Marrying asteroid defense and asteroid mining

--- In spacesettlers@yahoogroups.com, "Combs, Mike"
>
> From: spacesettlers@yahoogroups.com
> [mailto:spacesettlers@yahoogroups.com] On Behalf Of Robert
>
> I don't see that making a difference. A detached mirror could be
> brought very close (just short of touching, I suppose), or an
attached
> mirror could be mounted quite a ways away if the struts are very
long.
>
> At what distance the beam was brought to a focus would depend on the
> curvature of the mirror. A smaller curvature would result in the
focus
> being further from the mirror. (That might be advantageous, due to
a
> point coming up later.)
>

the problem with the unmounted one is that while the asteroid is
spinning, it is not the same distance from the mirror. that means
that the focus point (it would have to be relatively small to
vaporize the material very quickly) would be either above the
surface, doing no good or below the surface, therefore spreading the
area the energy strikes, and thus not heating the material up
sufficient for vaporizing before it moves out of range.

>
> It's in the same place all the time on the asteroid surface, but the
> point I was trying to make was that with a solar concentrating
mirror
> one might have the hot spot be consistently on /the same side of the
> asteroid/ (relative to the rest of the universe), even while the
> asteroid rotates under the beam and that hot spot describes a
complete
> circle around on the surface...

sorry, this is not possible. either the hot spot is in the same side
in relation to the path, or its moving around in a tumbling circle
with the asteroid. it cant be both simultaneously.

... For moving an asteroid, what's needed is
> continuous thrust along the same vector. If we've halted the
rotation
> of the asteroid, that vector is consistent with regard to the
surface.
> But if we seek to avoid the complication of de-spinning the
asteroid,
> the vector needed is constantly turning relative to the surface of
the
> body. (For example, MADMEN addresses this by landing a large
number of
> small thrusters which are fired sequentially.)
>

this would only be possible with a mirror if you had several dozen
mirrors rotating around the asteroid, each at a specific height above
its target spot, and each heating that area up in the specific time.
in order to move something away from a specific path, what you have
to do is make sure that all the vectors are away from the path, AND
that they do not cancel each other out. allowing the spot to turn
with the asteroid would cause the vectors to cancel. the end result
would be that the asteroid would not change its course.

>
> That might be a significant concern. Still, I'd say given
sufficient
> area of the concentrator, the material under the focused light
ought to
> explode to vapor in a second or less, I would think.
>

if the beam is hot enough, but that means the spot has to be
comparatively small, the smaller it is the better. problem is the
smaller it is, the faster that spot will move out from under the
spot. the obvious answer is to have a bigger mirror, but that means
that the controls on it are harder to maintain, and it has to have
more power to move.

>
> As with your first point, I don't see a difference here. Such a
disk of
> glass could rotate in front of an unattached mirror as easily as an
> attached one. The only difference is the requirement for RCS on
both
> the mirror and the disk of glass.

the difference is that the shield would cause a movement that would
affect the focus of the beam and would have to be constantly
compensated for.
>
> And I'm not so sure it would be all that easy to clean the glass.
> Depending on the temperature we're assuming, what we'd be doing is
> essentially vacuum vapor deposition, like what puts the silvering
on the
> back of a mirror. It wouldn't be like mud splattering on a surface.
>

actually that would depend on how far away it is. space is very, very
cold. within a matter of feet the vapor would liquefy, and even
congeal. if the shield is far enough away, it would be closer to mud
hitting it than a mist.

>
> I would rate the non-spherically of the asteroid as a bigger
challenge
> to maintaining focus than the course change, which would be a change
> occurring only very gradually over long periods of time, relative
to the
> rotation period of the asteroid. But yes, there are challenges to
be
> overcome here.

again it goes back to how big your mirror is and how big the spot is.
in order to move it away from earth, (depending on how long you have,
if its several years, it would be a small fraction of this) you would
have to move the object a matter of a foot per second, per second.

lets take a spherical (just for ease of calculating) asteroid a half
of a mile wide. lets say you get the mirror there with 8 months to go
(i did some calculating and that puts it very close to a foot per
second per second). lets say that you are using a 10 foot diameter
beam (very large). lets say you have it at a set angle to the path,
not perpendicular, or straight on. you have the beam hitting the
surface in the center as the mirror looks at it. (there are several
things wrong with this, but lets keep it simple). the beam is
sufficiently strong enough to vaporize the surface, causing a cloud
to explode outwards (lets not discuss the problems of that cloud
blocking the beam just yet) and pushing the asteroid a way a foot per
second. now depending on how long your focal length is, that hot spot
will be a bit bigger or smaller depending on where the actually focal
point is. the second second it does it again, and the asteroid is
moving away from the mirror at two feet per second, so the beam has
to travel 3 feet to reach the surface. now the mirror is still
following the original path, but the asteroid is now arching away,
meaning the mirror is no longer looking at the center of the asteroid
as it was before. that means one edge has to go even farther, albeit
as yet, not much. in a minute the asteroid would be 360 feet farther
away and the focus spot would not longer be any where near the
surface and would be lot bigger, that means it would no longer have
the energy to vaporize the material.

all of these (shape of the asteroid causing limited focal ability,
the movement of the asteroid as it is pushed off its course, and the
cloud of material blocking the beam) plus others would be a major
challange, which using a non attached mirror would make harder to
handle.

>
> Maybe. If we depend on cooling of the vaporized material, that
would
> suggest keeping the mirror as distant from the hotspot as
possible. To
> me, that makes it more likely to be free-flying than attached.
>

thats possible, but your forgetting that the gravity of the asteroid
would be very small. a huge tower would be very light.

>
> I think my reason for favoring mass driver over ion engine is that
the
> mass driver can use literally anything for fuel (reaction mass,
really),
> while ion engines require specialized fuel. It would be
advantageous to
> not have to launch the system to the asteroid with the reaction mass
> needed.
>
actually most elements can be ionized (not all of course), albeit
some dont do so well, and others are very heavy. some ionize
negatively and other ionize positively. the method of ionizeing some
will not work on others. the thing is they are all there and once
seperated, they can be dealt with accordingly. some of them might be
saved for taking back to earth after the mission. the difference
between the mass driver and the ion engine is that the ie fires it
mass at several hundred miles per second, while the mass driver only
gets a few. that means it takes a lot less material, and the engine
is a lot smaller.