Practical limits on the size of orbiting objects: could two pebbles orbit each other As I understand it, gravity is inherent to mass and therefore even a small rock has its own gravitational pull. It seems entirely plausible then that a rock 1" in diameter could orbit a bigger rock, say 10" in diameter. Is this actually possible? Are there any practical limits on the size of objects that could have another object orbiting them?
What kind of environment would be necessary to facilitate something like this occurring? Could it happen in a high or low earth orbit? Would it have to be far away from any stars, planets, or other large bodies? It seems to me that the speed of the objects would have to be very, very slow.
I have no background in physics and don't know a ton of math so answers without a lot of complicated equations would be appreciated. Thanks.
 A: If there were no external influences like the gravity from the Earth and stars and no light radiation to push things, then even tiny objects could orbit each other.
Assuming somewhat constant densities (which is generally true for small objects), the mass of an object grows as the cube of the radius: $Mass \propto r_{}^3$.  The gravitational strength falls as the square of the distance which must be greater than the radius: $Force \propto \frac{1}{r^2}$.  Following this reasoning you can expect the attraction and therefore the orbital speed to roughly increase proportionally as you increase the size of the objects.  Small objects means a low attractive force and therefor a very slow orbital speed.
This will break down for microscopic objects though.  At those sizes the uncertainty principle would start to have an effect (not to mention other forces like Coulomb repulsion).
I haven't done the calculation but I suspect if small objects on the order of a few inches in diameter would need to be outside of our galaxy for the gravity between them to dominate over the gravity of the galaxy.
A: As Brandon mentioned, two small objects couldn't orbit each other near a significant gravitational field. The Hill Sphere "approximates the gravitational sphere of influence of a smaller body in the face of perturbations from a more massive body."
Therefore, your pebble's Hill Sphere would be too small to permit orbits near Earth. The Wiki article has a calculation showing that an astronaut couldn't orbit the 104 tonne space shuttle 300 km above the Earth since the shuttle's Hill Sphere was only 120 cm.
A: There is a lower limit to size. Specifically, that imposed by light pressure. While the sun is very bright, it is also very big. A meter-wide object the density of the sun at room temperature has a considerably higher power density than the sun, and so exerts a lot more light pressure per gravitational force. A sufficiently tiny object orbiting another such object will have to compensate for this, meaning something a few centimeters across is probably the lower limit at room temperature.
