Yuri Milner breakthrough mission to Alpha Centauri Options

[Guest_mcmcmc_*]

https://breakthroughinitiatives.org/about

Laser-powered nanocrafts headed to Alpha Centauri:

Breakthrough Starshot is a $100 million research and engineering program aiming to demonstrate proof of concept for a new technology, enabling ultra-light unmanned space flight at 20% of the speed of light; and to lay the foundations for a flyby mission to Alpha Centauri within a generation.

Which are the engineering challenges?
http://breakthroughinitiatives.org/challenges/3

Yuri Milner twitter feed (official?): https://twitter.com/yurimilner

[Gerald]

Here is a pretty detailed study of a similar approach, called sail beam.
One of my major questions/concerns is the energy conversion efficiency, i.e. which fraction of the energy of the laser beam is converted into kinetic energy of the space probe? The referenced paper says 6.6 N/GW for an idealized mirror. How does this translate into the laser energy required to accelerate a 1 gram sail to 0.2 c? And how does this energy compare to the energy produced by a typical 1 TW power plant within a year? The straightforward idea, that the powerplant is producing the required TJ in one second doesn't hold, since just the momentum of the photons is used for propulsion, not their energy. Another comparison would be the mass equivalent of the laser energy required to accelerate a light sail of a given mass to 0.2 c.
A (sub-) monolayer graphene mesh would probably be considerably lighter, but also pretty translucent, such that only a small fraction of the light would be reflected, and realease its momentum.
The straightforward idea to stack such layers of graphene meshes, however, would result in multiple reflections between these layers and annihilate parts of the momentum of the photons.