L4, L5 Gravity Wells

>
> Is it the solar wind? I imagine that might have a non-negligible effect
> on thin metallic foil, compared to such a weak gravitational effect.
>

Almost correct, but not quite.

Probably worth three points :-)

The solar wind is a stream of subatomic particles and exerts a
negligible force.

However, radiation pressure from sunlight exerts a substantial force
hundreds of times stronger than the solar wind.

Each of the bits of chaff would also be a mini-solar sail. Figuring
out how much of the motion is caused by gravity and how much caused by
radiation pressure would be practically impossible.

Radar bounces would also cause some pressure, but a lot less than
sunlight, that would also be worth about another 3 points :-)