Planetary Dismantlement [Was SPACE: ...]

Robert J. Bradbury (
Fri, 30 Jul 1999 10:17:01 -0700 (PDT)

Well, Eugene raises some interesting points. I believe there are solutions for his objections. Discussion follows --

> Eugene Leitl <> wrote:
> It is just that Mercury already receives quite a
> lot of radiation. Even if you increase the amount if incoming power by
> one order of magnitude (doubtful the more delicate structures can
> survive it), does it really allow the dismanting process to complete
> in just 11 d? You must be hiding at least a few big envelopes back
> there.

The problem revolves around how "efficiently" you can convert the incoming power to a useful form (avoiding the production of heat). If you beamed the power back with diode lasers at say 520 nm, this is a particularly efficient absorption band for CCDs (or equivalent photovoltaic) structures. I would think that you get a minimum of 70% perhaps up to 90% conversion efficiency (light to electricity). Traditional PV conversion efficiencies are taken for the entire black-body spectrum of radiation (which is why they are so low). You could also beam the energy back in a wide-band form and heat "boilers" to very high temperatures (1500-3000 C probably depending on materials). The gas vapor pressure could be converted directly into mechanical energy for moving materials around or powering turbines. The thermodynamic efficiency of boilers operating at these high temperatures is quite high.

I suspect that if you had the energy coming in at a very specific wavelength, you could construct receivers with just the right thickness and bandgap so that "virtually" all of the photons are absorbed and generate electrons. What you would really want is a structure that allowed the electrons to be immediately moved from the PV conversion material into a superconductor.

You still have a problem of where to sink the waste heat. There are two solutions to this -- the planet and space. The planet turns out to be a good starting point. As you put more heat into the planet it becomes much easier to dismantle. In my calculations for dismantlement I assume a very conservative value that requires breaking every atomic bond in the planet. If you pump a lot of heat into the planet, it becomes soft enough to "flow", then you might centrifuge it to seperate the elements (or as RF points out in Nanomedicine, you could "weigh" each molecule). Then you "pump" the desired elements into containers on the mass drivers and launch it into space. This saves a lot of energy over breaking all of the atomic bonds.

The other possibility is radiating the excess heat into space. Diamond makes a great conductor and carefully engineered surfaces can be > 99% efficient at radiating the heat (see also below). It is worth remembering that only the sun-side of Mercury is hot. The non-sun side is pretty cold and it is thought that ice may exist in sheltered craters at the poles (just like on the moon).

> > You could vaporize the planet completely, but I favor a strategy
> Vaporization is sure easy, but I do not see a way to recapture the
> material quantitatively. The point is not destruction, the point is
> retaining the bulk of the material (especially volatiles) for
> constructive purposes.

You start with vaporization points on the planet, surrounded by a much larger (thin) condensation surface (that radiates the heat into space. The vapor coming off the planet condenses onto the cooler surfaces and is then manipulated into ever larger condesation surfaces. You could envision successive layers of condensation meshs/grids that are heated by the gas mixture but are cool enough to condense specific elements. As the distance from the planet increases, elements condense at lower and lower temperatures. This is not too much different from how the solar system actually formed.

> I grant you nanotechnology, but this does not address the maximum
> tolerable energy flux on planetary body's surface.

The thing is that using nanotechnology, you can "grow" the surface and therefore the radiator power. You put the radiator panels on top of diamondoid beanstalks. You grow the diamonoid beanstalks from the bottom, circulating cooling fluid up through the stalks and and through the radiators. The surface, keeps expanding outward as the heat dissipation requirements increase. There are holes in the radiators that allow the high density energy to be beamed through the radiators down to the surface. [Or a really clever design merges the MW/PV energy absorbers/converters with the radiators].

> You could carry off some energy as hot cargo, but how much can you
> carry off with mass launchers? These coils get awful hot after a
> while.

Yep, you could essentially "liquify" the planet, then "flash-freeze" the surfaces of the mass-pods long enough to launch them. The do the final heat radiation step in space. Alternatively you could launch hot stuff in something like sapphire or tungsten carbide "crucibles". Since there are relatively abundent poor heat conductors, you should be able to easily construct a big vessel, pour in some liquid planet, lanuch it and repeat the process. You don't really care if the thing melts "in transit" since wherever you collect it is going to have a large capacity to radiate the heat away. If you have energy to burn, you could ionize the molecules and simply run them through particle accelerators.

Since you have the problem of cooling the energy collectors (even if they are 99.99999% efficient at converting the incoming energy), you have the problem of cooling the mass drivers as well. You dump the heat first into the planet, then into space (either in the payload or through radiators, as pointed out above). Your comments do point out the need to do some more in depth calculations regarding how you partition the waste heat as the dismantlement occurs. My gut feel is that it goes: (1) Planet, (2) Departing Mass, (3) Radiators.

Re: collector areal density & times for collector migration
> I don't think these are relevant questions. The bottleneck is
> obviously elsewhere.

Without real designs, it is difficult to say exactly where the bottlenecks might be. I identified these because these have the greatest effect in the simulations I have run. As you point out the heat radiation problem is a significant concern, but with 3-4 possible solutions it it appears to be resolvable.

> Yes, and it is a very stringent constraint indeed. Either you have
> numbers I am unaware of, or you're glossing over the issue.

Well, I did "gloss over it" in my discussion, because I viewed "vaporization" as the final (ultimate) approach to the problem. As your comments point out, it may come down to the question of what is faster -- vaporization & condensation or mechanically growing the planet. The first is energy constrained, while the second is most likely element abundance constrained.

> It was most assuredly intentional. It is difficult to envision more
> efficient launch mechanisms than launching from top of beanstalks (and
> a large beanstalk sees mostly 4 K space, not Mercury surface, and,
> well, some of Sun's), Mercury is probably small enough for them (and
> doesn't have an atmosphere).

Good point, so it looks like you have beanstalk mass-driver launchers that circulate cooling fluid to thin radiators spanning the area between the tops of the beanstalks, possibly with some holes to allow highly focused energy beams back to the surface.

> How do you intercept a ~Mt/s stream of evaporated planet? (Or Gt/s,
> don't know how much mass Mercury has divided by 10^6 s).

You simply get far enough away. It turns out that you need to handle about 10^17 kg/sec (average). That works out to a sphere radius of ~10^5 km (less than the radius of the sun) from Mercury if you want to handle 1 kg/m^2. Since that seems rather conservative, I think the sphere could be much smaller. For comparison, average radius of Mercury's orbit is 5x10^7 km. You should be able to use the momentum of the launched mass (if you give it a little extra) to push the mass collectors away from the planet at the rate that matches the increase in mass volume (due to the increased power being beamed down to the planet).

> > > I would like to see an easy way of translating power into such a
> > > coordinated activity as planet dismantlement.
> > The easiest approach is to vaporize the planet and condense it.
> How do you duct it? How do you trap it?

No ducting required, condense it at the distances allowed by natural radiation (as the solar system did). Since you can can construct condensation radiators that radiate "outward" and are much more efficient than "natural" dust this should be easy to control. You could either orbit the condensers or "grow" them from the surface (more beanstalks). To radiate the heat effectively they would have to be thin, and therefore lightweight.

Diamondoid radiators (lower temp than say Al2O3, SiC or Fe2O3), require a sphere that is *smaller* than Mercury's orbit (2 x 10^7 km vs. 5 x 10^7 km). And since to minimize the radiation/heat damage, you want to construct your collector/radiator sphere *outside* of Mercury's orbit, everything seems to hang together. Of course, moving the mass from a Mercury encompassing sphere to a Sun encompassing sphere is going to be an orbital nightmare, but it doesn't seem impossible.

> Energy is cheap, very cheap. A 0.1 um metal foil would reflect ~95% of
> incoming radiation. One needs to tranform the planet into a fleet of
> solar sailers which try to keep the original body in focus. The higher
> the initial insolation the faster the initial growth. Difficult to do
> that trick with Pluto.

Agreed. I thought about doing the asteroids, because you can do more in parallel, but the problem is the insolation is an order of magnitude lower when starting out. For exponential growth you win big with large up-front energy contributions.

> Once again, energy is not a problem. The Sun produces sufficient
> amounts of power that it is difficult to get enough material to make
> the computers -- and then there is lots of fusable hydrogen out there
> in the Oort.

No, the material in Mercury or the Moon provide sufficient Mass (whether the element abundances are correct is an unresolved question) to build sufficient 1 cm^3 nanocomputers to use the entire power output of the sun. The *killer* for MBrains is the radiator mass, not the nanocomputer mass. The further out you build the radiators (for the outer MBrain layers) the more mass is required (heat radiation scales with T^4, so cool radiators suck mass bigtime). The denser you try to make your nanocomputers the more power you consume circulating the cooling fluid and the larger the radiator mass (due to the higher coolant pressures). There are clearly limits to how dense you can do the computing given the mass and energy available. What the exact tradeoffs are probably depends highly on the computer architecture.

When I have time (ROTFL), I've got the CAD/modeling program to do a real design of one of Eric's nanocomputers and a "real" radiator. With that in hand we should have at least one valid point for possible MBrain subcomponents. In current calculations, I assume collector/radiator masses of ~1 kg/m^2 which seems "reasonable".

> Asteroids are fundamentally much easier, because the volume/surface
> ratio and absolute gravitation are not an issue. There is even no need
> to launch, just transfer material to the rim, which flattens things
> and increases surface along with launching mirrors/building rectenna
> arrays/microwave beamers. The only minus is that most of them reside
> beyond Mars' orbit, but that just delays the matters somewhat if you
> have to start from a microgram seed.

Yep. In theory, you could do both Mercury & the Asteroids simultaneously. You could also do part of Mercury (say until it gets to the point where you have melted the planet and the nanomachines, then use that energy to do the asteroids one by one. No matter what strategy you choose, once you are up in the TW power range the whole thing goes pretty quickly. Asteroids do suffer from the requirement that you have to either make the collectors thinner or use more mass (interestingly enough, the estimates I have show Mercury as having more mass than the asteroid belt by at least an order of magnitude). You also have longer travel times for optimal positioning from Asteroidal orbits.

> I am not sure how you can integrate the multiple factors of surface
> increase into one simulation. Lot's of orbiting mirrors will sure
> produce lots of evaporating planet smoke....

True, perhaps better to ionize the smoke and put it into carefully focused particle beams. Energy directed down, Mass directed up, so long as nobody changes lanes it should work.