reflecting sunlight onto the Moon?

Joann Evans wrote:

If you really *must* do something like this, base these lasers in
stationary Earth orbit.

Good grief. Do you really think a laser in earth orbit is going to
be less than three times the cost of similar lasers on the ground?

Space is *expensive*. You don't do things there if you can avoid it.

Paul

"Paul F. Dietz" wrote in message ...
Joann Evans wrote:

If you really *must* do something like this, base these lasers in
stationary Earth orbit.

Good grief. Do you really think a laser in earth orbit is going to
be less than three times the cost of similar lasers on the ground?

Space is *expensive*. You don't do things there if you can avoid it.


Well, if we were on our way to building SPSs for Earth already, adding
a transmitter pointing to the Moon might be the most economical solu-
tion...

Problem is, we have a chicken/egg problem if we did it that way. We'd
need SPSs to support Lunar mining and we'd need Lunar resources to
build SPSs.


Maybe building an experimental SPS in GEO that could be pointed to-
wards either Earth or the Moon would be a good investment, though.
It wouldn't be strong enough to make any difference to Earth's energy
economy, so after it's demonstrated the practical feasibility of mi-
crowave power transmission it could be turned around and both trigger
and supply Lunar exploitation...

As long as it has to be built out of Earth resources it's certainly
cheaper to shoot it into GEO rather than L1. Focusing from 384,000km
rather than 27,000km away is a little tougher, though...



--
__ "A good leader knows when it's best to ignore the __
('__` screams for help and focus on the bigger picture." '__`)
//6(6; ©OOL mmiv :^)^\\
`\_-/ http://home.t-online.de/home/ulrich....lmann/redbaron \-_/'

Ool wrote:
| Space is *expensive*. You don't do things there if you can avoid it.
|
|Well, if we were on our way to building SPSs for Earth already, adding
|a transmitter pointing to the Moon might be the most economical solu-
|tion...
|
|Problem is, we have a chicken/egg problem if we did it that way.

No matter which way we do it, there's a chicken/egg problem.

|We'd need SPSs to support Lunar mining and we'd need Lunar resources
|to build SPSs.

If we are going to have Lunar mining going on, we need to know where
to dig. Surface fly-overs can only give you so much information,
we need to know a little bit more about how the Moon is put together.
Install a series of seismometers first, before doing any excavations.
(Do I have to state the obvious?) And since near-Lunar orbits are
inherently stable, save those for *after* the seismometers are deployed-
They can afford us the more information if they are delayed until *after*
the seismometers (communications beacons) are deployed.

Ool wrote:
| Second, how much light is a minimum for them to survive local 2-week
| night?

I had a Ki plant (also known to some as the Ti plant) that regularly
survived 2 weeks of pitch black night, you just had to keep them warm
during the night, they'll kick into overdrive once you give them light
again. This kind of plant is perfect for hydroponics because it is one
of those things that likes to suck water in through its roots, and pump
it upward through its stem. Unfortunately, it is a non-fruiting plant,
and it is only good to propagate through shoots and cuttings. Just be
sure you give the plant a week of constant light before turning off the
lights for 2 weeks. And don't let it get cold, it will suffer big time.

| Does the artificial light need to be 16 hour off/8 hour on, and
| at what minimum intensity? What wavelengths are most useful? The
| plants do not need to grow at night, just survive.
|
|Pass...

Broadband "solar" spectra were unnecessary for my "Ki" plant. I used
a host of ordinary 60 watt lightbulbs in my computer room, with one
of them poised 1 foot away from the plant.

Now, as for raising tomatoes, I will bet you would need more "candle
power" for those little things.

In article ,
Ian Stirling wrote:

Can anyone fill in some more details for this idea?

It may be a case of filling in the hole for it to be buried in.
It's got some problems.

That's OK; this is why I asked. Thank you for answering the question I
asked instead of going off into tangents about other ways to supply
power -- I know the other ways, I want to explore this one.

The moon has a diameter of 2000Km.
So, at best the light has to come at midnight around 2000Km.

Well, at best it'd be 1000 km (the radius, not the diameter, right?).
But I think I get your basic point here anyway -- the satellite would
have to be either very close to the horizon, or in a very high orbit (or
both) in order to be in sunlight while the base is at midnight.

The suns rays diverge at an angle of .01 radians, so the image of the
sun will be at least some 20Km in diameter by the time you get to the
base.

OK, I'm perhaps exposing my ignorance here, but this sounds like you're
assuming a flat reflector rather than a parabolic one. A flat mirror
would of course be silly. But the idea is to use a parabolic one to
gather light from a larger area and focus it down to a spot on the
surface of the Moon. That means, in principle, you could get
intensities much higher than you get during the day. But in reality,
this would rarely happen, since I assume you wouldn't be able to
maintain such focus for long.

This is a good match only if your base is some 20Km square, using some
400Gw of power.
The reflectors on the satellites would need to be around 30Km or so
across.
This has big nasty problems from masscons in the moon.

I think this all relates to the assumption of a flat reflector (or
series of reflectors), doesn't it?

What about another way.
If you built a polar station, you could probably get by with a string
of reflectors in polar orbit.

A 200m spot of sunlight, of some 30Mw power seems more likely
for a near-term base.
This can't be sent from much over 20Km distance.
Orbital velocity on the moon is around 2Km/s, so a reflector will be
in view for some 10 seconds, so you need some 180 of them.

Ouch. If I were going to build a polar base, it'd probably be easier to
just stick my solar panels an a tall tower, or to have them on a ring of
slopes around the pole connected by cables.

The really horrible problem is how do you accurately slew a mirror
200m wide by well over 90 degrees in 10 seconds.

Well you don't; you slew a secondary mirror.

Add to this that you'r going to be needing large amounts of thrust
to keep in a 20Km orbit, and things just go downhill.

Granted, this 180-satellites-in-20Km-orbit scheme doesn't seem worth it.
I still wonder about lighting a base at lower altitudes with satellites
in a higher orbit, using parabolic reflectors.

Thanks,
- Joe

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

In article ,
Joe Strout wrote:
The suns rays diverge at an angle of .01 radians, so the image of the
sun will be at least some 20Km in diameter by the time you get to the
base.

OK, I'm perhaps exposing my ignorance here, but this sounds like you're
assuming a flat reflector rather than a parabolic one...

No, there's a problem regardless, because the Sun is not a point source.
Light from the Sun reaches *each point* of the mirror over a range of
angles, not from a single direction. There is no magic way to get rid of
*that* divergence -- to make each point of the mirror reflect some light
at one angle and other light at a slightly different angle (relative to
the direction of its arrival).

...the idea is to use a parabolic one to
gather light from a larger area and focus it down to a spot on the
surface of the Moon. That means, in principle, you could get
intensities much higher than you get during the day.

Not with a reasonable-sized mirror at a long distance. Fundamentally the
best you can do is to give the mirror the same apparent brightness, per
square degree of sky it occupies, as the Sun. So to equal daytime
intensities, the mirror must occupy at least as many square degrees as the
Sun, i.e. at a distance of (say) 2000km it must be about 20km across,
minimum.

Like quite a number of other lunar-night-power concepts, this one actually
works reasonably well on a very large scale, but rather poorly on the
small scale of an early base.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Joe Strout wrote:
In article ,
Ian Stirling wrote:

Can anyone fill in some more details for this idea?

It may be a case of filling in the hole for it to be buried in.
It's got some problems.

That's OK; this is why I asked. Thank you for answering the question I
asked instead of going off into tangents about other ways to supply
power -- I know the other ways, I want to explore this one.

The moon has a diameter of 2000Km.
So, at best the light has to come at midnight around 2000Km.

Well, at best it'd be 1000 km (the radius, not the diameter, right?).
But I think I get your basic point here anyway -- the satellite would
have to be either very close to the horizon, or in a very high orbit (or
both) in order to be in sunlight while the base is at midnight.

The minimum distance is around the circumference of the moon, so a
quarter of the circumference (1550Km or so) in practice it'll be
a bit higher.

The suns rays diverge at an angle of .01 radians, so the image of the
sun will be at least some 20Km in diameter by the time you get to the
base.

OK, I'm perhaps exposing my ignorance here, but this sounds like you're
assuming a flat reflector rather than a parabolic one. A flat mirror
would of course be silly. But the idea is to use a parabolic one to
gather light from a larger area and focus it down to a spot on the
surface of the Moon. That means, in principle, you could get
intensities much higher than you get during the day. But in reality,
this would rarely happen, since I assume you wouldn't be able to
maintain such focus for long.

This is a very, very common misconception.
The sun is about 1/2 degree (1/100th radian across)
Consider a 100m parabolic reflector.
Now, consider a 1mm bit of it.
It's to all intents and purposes flat.
The reflection from this spot will be a half a degree image of the sun.
The addition of all the images from all the spots can mean that the
sun can appear bigger, but it can never appear brighter.

The reflection of any mirror system you can make can never exceed the
brightness of the source.
If it could, you can make a heat engine that violates the laws of
thermodynamics.
(you can fudge a factor of several by using refractive elements as it
is possible to concentrate the light more in a higher refractive index
material, but not much, as the highest RI is under 5)

Matthew Montchalin wrote:

If we are going to have Lunar mining going on, we need to know where
to dig. Surface fly-overs can only give you so much information,

Far too expensive. Taking a good guess backed by some data for regolith
content is about as much as you can do or afford for the first sites.

we need to know a little bit more about how the Moon is put together.
Install a series of seismometers first, before doing any excavations.

Seismometers won't tell you much useful about mineral deposits.

(Do I have to state the obvious?) And since near-Lunar orbits are
inherently stable, save those for *after* the seismometers are deployed-
They can afford us the more information if they are delayed until *after*
the seismometers (communications beacons) are deployed.


Since when is there a shortage of Lunar orbits?

--
Sander

+++ Out of cheese error +++

In article ,
Ian Stirling wrote:

This is a very, very common misconception.
The sun is about 1/2 degree (1/100th radian across)
Consider a 100m parabolic reflector.
Now, consider a 1mm bit of it.
It's to all intents and purposes flat.
The reflection from this spot will be a half a degree image of the sun.
The addition of all the images from all the spots can mean that the
sun can appear bigger, but it can never appear brighter.

The reflection of any mirror system you can make can never exceed the
brightness of the source.

Please bear with me, I really want to understand this. What you say
here doesn't make sense to me; perhaps we're using words to mean
different things.

I have a solar cooker kit which uses a parabolic reflector to
concentrate sunlight enough to cook hamburgers. Hamburger left out in
unconcentrated sunlight does not cook. Is the light not brighter at the
focus of the reflector?

Suppose I take a hand mirror and use it to shine a spot of sunlight into
the shadow of a tree. Now I take another hand mirror, and direct its
image of the sun onto the same spot. Is that spot not brighter with two
mirrors than with one?

Parabolic trough power plants use a mirror with a parabolic
cross-section to concentrate the sun's light onto a tube of working
fluid. Isn't this making a line of brighter light on the tube?

Whatever it is that these systems are doing, I want to consider doing
the same thing to a base on the night side of the Moon.

Thanks,
- Joe

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

In article ,
Joe Strout wrote:
I have a solar cooker kit which uses a parabolic reflector to
concentrate sunlight enough to cook hamburgers. Hamburger left out in
unconcentrated sunlight does not cook. Is the light not brighter at the
focus of the reflector?

Yes, but only because the reflector fills much of the "sky" around the
focus. The reflector is filling a good bit of the focus's view with
(loosely speaking) images of the Sun, which have the same brightness per
square degree as the Sun. This approach only works if the reflector is
very close, so it can fill a sizable fraction of the sky.

Suppose I take a hand mirror and use it to shine a spot of sunlight into
the shadow of a tree. Now I take another hand mirror, and direct its
image of the sun onto the same spot. Is that spot not brighter with two
mirrors than with one?

Correct. The more of the spot's sky you fill, the more light you can put
onto the spot. But you can't put very much on it from a single small
mirror held far away, no matter what shape that mirror has. Passive
optical systems (mirrors, lenses, etc.) can reduce the amount of light
coming to the spot from each square degree of sky, but can't increase it.

Parabolic trough power plants use a mirror with a parabolic
cross-section to concentrate the sun's light onto a tube of working
fluid. Isn't this making a line of brighter light on the tube?

More or less, but only because the trough mirror pretty much surrounds
the tube.

Whatever it is that these systems are doing, I want to consider doing
the same thing to a base on the night side of the Moon.

How much of its sky are you prepared to fill? If you're doing it from,
say, 2000km away, each half-degree circle of sky requires a mirror 20km
across.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |