APOD: LightSail A (2015 Jun 19)

[Nitpicker]

Question 2: If there was no solar wind (only solar radiation), would active comets in the solar system still display separated dust and ionised gas tails?

I don't see how. They'd have a dust trail (formed by solar radiation pressure), but no ion tail at all. Gas would expand spherically around the nucleus and largely disperse. Ultimately you might have a slightly higher density in a toroidal volume defined by the comet's orbit, and it could be partly ionized, although detecting it would be challenging, and it wouldn't be structured like a tail.

So, gas in the coma (ionised or not) is not affected by solar radiation pressure? I would have thought it'd be more affected than dust. And of course, some of the gas is ionised by the solar radiation, which I assume induces some kind of magnetic field in the vicinity of the comet. But I'm not sure of the effect in the absence of a magnetic field induced by a solar wind.

[geckzilla]

My impression of a sail (solar or otherwise) is that it is a tensile structure. The force imparted on it is transferred to the supporting vessel thru tensile members. When I look at this photo, it seems that the sail is a rather limp and rumpled structure without tensile integrity. I am wondering how the energy caught by the wail isn't wasted on bending the fabric or just having the fabric flop around due to small differential pressures across the surface of the fabric? Was this photo taken prior to, or after, the main test were conducted and therefore give a false impression? Or was there a flaw in deployment of the sail?

A normal sail, used in an atmosphere (as on a boat), is tensile, but not flat. Sails are designed to be curved (camber) with carefully designed leading and trailing edges. They act like wings, generating lift, and that is an important aspect of their efficiency and ability to provide not just propulsion, but also direction. But a light sail is operating in a medium where you can't have lift in a significant way, so I'd expect the most efficient structure to be a flat one. A loose solar sail will still "fill with the wind" after a while and become taut, but without the entire surface being normal to the incident radiation it won't be producing maximum thrust. Of course, since these sails in practice would utilize tacking, they wouldn't produce maximum thrust at all times in any case. A bit of camber probably wouldn't matter all that much.

I read one of Emily Lakdawalla's tweets when they were getting the first scrambled pictures back from it and she mentioned that the sail was not fully deployed.