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Comment by sandworm101 | Hacker News Reader

>> 2D sails pave the way for reaching other star systems in a human lifespan.

Pave the way? Explain to me how the laser is going to work? There are some fundimental size/optics restrictions here that keep the sail very much a seconday concern in terms of interstellar travel.

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IIRC the breakthrough starshot project is proposing accelerating a small probe (cubesat sized) to relativistic speed in a very short timespan.

The old Starwisp (I think) proposal involved accelerating a lens as well, keeping it positioned between the laser and the payload sail, to refocus the beam. (A 2D diffractive lens, of course, not a giant piece of glass.)

What are the size/optics restrictions?

For a given distance to the target, there is a minimum required lens size to focus the laser so (most of) it hits the target. The required size is proportional to the target distance.

When the distance to the target is interstellar-scale, the required lens gets pretty big.

You have to focus the outgoing light, the laser energy, just as a telescope does inbound light. To hit a small target (say a 1x1km light sail with a 4x4km beam) at interstellar ranges would require laser emitting equipment many kilometers in size repeated across multiple locations.

How large of a telescope is needed to focus the image of a 4km feature on Pluto? Then multiply that by a few orders of magnitude. Then plug in a laser powerful enough to vaporize a small town. Those are the real problems, not the mass of the sail.

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