# Experimental considerations for measuring the gravitational constant/effects, with two large (1000kg) masses orbiting each other? [closed]

So @uhoh, wanting to get some experimental data to verify Limits on non-Newtonian gravity at length scales larger than 1 meter?, proposed an experiment:

Place two large (1000kg) spherical masses in space (where there is as little disturbance from gravity gradients as possible), have the spheres orbit each other at a distance of less than 10 cm. Then, integrate over time, to measure the orbital period and distance between the spheres.

So my question:

What secondary effects would mess up this experiment (e.g., solar electrostatic charging, angular momentum changes due to impinge photons etc) and how would you measure the influence of those secondary effects?

• Just because you are in space doesn't mean you don't have gravity, you'd need to be as far as possible from any mass distribution to feel little gravitational effects – Triatticus Jan 31 '19 at 2:05
• Agreed! So we could create two copies of this experiment place one close to earth and one further away (by a known amount) . That way we can measure the secondary effect and account for it in our measurements. Good comment. Maybe you would like to convert it into an answer? – DarcyThomas Jan 31 '19 at 2:09
• Oh no it's more of a constructive thing so I'll just leave it as a comment, at least it makes you think more about the question for sure. – Triatticus Jan 31 '19 at 2:11
• – rob Jan 31 '19 at 4:01
• @rob it really isn't a near-duplicate of the question I've actually asked. This pseudo-cross-posting to prove a point in meta probably isn't the best way to go either. I think your answer would receive a welcoming audience there, consider writing something there as well? Thanks! – uhoh Jan 31 '19 at 4:20

When I was an undergraduate, some friends were involved in a proposal ("project SEE") to do exactly this, albeit with smaller masses. The proposed method was a satellite specially crafted to produce negligible gravitational forces on the objects inside (by having the satellite's mass distribution match the charge distribution on the surface of a conductor of the same shape, since electric and gravitational fields both go like $$1/r^2$$). Then test masses would be released in a cavity inside the satellite, where the dominant gravitational interactions would be with the Earth and with each other. There is a neat effect where the gravitational interaction between co-orbiting satellites is effectively repulsive, leading to horseshoe orbits. Needless to say, the number of "secondary effects" was very large. As a fundamental test of gravitation, the project never launched.