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newtonslovechild
2011-Sep-25, 08:15 AM
Hi,

I’m writing to ask whether GPS and other satellites revolve around the earth purely by Newton’s planetary laws of motion or whether their positions need to be adjusted from time to time, and if so how? Do they have on-board rockets?

Also, why did a satellite crash to earth recently? Did it gradually fall out of orbit, and if so why? Or did NASA bring it down?

My understanding is a satellite is carried into space via a rocket launched at a precise trajectory. It eventually reaches a point where gravity pulls it back to earth but because it falls at an angle to the earth, it remains at a constant distance from the earth. Is this correct?

nlc

cjameshuff
2011-Sep-25, 05:30 PM
My understanding is a satellite is carried into space via a rocket launched at a precise trajectory. It eventually reaches a point where gravity pulls it back to earth but because it falls at an angle to the earth, it remains at a constant distance from the earth. Is this correct?

Not always at a constant distance. Some orbits can be quite precisely circular, but practically speaking all orbits are at least slightly elliptical.

Satellites in low orbit pass through the uppermost regions of the atmosphere and experience drag, causing them to slowly spiral in. The ISS is in this situation as well, and requires periodic nudges back into a higher orbit. Solar activity can heat the upper atmosphere and cause it to extend outward further, making the precise amount of drag difficult to predict.

GPS satellites are in fairly high near-circular orbits, and don't have much drag to worry about. They do have to maintain position in their orbits relative to other GPS satellites in the same orbit, and so must have some limited propulsion capability...I haven't found details about it though.

slang
2011-Sep-25, 11:22 PM
The ISS is in this situation as well, and requires periodic nudges back into a higher orbit.

Here (http://heavens-above.com/IssHeight.aspx?lat=0&lng=0&loc=Unspecified&alt=0&tz=CET) is a nice graph that shows this nicely.

Hornblower
2011-Sep-25, 11:45 PM
From the launch of Vanguard I in 1958 until his retirement in 1974, my father monitored U.S. Navy satellites and the evolution of their orbits. He saw the atmospheric drag coefficient at 500 miles diminish by a factor of about 1,000 from the solar maximum in 1958 to the next minimum around 1963, and then go back up by 1969. That made a huge difference in orbital decay rates. Another decade later, increased drag during a solar max hastened the fall of Skylab, before any shuttle missions could get there either to attempt a reboost or to bring it down in a controlled plunge.

Hornblower
2011-Sep-25, 11:51 PM
Hi,

I’m writing to ask whether GPS and other satellites revolve around the earth purely by Newton’s planetary laws of motion or whether their positions need to be adjusted from time to time, and if so how? Do they have on-board rockets?Also, why did a satellite crash to earth recently? Did it gradually fall out of orbit, and if so why? Or did NASA bring it down?

My understanding is a satellite is carried into space via a rocket launched at a precise trajectory. It eventually reaches a point where gravity pulls it back to earth but because it falls at an angle to the earth, it remains at a constant distance from the earth. Is this correct?

nlc

My bold. As I understand it, all satellites whose positions are critical have station-keeping thrusters with enough fuel for a service lifetime of several years. This includes geosynchronous satellites which are unaffected by atmospheric drag but are gravitationally perturbed by the Sun and the Moon. A typical large satellite will have several hundred pounds of fuel.

swampyankee
2011-Sep-25, 11:59 PM
Hi,

I’m writing to ask whether GPS and other satellites revolve around the earth purely by Newton’s planetary laws of motion or whether their positions need to be adjusted from time to time, and if so how? Do they have on-board rockets?

Also, why did a satellite crash to earth recently? Did it gradually fall out of orbit, and if so why? Or did NASA bring it down?

My understanding is a satellite is carried into space via a rocket launched at a precise trajectory. It eventually reaches a point where gravity pulls it back to earth but because it falls at an angle to the earth, it remains at a constant distance from the earth. Is this correct?

nlc

There are at least four things which perturb satellite orbits. One is that low-Earth orbit is not in a perfect vacuum: there is still some remnants of the Earth's atmosphere. It's not very much, but it's enough so that at orbital velocity, there is a non-zero retarding (drag) force present. The second is that there is a small force produced by solar radiation. The third -- and this is probably much smaller than the first two -- is that the Earth has a magnetic field, and most satellites are made of conductive materials, so the induced currents will dissipate some energy. The final are forces from other bodies in the Solar System.

So, yes, satellites have on-board rockets. There would be two reasons why satellite orbits decay, and the satellite eventually crashes to Earth: the first is that it's done deliberately, to reduce the accumulation of orbital debris; the second is that there are drag forces which reduce a satellite's velocity, and this drag force will increase as the satellite's altitude is decreased.


(yes, I know about GR, which predicts that an accelerated mass will lose energy via gravitational waves. I believe these are so small as to be negligible for Earth-orbiting objects)

cjameshuff
2011-Sep-26, 12:13 AM
There are at least four things which perturb satellite orbits. One is that low-Earth orbit is not in a perfect vacuum: there is still some remnants of the Earth's atmosphere. It's not very much, but it's enough so that at orbital velocity, there is a non-zero retarding (drag) force present. The second is that there is a small force produced by solar radiation. The third -- and this is probably much smaller than the first two -- is that the Earth has a magnetic field, and most satellites are made of conductive materials, so the induced currents will dissipate some energy. The final are forces from other bodies in the Solar System.

The Earth is also not a point source or perfect sphere, and irregularities in its gravitational field will perturb satellites.

swampyankee
2011-Sep-26, 12:17 AM
The Earth is also not a point source or perfect sphere, and irregularities in its gravitational field will perturb satellites.

True, but as these irregularities in the gravitational field are conservative, they won't cause the satellite to lose any of its kinetic plus potential energy.

korjik
2011-Sep-26, 12:18 AM
GPS sattelites have their orbits plotted out using general relativity, not newtons laws. There would be a small but noticable error in GPS otherwise

cjameshuff
2011-Sep-26, 12:52 AM
True, but as these irregularities in the gravitational field are conservative, they won't cause the satellite to lose any of its kinetic plus potential energy.

They can nudge it into a more elliptical orbit that cuts deeper into the atmosphere, though. Or in the case of the moon (which is rather "lumpier" than Earth), into an orbit that intersects the surface.

newtonslovechild
2011-Sep-26, 04:15 AM
All answers were very helpful, so thanks.

nlc