Re: Solar powered lasers in space

On 18 Sep, 19:05, Willie.Moo...@xxxxxxxxx wrote:
On Sep 18, 11:56 am, Ian Parker <ianpark...@xxxxxxxxx> wrote:

One further point on phase mirrors. They will not work in deep space.
In the case of an asteroid one wouold have to have a pilot spacecraft
where the Asteroid will be a double journey into the future.

- Ian Parker

On my google groups tree this is response 41 when sorted by reply, and
for the life of me I cannot bring up response 40 - so I don't know
what to say there.

I'm sorry. I had that rather as an afterthought. The pure phase mirror
will work only for stationary targets. For targets aroung the Earth
you have about 1/3 sec. For the asteroid it might be up to 1hr.

Beaming a tiny spot to an asteroid is a matter of figuring out the
wavelength and the Rayleigh criterion. The assertion you make here -
and the conclusion - doesn't make any sense - so you've missed
something friend.

If we are to seriously consider asteroid capture, or its twin,
asteroid deflection, using sunlight, then we need lots and lots of
power. That means we'll have to dip in close to the sun - this is
several powersat generations away - and it won't be the first thing we
build. But we WILL use micro-nuclear triggered fusion pulse - to get
rid of nuclear materials relatively cleanly - in a variety of ways in
space - one of which is to develop techniques of moving asteroidal
materials swiftly around the solar system.

I don't think I have missed anything. At 200m km you have 2*10^11
wavelenths. This means that 1.22d1.d2= 2*10^11 wavelengths or
2.44*10^5m^2. If we focus a 10m beam on asteroid this gives us a
diameter of 2.44*10^4m. 24km. Is that possible. One point which is
often missed when discussing this is the fact of phase coherence
across an array. This is really the point I am trying to get across.
If you have a single laster with a 10cm mirror that will extend to
500m at 42,000km. If you have a phased array however you can focus
onto points < 1m in size.

Consider though in the far future, a thin film automated system of
cells, that use a combination of solar wind and light pressure to
navigate to a region inside the orbit of Mercury. These cells -
manufactured and sent into space - operate at a relatively high temp,
and so can withstand being a few million km from the surface of the
sun. They are stationary held above the sun by a combination light
pressure and solar wind. The cells join together to form a mat - by
self-assembly - and they coordinate with each other by all seeing the
same reference laser from the target - beamed from anywhere in the
solar system. In this way laser emitters several hundred kilometers
across - operating at 1 MW per sq meter or more - can be contemplated.

Forward contemplated using fresnel lenses to collimate large solar
pumped lasers - in the TW range - to project light efficiently to
laser light sail spacecraft.

Here, we are using advanced laser beaming technology at the emitter
itself, combined with very large structures, located close to the sun,
to produce similar beams.

sin theta-r = 1.22 * lambda / diam

where lambda = 1 micron (1e-9 m)
diam = 20 km (2e4 m)

so, sin theta-r = 6.1e-14

Now, a spot 20 cm in diameter can be formed 1.2 billion km away.from a
20 km diameter emitter using 1 micron wavelength radiation. Emitting
1 MW per sq m - a 20 km diam disk emits a total of 314 TW. This
heat source operating a thermal rocket having an ejection speed of 10
km/sec - can produce a steady thrust of;

F = P/(2V) = 3.14e14 / (2*10,000) = 15.7 GN = 1.6 million metric
tons force

So, a spherical mass of 1 km with a density of 2 g/cc - has a mass of
8 billion metric tons. - and can be accelerated continuously at 7 m/s
per hour. (1/5000th gee)

To impart a delta vee of 7 km/sec (which is typical of moving an
object from the asteroid belt) requires 1000 hours of illumination by
this source. 8 objects per year can be handled by this single source
- harvesting 64 billion metric tons of material into MEO each year -
10 tons for every man woman and child on the planet.

Since the asteroid itself is ejected as the rocket exhaust - we can
estimate how much of the asteroid will be used up;

u = 1 - 1/exp(7/10) = 0.5034 = 50.34%

About half.. part at the outset, part when braking into Earth orbit.

So, 5 tons for every man woman and child on the planet.

First, there would be a survey of all the small bodies inthe solar
system, and then they would be rated for their value. The highest
value objects would then be harvested.

While the survey is going on the sun-centerd solar powered laser
system is built.

Of course we do everything in parallel. We are looking at orbits as of
now. As I said in my first contribution to the thread on the NASA
report, a laser system would determine the orbit more precisely, give
greater warning and add up to a far lower delta v.

The solar powered laser then beams energy to a manned spacecraft that
travels to the asteroid, and erects a solar powered thermal rocket
describes - it also uses laser energy to process the asteroid into
portions to keep and portions to eject in the rocket.

The idea of a nuclear bomb is that it vaporizes the surface thereby
proving a small delta v. A laser would essentially do the same thing
but act over a longer time period.

A small safety team stays on the asteroid, riding it back to Earth on
a minimum energy orbit - and making sure all system operate as planned
- and the main spacecraft, goes to the next target.

You don't send it back to Earth, you simply deflect it so that it goes
close to the Earth but does not collide. That is the basic idea.

Five years - R&D - Five years construction - Five years - mission.

A five year mission - haha- would collect 40 asteroids totalling some
160 billion metric tons of highly useful materials into MEO.

Those asteroids would then be processed by orbiting factories - solar
powered of course - via telerobotics (light delay negligible) - which
then deorbit products directly to users on Earth - or deliver them
anywhere in the cislunar system they're needed.

This will form the basis of the first space farm, and space home
developments. And support the large scale movement of people off
Earth into space - aboard their own space colonies.

This sort of scenario provides a way of gradually building up. In any
project you need to have intermediate stages or it wil never be built.

As all of you are probably aware my "hobbyhorse" is AI and robatics
and there seems little doubt that a solar complex would rapidly
develop into a Von Neumann complex in the way that you suggest. I
thought at one point that a VN machine would be needed to build
lasers. If you have a carbon slurry in liquid hydrogen and low launch
costs this may not be the case. Any engineering on asteroids has to be
thought of in a Von Neumann context.


- Ian Parker

.

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