# [ExI] No mirrors, was Deceleration mirrors

Anders Sandberg anders at aleph.se
Mon Sep 21 08:20:18 UTC 2015

```On 2015-09-21 07:45, Keith Henson wrote:
> I don't know if he ever published it, but heard Eric Drexler proposing
> to send out laser pushed vehicles that would reorganize themselves in
> flight as a coilgun.

It is on our to-do list, but rather far down, unfortunately.

> When they were close to a target system, they would fire bacteria
> sized nano seeds backwards, scattering them in the direction of
> planets with air at a low enough relative speed that they would not
> burn up.

One can view the main seedship as the exhaust of the seed rockets: the
rocket equation quite neatly ignores whether the expelled material is
dumb exhaust or an entire starship containing the actual engine.

We have looked both at the laser launch and in-flight damage
deflection/repair. Some cool problems there (interstellar hydrogen
becomes a proton beam when you move fast enough).

Rafal:
### Hm, if you can use stationary lasers to accelerate a moving target,
maybe you could use a moving laser or even a non-coherent but
directional light source to decelerate itself? What is the light
intensity per kilogram of laser needed to decelerate a laser at 1 g? How
many orders of magnitude away is it from current specs?

When launching with a laser, as the target accelerates, you both have
the problem that it gets further away (eventually the beam gets too
dispersed to be effective and starts falling off as 1/r^2) and it
receives the light redshifted. Slowing down by ejecting light is great
from this perspective, since you do not care about what the beam hits
(much; neighbor civilizations might be annoyed by light pollution).

The light pressure of a power P beam is P/c, so if you want 1 G
acceleration just plug it into F=ma and get P/c = mG. That is, P=mGc =
1e-3*9.82*300e6 = 2.9e6 W. So a 3 MW laser would work.

The naval free electron laser ONR is working on is 14 kW, but they plan
to scale it up to the megawatt range. Unfortunately it is not continous
mode. The Mid-Infrared Advanced Chemical Laser (MIRACL) at the High
Energy Laser Systems Test Facility at White Sands Missile Range
apparently reached megawatts for 70 seconds. The movable part weighs
about 8 tons. So we are a few orders of magnitude away in weight and
firing time. (Were a 1 g ship to do 3000 seconds of 3MW - the current
total time of MIRACL - it would get a delta-v of 29 km/s).

--
Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

```