
#1
Feb1112, 03:13 PM

P: 69

stationary or not?
I understand that satellites orbit the earth at a speed and angle that allows them to "free fall" the entire time. That is they are going so fast that they are always cresting the edge of the planet and always in a state of free fall. isn't it possible to place a satellite or station that is pretty much stationary over the poles? yes the orbit of the earth is not round. yes the earth tilts. 



#2
Feb1112, 03:58 PM

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Satellites can be placed stationary over a point on the equator, not at the poles. The satellite must orbit the earth  over the equator, the orbital period can be made coincident with the earth's rotation.




#3
Feb1212, 01:06 AM

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A stable orbit is impossible unless the satellite is traveling at least at escape velocity wrt to the body it orbits  this is orbital mechancis 101.




#4
Feb1212, 04:07 AM

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stationary or not?Any other orbit must lie in a plane which contains the Earth's center of gravity (mass) so polar orbits are possible, but they cannot be geostationary. 



#5
Feb1212, 04:43 AM

P: 69

thank you.




#6
Feb1212, 06:06 AM

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#7
Feb1212, 08:47 AM

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#8
Feb1212, 11:31 PM

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So, regardless of what a hyperbolic orbit is (and I haven't googled it), the statement that the escape speed is a minimum speed required for a stable orbit seems manifestly wrong. 



#9
Feb1312, 12:49 AM

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Agreed, I was thinking of escape velocity at the surface of earth vs orbital velocity around earth. Obviously orbital velocity is always less than escape velocity by a factor of sqrt 2.




#10
Feb1312, 04:58 AM

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#11
Feb1312, 11:13 AM

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#12
Feb1512, 12:23 PM

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With a constellation of satellites in Molniya orbits, you can stagger their orbits so there's always some satellite in the same given location all of the time. From the point of view of the tracking station, they point their antenna roughly in one direction and each satellite moves into that location in a kind of relay race. The antenna barely has to move to pick up the next satellite coming along. Typically, this is done with six satellites in a semisynchronous orbit (two orbits per day, which means each satellite is used for two periods per day, giving you the equivalent of a 12 satellite relay race). You could do this with a minimum of four satellites if you were willing to move your antenna just a bit further to pick up the next satellite coming along. And, the satellites wouldn't be directly over the poles. They have to have an inclination angle of 63.4 degrees, so you'd actually always have a satellite at about 63.4 degrees latitude with some nearly constant longitude. The 63.4 degree requirement is because you create this relay race with satellites in highly elliptical orbits. The satellite is used when its at apogee and is moving very slowly. Because of the equatorial bulge, perigee and apogee will move forwards or backwards depending on the inclination angle, with 63.4 degrees being the angle where perigee and apogee remain stationary. 


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