earth and the Sun. If satellite sensors detect that the satellite is moving away from the proper orientation, the spinning wheels speed up or slow down to return the satellite to its correct position. Some spacecraft may also use small propulsion-system thrusters to continually nudge the spacecraft back and forth to keep it within a range of allowed positions. Voyagers 1 and 2 stay in position using 3-axis stabilization. An advantage of 3-axis stabilization is that optical instruments and antennas can point at desired targets without having to perform “despin” maneuvers.

 

 

 

 

 

 

 

 


Thanks -- but I get all that.  It's pretty easy to understand that the use of gyroscopes and other 'active' control systems can allow the satellite owner control the orientation.  The question is: what happens to a dead / unguided object with no spin, etc.?

 

EDIT - I realize my OP was insufficiently worded; going back to make it clearer!  Thanks for the initial responses!


Edited by BlackFive, Mar 24 2020 - 17:26.


GeorgePreddy #5 Posted Mar 24 2020 - 17:31

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View PostBlackFive, on Mar 24 2020 - 13:22, said:


Thanks -- but I get all that.  It's pretty easy to understand that the use of gyroscopes and other 'active' control systems can allow the satellite owner control the orientation.  The question is: what happens to a dead / unguided object with no spin, etc.?

 

"... spacecraft resists perturbing forces, which tend to be small in space"

 

So, if "perturbing forces", even small ones, exist in space, then a "dead' satellite would most certainly not maintain its originally desired orientation for very long after becoming "dead".  This would be far more true for actively corrected satellites like you are interested in (those that are designed to maintain an orientation "facing" a certain direction rather than spinning).

 

How it would de-orient and by how much would depend on what "perturbing forces" it actually suffers.

 

Perturbing Forces in Space:

 

https://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv4/IAA-B4-1308P.pdf

 

 

 

 



Christojojo #6 Posted Mar 24 2020 - 17:39

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View PostBlackFive, on Mar 24 2020 - 11:11, said:

Just a random question about physics... and not game physics.

 

What happens to a 'dead' satellite in orbit?

 

See the figure below:

 

Orbit Question.jpg

 

i.e. WITHOUT any active correction, would a satellite in orbit follow the path of Fig 1, or Fig 2?  (Or... neither, and if so, why?)

 

-- Presuming no atmospheric drag: I'm struggling with the behavior of an inertial mass in orbit... like how would a 'dead' satellite behave.

 

Thanks to anyone willing to comment!

 

EDIT!  Please note: this is not a simple "How do spaceships / satellites work" question!  

 

If I may assume this is a simple momentum/ Newtonian question. Without corrective action, it would be fig 2. Simply because it is not being oriented towards the earth's surface. That is unless the satellite has an ever so slight revolution of one revolution per orbit. I would also point out that in the diagram that the Earth is the pulling force for the orbit but not the rotation of the satellite.



BlackFive #7 Posted Mar 24 2020 - 17:47

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View PostChristojojo, on Mar 24 2020 - 17:39, said:

 

If I may assume this is a simple momentum/ Newtonian question. Without corrective action, it would be fig 2. Simply because it is not being oriented towards the earth's surface. That is unless the satellite has an ever so slight revolution of one revolution per orbit. I would also point out that in the diagram that the Earth is the pulling force for the orbit but not the rotation of the satellite.


Well - the underlying question is both Newtonian and Einsteinian...  i.e. in Newtonian physics, the body at rest stays at rest, thus Fig 2.  But if 'Curved Space' is the answer... why doesn't Fig 1 describe the orbit?



stevezaxx #8 Posted Mar 24 2020 - 17:48

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there is also something called "Gravity gradient stabilization" where a satellite that is "long enough" will tend to orient itself with it's long axis pointed toward the Earth (center of orbit). this is of course a gross oversimplification, but hopefully gives you an additional thing to search for.

MagillaGuerilla #9 Posted Mar 24 2020 - 17:50

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View PostBlackFive, on Mar 24 2020 - 11:22, said:


Thanks -- but I get all that.  It's pretty easy to understand that the use of gyroscopes and other 'active' control systems can allow the satellite owner control the orientation.  The question is: what happens to a dead / unguided object with no spin, etc.?

 

EDIT - I realize my OP was insufficiently worded; going back to make it clearer!  Thanks for the initial responses!

The video states that a dead satellite in any atmosphere at all will eventually return to earth.



Pipinghot #10 Posted Mar 24 2020 - 19:01

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View PostMagillaGuerilla, on Mar 24 2020 - 11:50, said:

The video states that a dead satellite in any atmosphere at all will eventually return to earth.

That answer is true, but completely misses the question, which is obvious from looking at the image that he posted.

 

He's not asking what happens to the orbit, he's asking what happens to the facing/orientation.



Helpless #11 Posted Mar 24 2020 - 19:39

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figure 1 is correct..... if the satellite had been oriented facing the planet it's momentum would continue until a force acted upon it.

 

as simple as i can put it.... the satellite itself makes one full momentum based rotation per orbit and would continue to do so until a force acted upon it regardless of fuel or control.

 

 

 

 

 

 


Edited by Helpless, Mar 24 2020 - 19:47.


Pipinghot #12 Posted Mar 24 2020 - 19:52

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View Poststevezaxx, on Mar 24 2020 - 11:48, said:

there is also something called "Gravity gradient stabilization" where a satellite that is "long enough" will tend to orient itself with it's long axis pointed toward the Earth (center of orbit). this is of course a gross oversimplification, but hopefully gives you an additional thing to search for.

^^ BlackFive, this is the point that leads to the answer for your question.

 

Let's start with two assumptions (that I'm making):

* Based on the two figures in the illustration in your OP, you're not asking about orbital decay. What you're basically asking is, "If orbital decay doesn't exist, which of these diagrams is right?".

* You're not asking about a satellite that was originally deployed to deliberately take advantage of gravity-gradient stabilization. It a satellite was deliberately deployed to take advantage of gravity-gradient stabilization (like the examples that can be found in the wikipedia article on the topic of gravity-gradient stabilization) then they didn't need power in the first place and therefore there can't be a loss of power. Your question is specifically aimed at satellites that use some sort of powered stabilization method.

 

Keeping in mind these two assumptions, the answer depends on

a) Timeframe and

b) shape & weight distribution of the satellite

 

1) Short timeframe

In a short timeframe the answer is... it depends on what it was doing before it lost power.

* If a satellite is deployed with no spin of any kind, and only changes orientation when power is applied, then it will look very much like Figure 2 when it loses power. It will continue to maintain the same orientation it had at the last moment before power was lost.

* If a satellite is deployed with some degree of spin to help it maintain orientation and only needs occasional applications of power to maintain the rate of spin and orientation, then it will continue to spin at the current rate and maintain the current orientation when it no longer has power. So if it had just enough spin to look like Figure 1 before it loses power, then it will continue to look like Figure 1 after it loses power.

 

This is essentially your Newtonian answer, and the shorter the time frame the more your answer will look like a perfect Newtonian solution.

 

2) Long timeframe (Einsteinian) answer

 

I'll tell you up front I don't know how long this would take, I've never done enough work with this kind of physics/math to even make an educated guess. Like, I don't know if this would take dozens, hundreds, thousands or... ??? of years.

 

Over time, gravity will start to affect the orientation of the satellite. The exact details of how gravity would affect it depend on the shape, size and weight distribution of it the satellite. If it is elongated then gravity will affect it more quickly, if it's essentially spherical or cubical then gravity will affect more slowly based on the distribution of weight within it, if it's essentially rectangular then the more elongated the rectangle is or the more that weight is distributed to one end the faster gravity will act on it, and so on, you get the idea.

 

Just as we can deliberately use gravity-gradient stabilization when we deploy a satellite, so to will gravity cause this to happen over time. It it was spinning at a rate other than 1 rotation per revolution around the earth gravity would eventually cause it to slow down and adopt an orientation based on the gravity profile of the object. If it has no spin, gravity would eventually cause it to adopt a spin of 1 revolution per orbit, so that it stays constantly aligned based on the gravity profile.

 

As far as I know, this longer time frame is purely hypothetical, it can't ever happen in real world. This answer is only true in the controlled thought experiment that you have set up, in which we choose to ignore orbital decay. Any man-made object that is close enough to earth that it's orientation & spin could be affected by earths' gravity is also close enough to earth that gravity would pull it down to destruction long before the gravity would significantly alter its spin/orientation.

 



SymbiosisBC #13 Posted Mar 24 2020 - 20:19

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Up next on Stuck-At-Home-Taking-A-Battle-Break forum:

 

Why is 42 the "Answer to the Ultimate Question of Life, the Universe, and Everything"?

 

:hiding:



frizixt #14 Posted Mar 24 2020 - 20:58

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The answer to your question depends on how much you want to ignore.  If the satellite is small and not rotating when it goes dead, then figure two is your answer for the short term future of the satellite.  If it went 'dead' with some initial rotational speed, then that rotational speed would be preserved over the short term.  So it would continue facing the ground, but any perturbation will propagate with time and the likelihood of it pointing at the ground will decrease with time.  

 

The more time you give the perturbation to act, the larger the effect will be.  If you are just looking at the next orbit, you can pretty much ignore them all.  If you are asking about 100 years in the future, then it becomes complicated.

 

As for general relativity curving the space around the planet, the effect is really small, and falls into the category of what happens after a long period of time.  The effect is especially small for a circular orbit, mostly just resulting in a time correction of tens of microseconds per day.

 

 



Christojojo #15 Posted Mar 24 2020 - 21:16

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View Postfrizixt, on Mar 24 2020 - 14:58, said:

The answer to your question depends on how much you want to ignore.  If the satellite is small and not rotating when it goes dead, then figure two is your answer for the short term future of the satellite.  If it went 'dead' with some initial rotational speed, then that rotational speed would be preserved over the short term.  So it would continue facing the ground, but any perturbation will propagate with time and the likelihood of it pointing at the ground will decrease with time.  

 

The more time you give the perturbation to act, the larger the effect will be.  If you are just looking at the next orbit, you can pretty much ignore them all.  If you are asking about 100 years in the future, then it becomes complicated.

 

As for general relativity curving the space around the planet, the effect is really small, and falls into the category of what happens after a long period of time.  The effect is especially small for a circular orbit, mostly just resulting in a time correction of tens of microseconds per day.

 

 

 

Thank you. yOu saved me some typing about the EInstein curving. (I took a nap.) SOrry about that

 






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