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View Full Version : Are gravitational systems scalable?

Ekim
2007-Oct-26, 03:15 AM
If so how small can you go and still maintain a stable system?

Jens
2007-Oct-26, 03:42 AM
I think the problem you will get is that at closer distances, other forces become important. Elecromagnetic effects, specifically. So for example, two very large magnetic objects at large distance will orbit based on gravity, but as they get closer you will start to get a magnetic attraction or repulsion. So no, it can't be scaled down as far as you want. In atoms, I think the gravitational effects are so small that they are not important.

JohnD
2007-Oct-26, 09:12 AM
Aren't there plans to put a probe in orbit about an asteroid?
That's pretty small in relation to a planet/star system.
John

astromark
2007-Oct-26, 10:14 AM
Yes John., That is true.. but the asteroid is Orbiting the sun. The space probe will be moving very slowly or the little gravity of that asteroid will be wasted... Its a mater of matching velocities to allow gravity to work. Scaling does work... BUT. Its the other parameters that stop small scale scaling... mark.

antoniseb
2007-Oct-26, 10:29 AM
If so how small can you go and still maintain a stable system?
There was a Universe Today article (blog entry?) about a proposed mission to launch a sphere that would have a heavy metal ball, and several smaller metal balls orbiting it to test some theories related to gravity. The implication is that we expect it can scale down to pretty small scales.

Ekim
2007-Oct-26, 12:17 PM
So in theory it is possible to take a bowling ball like object and several golf ball sixe objects (non magnetic of course) and create a stable gravitational system?

antoniseb
2007-Oct-26, 12:25 PM
Please note that the mission was in a sphere to keep solar wind and other effects from disturbing the system. Also note that the golf balls (or beebees) might take months or years to orbit the bowling ball (depending on their distance), just like the planets do around the Sun.

Nicias
2007-Oct-26, 01:43 PM
One problem with this scaling is that tidal forces don't scale the same as the attractive force.

So if you take a binary star system, and scale it down, to half size, the stars have to be twice as close, and half as massive to get the same force, but then the tidal force actually doubles.

The other problem has to do with the interference of nearby bodies. The tidal forces of the earth prevent any object closer than about twice geosync orbit from having a satellite. See the wiki article on Hill Sphere (http://en.wikipedia.org/wiki/Hill_sphere#Further_examples)

Nicias
2007-Oct-26, 01:53 PM
There are two problems with scaling gravitation systems, and both have to do with tidal forces.

Firstly tidal forces scale differently then the attractive force. So if you take the earth/moon system and half both masses, move them twice as close you should have similar dynamics. However, the tidal force will have doubled! So the earth might have locked already by now.

Secondly tidal force of nearby objects can ruin the system. For instance it is impossible for any object in LEO or even geosync orbit to have a satellite. According to the wiki page on "Hill Sphere" (http://en.wikipedia.org/wiki/Hill_sphere#Further_examples) a lead sphere would have to be at triple geosync distance. (Iridium, DU, or Plutonium might get a little closer, but not much.)

Jerry
2007-Oct-26, 03:26 PM
So in theory it is possible to take a bowling ball like object and several golf ball sixe objects (non magnetic of course) and create a stable gravitational system?
No. The gravitational attractions are so small, that they would be completely overwhelmed by subtle EM effects - hydrogen bonding, Vanderwalls forces, stuff like that. The smallest system that could be stable would depend upon the proximity of the system to gross objects like the sun; but in any case don't expect a stable system to be less massive than many kilotons.

dtilque
2007-Oct-29, 09:03 AM
Aren't there plans to put a probe in orbit about an asteroid?