Counter Orbital Mechanics

[RainOfSteel]

I'm familiar with the old metaphor of firing a canon ball out of a canon at just the right power so that it would continue on and on, around the world (ignorning air and such, for the purposes of the example).

Naturally, there is a minimum speed for objects to achieve to reach and maintain an orbit.

I've read here and there about "counter orbits", an orbit around a world in the opposite direction of the world's rotation.

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• How are "counter-orbits" achieved (is it just a matter of going sufficiently faster in the other direction)?</font>
• For low-earth counter orbits, would the atmosphere have an even greater effect in degrading orbits?</font>
• What other issues are involved that I can't even think up the questions for?</font>

 

[veltyen]

Once in near vacuum the spin of the planet becomes academic. Gravity doesn't care what spin the central mass has.

Hence polar orbits, and counter orbits (I belive you mean Retrograde)

Without the spin of the planet mattering a retrograde orbit is really easy to set up. Yes, a leo probably has too much material heading the wrong way, but considering the effective velocity of an object in orbit, the difference between going with the wind and against the wind is slight. You need slightly more thrust to go against the orbit, but you would need almost that much thrust to maintain velocity with the orbit.

From the ground a retrograde orbit requires a lot more energy to achieve, but thats all it requires.

Polar orbits are therefore possible. This is where a sattelite passes over generally both poles.

 

[Malenfant]

You mean retrograde orbits? (like Triton around Neptune)

You can launch a satellite into a retrograde orbit, but it's more expensive in terms of fuel and energy (and therefore money). If you launch it into a prograde orbit (ie into the same direction as the planet is rotating) then you can use the planet's rotation to help get it into orbit. If you're launching into a retrograde orbit, you can't do that and indeed you have to work against the planet's rotation.

http://scienceworld.wolfram.com/physics/RetrogradeOrbit.html

 

[TheEngineer]

Hi Rain !

Taking the physical view an object travelling on any orbit level with a surface parallel velocity v is affected by a force pendicular to the surface.
This force is calculated that way:
F= (mass*velocity) / r^2
r is the orbit radius

Now, the counterforce is caused by gravity, calculated simply by Fg= mass * local g.
If both components are equal a stable orbit situation is achieved.
If you take a look at the variables, you also see the things, which influence orbit stability.
Well, mass is cancelled out here.

The only aspect which makes counter orbits different, is that you first need to overcome the initial velocity vector given by planets rotation, which is directed in the opposite direction you want to go.

Regarding the second question its just easy.
Travelling in an athmosphere constantly would decrease speed and you have to compensate that all the time. Its no longer possible to orbit without doing anything.

Regards,

Mert

 

[RainOfSteel]

Originally posted by TheEngineer:
Regarding the second question its just easy.
Travelling in an athmosphere constantly would decrease speed and you have to compensate that all the time. Its no longer possible to orbit without doing anything.

So . . . the extremely thin extents of the upper atmosphere which, in theory, are "rotating" prograde along with the Earth, exact no "added" braking on long-term orbital stability on a retrograde low-earth (125 miles or less) orbit?

 

[RainOfSteel]

Thanks to those who responded.

 

[aramis]

One other thing:

Retrograde orbits have differing tidal effects from prograde.

not significant for most satellites, but a long term concern. And only really significant for natural satellites.

Orbiting bodies to induce a small, almost negligible, amount of rotational energy upon the crust below their path, inducing spin in theory... they lose a similar amount. For the Earth-Moon, the moon is gaining velocity from the loss of spin velocity of the earth. Fast (for this purpose, fast is anything more than geosynch) Prograde orbits impart additional spin on the body, taking this as a trivial drag. Retrograde orbits all experience tidal drag as a slowing of orbit, and induce a slowing of world spin. For a planet, the amount is hardly even measurable until you get large-body interactions. For satelites, it can be a long term concern.

Additionally, escaping atmospheric gasses induce similar drag forces on satellites. Inconsequential on the scale NASA thinks in (They said so), but for long term (century+) both of these forces can add up. A small ion thruster can correct for the drag for even fairly large structures.

 

[RainOfSteel]

Ok, no laughing . . .

1) How difficult would it be, labor/effort wise, to put a "particle field" into fast retrograde orbit above the Earth?

2) Just how difficult would this make it to get off the Earth with conventional launching (no trans-dimensional movement, by technology or magic, etc.)? (Or would the whole effort be pointless?)

 

[TheEngineer]

Hi Rain !

Of what size/mass would You like the particle field to be ?
Guess I would just launch the normal way, but carry a second rocket with the particle load in order to reach retrograde orbit and release the particles at the appropriate time.

So, for both 1) and 2) essentially not more difficult than any other real life orbital operation

Regards,

Mert

 

[RainOfSteel]

Originally posted by TheEngineer:
Hi Rain !

Of what size/mass would You like the particle field to be ?

Pole to pole, 360 degrees around the equator. Let's give it at least ten miles depth.

 

[mike wightman]

Why do you want to have a ten mile thick plasma/particle layer around a planet?

 

[RainOfSteel]

<cough, cough>

That was the no laughing part.

Over on the Palladium Forum, there is an extensive and vigorous discussion over the "Counter Orbital Particle Field" (COPF) encircling the Earth in the Rifts game. It's supposed to be the "key factor" preventing the [super high-tech, mecha & super-weapon equipped] societies of Earth from launching spacecraft. The "field" is backed up by various k-sats, but the field is "put-forth" as the primary preventitive mechanism (from getting off Earth).

Several people made some telling points about the whole thing being ridiculous. I wanted to ask some people who might be in a better position to answer space-related questions, so I made an open post here.

Oh, and because I was making some assumptions which I didn't point out, I'll add in: it would be substantial physical granular particles, sand sized; not microscopic plasma.

 

[Simone]

You are describing a weapon system, simplified, something like sand particles to mess up anything leaving the planet.

The issue on a more diffuse scale, right now Nasa "keeps up" with orbits of thousands of orbiting items, from paint fleck sized up to many kilos, they are a serious navigation hazard now.

By counter orbital, I think you mean, to prevent orbit from being achieved. Try it (leaving atmosphere) and get holed.

 

[RainOfSteel]

Originally posted by Soldiurnare:
You are describing a weapon system, simplified, something like sand particles to mess up anything leaving the planet.

The issue on a more diffuse scale, right now Nasa "keeps up" with orbits of thousands of orbiting items, from paint fleck sized up to many kilos, they are a serious navigation hazard now.

By counter orbital, I think you mean, to prevent orbit from being achieved. Try it (leaving atmosphere) and get holed.

Yes.

 

[mike wightman]

So you want a cloud of small ball bearings in a retrograde orbit so that anything launching though the cloud gets hit by particles with a ridiculously high relative velocity, and gets sand blasted to death.

I would be tempted to have a layered structure, with each layer a couple of miles thick, and going in different directions. That way a launch vehicle can't avoid hitting something going really fast.

As to if such a thing is possible given your parameters, I don't see why not. Provided you have the huge number of heavy lift vehicles to put the particles in orbit in the first place.
A better bet would be to manufacture the particles from asteroids, or the like, and put them into their orbits from deep space, rather than trying to lift them all to their orbits.

 

[TheEngineer]

Hi Rain,

just over the tumb...

Thinking of a layer of 0,01 kg high density spheres moving around with ~ 7000 m/s on a stable orbit, you would roughly need 500E9 of them to somehow cover one layer surface (about 1 sphere per 1000 m2).
I had chosen this weight in order to provide some effect on impact (245kJ).
This would result in a mass of 5 million tons to be transfered to orbit and accelerated to orbital velocity for one layer.

So this might result in an interesting contract for a shuttle company

Regards,

Mert

 

[BetterThanLife]

Actually it wouldn't have to be that thick. After all if a layer were dense enough it would work like chaff and you couldn't see past it. So you could put laser equipped sats above the layer and pick off anything coming through without anyone groundside being able to target them. In all honesty though it seems like a serious waste of time and material.

If you had a gross of satelites in polar orbit backed up by say 60 in geosync orbit all armed with powerful lasers, you could keep anything on the planet, on the planet. The only reason you would need so many would be to swat any locations attempting to fire on your satelites in the first place. You would need a base and a couple of ships to replace lost, damaged or malfunctioning satelites. But otherwise that should be sufficeint to blockade a planet.

 

[TheEngineer]

Truely there are many more efficient ways to set up a blockade, but the way Rain described is a very interesting method. And very persistent one and absolutely maintainance free for quite a long time.
Something like a bad joke of the ancients....

 

[BetterThanLife]

Originally posted by TheEngineer:
Truely there are many more efficient ways to set up a blockade, but the way Rain described is a very interesting method. And very persistent one and absolutely maintainance free for quite a long time.
Something like a bad joke of the ancients....

But with ground based lasers, or even shotgun shell style KE missiles you could punch predictible holes in in fairly easily.

Actually, thinking about it, the shotgun warhead missiles would tend to work better. Like the break shot on a pool table.

I mean if what you sent up needed to survive the encounter then it is effective, but not everything sent up would need to survive. YOu could even launch a bunch of these missiles, back them up with ground based lasers, and launch your starship along a preplanned route and clear the path right in front of the starship without giving the people that blockaded you time to react. If you punch several holes you could sneak your ship through one and coast through powered down and spring a nice surprise on the clowns trying to patch the holes you created.

 

[TheEngineer]

Hi !

Bhoins, dont You think that its quite complicate to punch a hole in a layer of 900E9 or more particles orbiting a planet at different directions ?
Its like trying to punch a hole in rain or a squadron of Point Pelee flies

(THAT was a holiday trip !)
Not hard to hit some individuals, but hard to hurt the group.