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Justin Cyder

12,379 posts

34 months

[news] 
Friday 20th April 2012 quote quote all
Can we get over the 200km typo & move on? yes

SystemParanoia

9,696 posts

83 months

[news] 
Friday 20th April 2012 quote quote all
isnt geo-sync orbit 26,000km ... not 200km

annodomini2

5,082 posts

136 months

[news] 
Friday 20th April 2012 quote quote all
SystemParanoia said:
isnt geo-sync orbit 26,000km ... not 200km
Geostationary is 35786Km.

Geosynchronous varies depending on the required position and can be elliptical.

BarnatosGhost

7,741 posts

138 months

[news] 
Friday 20th April 2012 quote quote all
annodomini2 said:
SystemParanoia said:
isnt geo-sync orbit 26,000km ... not 200km
Geostationary is 35786Km.

Geosynchronous varies depending on the required position and can be elliptical.
Does that depend on the mass of the satellite?

MartG

3,652 posts

89 months

[news] 
Friday 20th April 2012 quote quote all
SystemParanoia said:
isnt geo-sync orbit 26,000km ... not 200km
The ISS is not in geosynchronous orbit - it is only 200 - 300km up
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Eric Mc

76,625 posts

150 months

[news] 
Friday 20th April 2012 quote quote all
Currently the ISS's maximum altitude is just under 400km.

That is the highest we are going with manned spacecraft technology at the moment.
Obviously, 40 years ago we were taking people out 250,000 miles.

Unmanned is a different story. The "highest" unmanned probes are the Voyagers and Pioneers 10 and 11.

Simpo Two

59,618 posts

150 months

[news] 
Saturday 21st April 2012 quote quote all
Eric Mc said:
That is the highest we are going with manned spacecraft technology at the moment.
Obviously, 40 years ago we were taking people out 250,000 miles.
Won't be long before H&S limit this to a 3m scaffold tower surrounded by rubber matting and 8 blokes in hard hats.

Eric Mc

76,625 posts

150 months

[news] 
Saturday 21st April 2012 quote quote all
I wonder how well a rubber mat would cope with a 25,000 mph impact.

Simpo Two

59,618 posts

150 months

[news] 
Saturday 21st April 2012 quote quote all
If you fell off the top of the 3m tower you wouldn't be doing 25,000mph (unless it was a neutron star, in which case the tower would be a flat disc along with the blokes in hard hats so the question doesn't really arise)

Eric Mc

76,625 posts

150 months

[news] 
Saturday 21st April 2012 quote quote all
Simpo Two said:
If you fell off the top of the 3m tower you wouldn't be doing 25,000mph (unless it was a neutron star, in which case the tower would be a flat disc along with the blokes in hard hats so the question doesn't really arise)
And a very, very thin mat.

annodomini2

5,082 posts

136 months

[news] 
Saturday 21st April 2012 quote quote all
BarnatosGhost said:
annodomini2 said:
SystemParanoia said:
isnt geo-sync orbit 26,000km ... not 200km
Geostationary is 35786Km.

Geosynchronous varies depending on the required position and can be elliptical.
Does that depend on the mass of the satellite?
No, hence why a feather and a hammer fall at the same rate on the moon.

The mass only has an impact for the launcher and orbital maintenance.

BarnatosGhost

7,741 posts

138 months

[news] 
Saturday 21st April 2012 quote quote all
annodomini2 said:
BarnatosGhost said:
annodomini2 said:
SystemParanoia said:
isnt geo-sync orbit 26,000km ... not 200km
Geostationary is 35786Km.

Geosynchronous varies depending on the required position and can be elliptical.
Does that depend on the mass of the satellite?
No, hence why a feather and a hammer fall at the same rate on the moon.

The mass only has an impact for the launcher and orbital maintenance.
When you say orbital maintenance, do you mean they need to exert a force to stay in position (beyond small tweaks here and there)?

I ask because that doesn't seem intuitive to me. Swinging a 1kg weight on a 1m string around my head at 60rpm requires a different input of energy to a 2kg weight on the same string at the same speed, doesn't it?

Or doesn't it?

Eric Mc

76,625 posts

150 months

[news] 
Saturday 21st April 2012 quote quote all
Once the object has achieved a stable orbit, it will stay in that orbit indefinitely (ignoring any other influences that might affect the object once it is in space).

The energy required to get that object to the stable orbit will have been set by the mass of the object. That is why a 7.5 million pound thrust Saturn V was needed to launch 100 tons into a low earth orbit whereas the much less powerful Atlas was required to put 1.5 tons into the same type of orbit.

BarnatosGhost

7,741 posts

138 months

[news] 
Saturday 21st April 2012 quote quote all
But why does a 1 ton satellite have a geosynchronous orbit at the same distance as a 2 ton satellite?

Isn't there a difference? It 'feels' like there should be.

MartG

3,652 posts

89 months

[news] 
Saturday 21st April 2012 quote quote all
BarnatosGhost said:
I ask because that doesn't seem intuitive to me. Swinging a 1kg weight on a 1m string around my head at 60rpm requires a different input of energy to a 2kg weight on the same string at the same speed, doesn't it?

Or doesn't it?
A string isn't gravity. While swinging a 2kg weight on a string will put more force onto the string than a 1kg one will, gravity 'automatically' puts more force on a 2kg mass than a 1kg one.

The two important equations you need to know to work it out are:

Force due to gravity F=GMm/r^2 ( G=universal gravitational constant, M=mass of Earth, m=mass of object, d^2=sqaure of the distance between the centre of gravity of the two objects )

'Centrifugal Force' ( I know, it doesn't really exist ) F=mv^2/r (m=mass of object, v=velocity ( parallel to planet surface ) r=radius of the orbit )

For something to be in a stable orbit the centrifugal force must equal the force due to gravity, so the equations simplify to GM=rv^2 i.e. the radius of the orbit multiplied by the square of the velocity equals the mass of the Earth multiplied by the gravitational constant. Notice that the mass of the satellite doesn't appear in this equation, as it doesn't affect the result

Eric Mc

76,625 posts

150 months

[news] 
Saturday 21st April 2012 quote quote all
BarnatosGhost said:
But why does a 1 ton satellite have a geosynchronous orbit at the same distance as a 2 ton satellite?

Isn't there a difference? It 'feels' like there should be.
"Feelings" aren't very scientific.

MartG

3,652 posts

89 months

[news] 
Saturday 21st April 2012 quote quote all
BarnatosGhost said:
But why does a 1 ton satellite have a geosynchronous orbit at the same distance as a 2 ton satellite?

Isn't there a difference? It 'feels' like there should be.
If you had two 1 tonne satellites tied together with string, do you think they would be in a different orbit to a 2 tonne satellite ?

( incidentally, it's this concept that Galileo used to show that objects of different mass fall at the same rate )

BarnatosGhost

7,741 posts

138 months

[news] 
Saturday 21st April 2012 quote quote all
MartG said:
BarnatosGhost said:
I ask because that doesn't seem intuitive to me. Swinging a 1kg weight on a 1m string around my head at 60rpm requires a different input of energy to a 2kg weight on the same string at the same speed, doesn't it?

Or doesn't it?
A string isn't gravity. While swinging a 2kg weight on a string will put more force onto the string than a 1kg one will, gravity 'automatically' puts more force on a 2kg mass than a 1kg one.

The two important equations you need to know to work it out are:

Force due to gravity F=GMm/r^2 ( G=universal gravitational constant, M=mass of Earth, m=mass of object, d^2=sqaure of the distance between the centre of gravity of the two objects )

'Centrifugal Force' ( I know, it doesn't really exist ) F=mv^2/r (m=mass of object, v=velocity ( parallel to planet surface ) r=radius of the orbit )

For something to be in a stable orbit the centrifugal force must equal the force due to gravity, so the equations simplify to GM=rv^2 i.e. the radius of the orbit multiplied by the square of the velocity equals the mass of the Earth multiplied by the gravitational constant. Notice that the mass of the satellite doesn't appear in this equation, as it doesn't affect the result
Thank you, great answer. That makes sense.

Now the crucial test-your-understanding second question:

So if the moon were to be geosynchronous, it too would have to orbit at 37,000 (or whatever the figure was) km?

BarnatosGhost

7,741 posts

138 months

[news] 
Saturday 21st April 2012 quote quote all
Eric Mc said:
BarnatosGhost said:
But why does a 1 ton satellite have a geosynchronous orbit at the same distance as a 2 ton satellite?

Isn't there a difference? It 'feels' like there should be.
"Feelings" aren't very scientific.
If you never intuit that anything seems right or wrong (regardless of whether it is actually right or wrong) I'd suggest that you were unusual...

Or did you know exactly what I meant and were 'feeling' a little supercilious? smile

MartG

3,652 posts

89 months

[news] 
Saturday 21st April 2012 quote quote all
BarnatosGhost said:
So if the moon were to be geosynchronous, it too would have to orbit at 37,000 (or whatever the figure was) km?
Yes - well its centre of gravity would be at that distance, but the surface would be nearer.
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