[ExI] Deceleration mirrors

Anders Sandberg anders at aleph.se
Thu Sep 17 10:38:23 UTC 2015

On 2015-09-11 01:46, Rafal Smigrodzki wrote:
> Let's say we build a series of large mirrors and possibly Fresnel 
> lenses and send them into space in the direction of a planned ship 
> cruise. Then we send the sail ship, followed by a series of lenses 
> focusing the laser beam on the ship until midway to target, at maximum 
> acceleration, and then switch to the mirrors ahead of it, to effect 
> deceleration. The mirrors eventually crash into the target star or 
> perhaps are left to sail forever but the ship is rapidly slowed down 
> to reach target in the shortest time.

This is very similar to the Landis interstellar lightsail model. You use 
a big sail to accelerate, then detach it and use it as a mirror to slow 
the payload section that has a smaller center lightsail.

The lenses only allow you to accelerate very efficiently for longer, 
they are unlikely to be helpful near the destination. However, 
sacrificial lenses and sails make for good SETI targets.

> This would be a complex scheme, and I am not sure if it would offer 
> any benefits over the alternatives. I feel that having a ship capable 
> of consuming its sail and using it as reaction mass in a plasma or ion 
> retro-rocket would still be overall faster and more reliable but 
> future starship builders should do the math on the deceleration mirror 
> idea as well.
The rocket equation is nasty, which is why light sails and laser 
propulsion is so neat. But if you are going to expel mass anyway, if you 
can do it with a very high velocity you can get some impulse from it.

In my "spamming the universe" scheme I looked at enormous coilguns, 
essentially a Dyson sphere with light-second long coilguns (a Dyson 
urchin?) since they also get around the rocket equation. Eric D pointed 
out that lasers would be even more effective, and the lens trick could 
allow focused acceleration for a surprisingly long time.

Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

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