13 hours ago, TheVat said:

From the Rocheworld article in wiki: To catch the energy, Forward uses a 1,000-km-diameter circular aluminum sail. The sail resembles a flattened disk with a 300-km-diameter removable center portion. When traveling to Rocheworld, the entire sail is used. When the ship needs to decelerate, the smaller sail is separated from the larger outer sail. The large sail is used as a reflecting lens, focusing light onto the smaller sail, slowing the craft. (end quote)

Aside from the absorptive heating issues, I am unclear how this double-reflection gimmick would be sufficient delta-v. I recall Forward as the sort of SF writer to really thrash out all the RW science issues, so maybe reading the novel would tell me more. Maybe there's other shipboard mass that gets dumped.

I don’t see how this would work. The detached sail will now have a higher acceleration, so it will separate from the craft. It will reflect less light, so any deceleration from it is smaller, and the remaining sail is still reflecting light, giving forward propulsion. The detached sail will be much more prone to tumble if there are any intensity variations over its profile.

20 hours ago, TheVat said:

In any case, the thread topic is on the possibility of aliens visiting here. Whatever the nature of UAPs, they don't seem to colonizing Earth. (and we would be a poor choice for a colony - imagine if we humans went looking for a terrestrial planet to colonize; overpopulated, polluted, bristling with nukes, and constantly having wars would be our last choice)

With low speed travelling, they couldn't know at the departure that the Earth is inhabited by an intelligent species. I don't know about them, but I wouldn't colonize a planet already occupied. So, I'd either abort the mission completely or choose some moon to dig and build habitable places and so on. Therefore it is possible to have aliens on some moon in our solar system.

16 hours ago, swansont said:

The limitation is that reflective surfaces are not 100% reflective, and the high power needed for any appreciable acceleration makes absorptive heating a huge obstacle

We may solve this problem by improving and extending the surface of the sails, by increasing de time for acceleration (using not only the lasers near the Sun, but also lasers situated much further, and powered from the belt of power stations near the Sun) and by improving the resilience of the shipment (robots, devices, etc.), in order to withstand longer travel time, with lower speed.

I also thought about a way to flip the sails when overheated (if sails are divided in smaller sections).

15 hours ago, exchemist said:

How would you decelerate the spacecraft at its intended destination?

Due to the fact that the acceleration is provided by external sources, we can send a big multistage rocket for the deceleration. After that, we redeploy the solar sails. If we still need deceleration, we may consider skimming through some planet(s) atmosphere.

I want to add another use for the solar power stations belt: we can use it to direct heat to some frozen moon, in order to make it habitable. So, the investment would not be solely for colonizing other solar systems.

4 hours ago, swansont said:

I don’t see how this would work. The detached sail will now have a higher acceleration, so it will separate from the craft. It will reflect less light, so any deceleration from it is smaller, and the remaining sail is still reflecting light, giving forward propulsion. The detached sail will be much more prone to tumble if there are any intensity variations over its profile.

Thanks, I had wondered how that could work with still retaining some forward thrust. And I still am not clear about research on embrittlement of sail materials from ionizing radiation. Both polymers and metals are subject to embrittlement. This could be fatal for a structure like a sail.

I don't have access to the full paper here but it may have some cited results that address materials such as sails could be composed of.

https://www.sciencedirect.com/science/chapter/edited-volume/abs/pii/B9780323524728000290