Couldn’t you place the satellite in a very slightly lower orbit to compensate for that, at least for the expected average power?
You can do that, but you have to remain in your orbital slot. As it stands, you’d have to keep firing your earth pointing thrusters periodically to speed up the orbit after so much time.
It’s been a while since I’ve read up on this and talked to folks about the in’s and out’s of station keeping, so I’ll give you a rather crude disertation (I’ve since moved away from geosatellites into another career).
The biggest affect on a geosatellites orbit (outside of earth’s) is the moon’s gravitational pull, followed by the sun’s gravity. Other things such a radiation pressure from the sun (and the geo bird), solar wind, and earth’s geomagnetic field will perturb the orbit. Each force causes the orbit to drift away from the original orbit.
Each geosatellite is assigned a specific slot of a specific width and length by the ITU. Essentially, it’s your little piece of the heavens and no one else can occupy that area. Conversely, you can’t allow the geo to drift into someone else’s assigned spot. And that’s for two reasons: 1) Collision (duh!) and 2) to avoid your transponders from inadvertently intercepting another uplink station (imagine if an transponder assigned to HBO, starts broadcasting Chinese dragon porn).
In the meanwhile, you’ve just gotten a Wonder-Buster telescope and enjoying a holiday at the equator…you decide to point the telescope straight above and that allows you to see a geo bird. As it stands, you would expect it to remain fixed in the sky, while the stars, moon, and sun move by…(the satellite is officially in a geosynchronous earth orbit).
You could tell that satellite to remain precisely affixed to that point, but then you’re burning fuel at an alarming rate. That would severely limit the actually life of the geo, (and you’re required to have enough residual fuel to place it in graveyard orbit at the end of geo bird’s life so that it 1) prevents a collision, and 2) opens the slot for a replacement satellite. These birds are expensive and they want to max out the broadcast life as much possible so conserving fuel is key to that).
(Ion propulsion, I believe, is now being phased into the latest birds, but I don’t know what the trade offs are in terms of advantages/disadvantages. I’m sure its as much about trying to get the extra life; why they weren’t used earlier–I’ve been told, but if there’s a possibility of violating ITAR [see below], I’m zipping my lips.)
So to avoid this burning of excess fuel, you let the drift take place naturally (see, even in space, we’re striving to be organic). Overtime, as you look through the telescope, you start to see the fixed point start moving around in a figure eight pattern (literally an analemma) which will increase in size over time. As long as you stay in your slot, you’re fine. Theoretically, you can hit edges, but nobody dare does that because you’d be risking a collision, and a lot of hate mail (plus, they’ll never allow you launch again, and the slot will go to someone else. So, long before you risk an international incident, you start firing the thrusters to perform a station-keeping maneuver to reduce the analemma to a reasonable size. Usually it’s the north-south thrusters that burn the most, followed by the east-west, and a tiny bit of high-low.
As for why the north-south is requires the most burns: as the geo rotates around the earth, the sun and moon tend to be above and below celestial equator every 24 hours. Hence, the orbit gets pulled up and down more than it gets pulled east and west. E-W drift is affected by the position of the sun and moon relative to each other and varies throughout the year. Depending on the location of the orbital slot and the mechanical layout, each satellite has its own distinct analemma.
Are there cases where satellites have drifted into other slots?
Yes, at least in the last few years. I worked on this one that was launched successfully and operated for years without a hitch. Then one day, the satellite could no longer hear the ground station. All sorts of things were tried to get it to respond, but nothing worked. This went on for months. Eventually, it started drifting into another slot. Fortunately, the other operator and the company I worked for worked out an elaborate ballet to ensure that no collision resulted while the broadcasts remained uninterrupted by the put upon operator. Eventually, the rogue satellite finally it reset and went into safe mode (sort of like Windows safe mode). A patch was uploaded to fixed the underlying problem and then eventually move the now legally sane satellite back to its original slot. But it took about a year to get back to normal operation. IIRC, the insurance company had to reimburse the operator for loss of revenue.
(As for the cause–I don’t know. It’s not to say I couldn’t find out, or have my suspicions–but I wouldn’t tell anybody because it’s proprietary to the company, and far more consequential–it would violate ITAR. Those penalties are not worth it; even hinting at the cause could get me in trouble with a capital T in Federal City.)
ETA–“Database” error. Well, my finely tuned essay got eaten when I tried to “Preview Post”. So it’s regressed to a earlier, less articulate piece. I’m tired and going to bed, so this is what you get.
Hope this helped.
Thanks, that was interesting. I do recall when that Geo satellite that drifted, it was in the news. Didn’t they end up actually switching Geo slots with the other satellite? Or was that only a temporary swap until the fix?
ObSF: Arthur C. Clarke’s “I Remember Babylon” I Remember Babylon - Wikipedia
No, I found a link, it was different (and more complex) than I remembered.
You found the link. I don’t know if they could switch slots. If they could, knowing the ways of international bureaucracies it would take quite a while. Enough time that the revenue loss wouldn’t be worth switching.
I can think of one technical reason: with respect to all of geos I worked with, the parabolic antennas are custom made to map the signal to a specified geographic region (if you could see the actual geo bird parabolic antenna, forget the smooth concave you see on the old giant C-band antennas in the backyard; instead, you’d see a warped surface.) Moving it to another slot would alter that mapping and require pointing the antennas in a less-than-optimal signal strength and distribution. It would also probably violate a licensing agreement with satellite’s host country. Another thing to consider is if the signal spills over to another country. So switching slots ad hoc isn’t likely to happen for commercial, technical, and political reasons.
So to get back to the OP question…
See, even in an orbit of 22,300 miles up (geosynchronous) plus almost 4,000 miles earth radius, for orbital radius of 26,300 miles; circumference of about 165,000 miles; each degree of separation is about 460 miles; IIRC current satellite eparation is about 1.5 or 2 degrees in he busy areas… So with a separation of over 600 miles each, there’s still problems with drift and possible collisions.
If you drop a satellite to a few miles above the lunar surface and fly it over denser lumps that could alter the orbit too- there’s a good chance the chaotic influences from different factors could alter the orbit enough that eventually the low point will clip the surface. There’s not atmospheric drag to speak of, but there is some short amount of drag when it hits the surface.
Drag? Don’t you mean collision?
I believe the technical term is “lithobraking”.
With airplanes it’s called “controlled flight into terrain”. But with a satellite left on its own, I don’t know about the “controlled” part. Maybe “uncontrolled flight into terrain” depending on the circumstances.
It’s a shame nobody has explained what tides are yet. Tides are the differential pull on an object caused by the different distance between the sides of the object and the respective gravity source.
Consider the tides on Earth caused by the Moon. We can treat the Moon as a point gravity source ~385000 km from Earth, in a circular orbit. Its gravity is pulling on the Earth. But the reality is that it is not pulling on the Earth as a solid rigid object connected at the center, but rather is pulling on each part of the Earth separately. So the near side of the Earth is getting pulled slightly more than the far side of the Earth, because the far side is 12,700 km farther away. This differential in Moon’s pull on the sides of Earth effectively creates a stretching of Earth in the direction of the Moon. The solid ground part literally swells a bit. But water is less rigid than dirt, so it flows even more, and thus the tidal action of coastal water height changing.
This differential pull of gravity is much more noticable in big objects like Earth rather than small objects like satellites, because the distance between the sides of the satellite are considerably smaller (;)). But that differential pull does exist. That is a tidal pull.
As James Nicoll said "You may have trouble getting permission to aero or lithobrake asteroids on Earth. " James Nicoll - Wikiquote
Duh. Decrepit memory now recalls true meaning of tidal forces.
When I first got into satellite work, I was use to active stabilization schemes (magnetic torque rods, momentum wheels). I asked if there were any passive methods and one example tossed my way was a “gravity gradient boom”. Which works precisely in the matter described by Irishman.
Magnetic torque rods can be either active or passive.
Eh…you’re right. But make sure you don’t use passive torque rods in a polar orbit.
Some friends of mine did, without any problem. For their purposes, any orientation of the satellite was acceptable, so long as they knew what it was, and it wasn’t tumbling erratically.
Your friends, they wouldn’t happen to be from Best Corea? 
It was in the early days when we were just getting into satellites. It was our first polar orbiting S/C and nobody thought about that scenario until we got some screwy telemetry. :smack: A fix was made, and things got back to normal.
No, actually, Montana State University, and it was designed with that in mind.