[polyfractal]

A minor note: the LightSail team says the perigee is lowering due to atmospheric drag not due to the sun slowing it down:

> The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft.

That's not to say the solar pressure isn't potentially slowing it down too, but it sounds like the main driver is drag in this case.

[e12e]

I wonder if it'd be feasible to just have the sail be a one way mirror? Transparent on the "front".

[doctorphil]

There are no known materials that act as a one way mirror. We don't expect to find any such material. With a one way mirror it would be easy to lower global entropy.

[cben]

However, one side reflective/white one absorbing black is possible. I guess the weight of extra paint is not attractive, compared to attitude control which you need anyway.

EDIT: this matters because full 180° reflection is like "bouncing" the photon to opposite velocity, can up to double exchanged momentum vs absorption. (This is why reflective mylar is nice)