Microwave Rock Fracture on Asteroids

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Do not attempt this unless you know what you are doing. YOU WILL DIE. I fused about $10,000 worth of (barely obtainable surplus) equipment in about thirty seconds. I didn’t know what I was doing and only escaped injury or death by sheer luck.

The massive machinery used in terrestrial mining is nearly worthless on an asteroid. Mechanical (kinetic) methods of fracture encounter both rebound from the surface and breakage and wear of tools that cannot be replaced or manufactured locally. A massive bulldozer will spin its treads and launch itself right off the surface of an asteroid.

To transport mining equipment to an asteroid it has to be low mass. Equipment for asteroid mining needs to replace common terrestrial mechanical methods with other methods. The tradeoff for using low mass mining systems is that it takes longer to excavate the same amount of material.

Microwaves and radio frequency methods for delivering heat into rock are far superior to lasers. A Laser’s energy stops at the surface and the heat has to travel from the surface to the interior by thermal conduction. Microwaves can directly place heat into the interior of a rock.

If you carefully impedance match the output of a microwave horn to the surface of a rock the microwaves will proceed into the rock as a microwave beam. The beam width might be around 20 to 30 degrees wide if you did a very good job.

Microwaves deposit some of their energy directly into the deeper volumes of the rock. Even with microwaves the normal thermal gradient is highest on the surface decreasing with depth. If multiple microwave beams interfere at some depth it may be possible to generate a thermal inversion. You would be able to create hot spots below the surface. If you are able to heat a volume of rock 5 to 10 centimeters below the surface, the thermal expansion of that rock might fracture the rock between the hot spot and the surface.

Microwaves are best used as a beam in order to control the placement of thermal energy into the rock. If you do not match the impedance of the microwave horn to the rock properly (a deformable sand bag with a correct mixture of dielectric materials can bridge the gap) you will create a circular wavefront. A circular wavefront is like a rock dropped in water radiating in all directions from the microwave’s point of contact. A circular pattern energy dispersal is far less controlled or effective as using a beam.

Many attempts at fracturing rocks with microwaves did not try to maintain the microwaves as a beam (or fan) within the target rock. The heating produced was localized and non-directional. Even so they did obtain limited results.

An antenna or horn would be pointed at the rock surface and the heating would form a spherical gradient at the point of maximum heating. Sometimes the rock would melt at the microwaves highest point of intensity. In the case of small rocks or boulders with low microwave absorption the microwaves could reflect back from the rear surface of the rock (total internal reflection) and then interfere with the incoming microwaves. This standing wave would create pockets of heated rock which would occasionally shatter the entire rock by thermal expansion.

You can even shatter a rock in your own microwave. By choosing a spherical rock made from the right material you can create a perfect standing wave in the center of it. The rock will detonate. If you actually tried this and did not concurrently earn a posthumous Darwin Award, you will still have to pay for a new microwave.

Heating a rock is just the first step in being able to excavate and tunnel on an asteroid.

(Part 1)​
 
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The Concept of Thermal Structures and Their Application to Rock Fracture

How to rapidly heat rocks (part 3) is another separate topic but for these following examples assume that we are using microwaves.

A thermal structure is a volume of material extending below the rock’s surface whose temperature has been rapidly elevated. A thermal structure is a temporary structure that when used in a planned manner can cause internal stresses which will disintegrate the rock.

I will start by heating a line along the surface of a rock.

By rapidly heating a line on the surface of a rock you are producing a 3d column of heated material. That column of heated rock will thermally expand.

The maximum compression force will be at the ends of the thermally expanding column. The maximum shear will be parallel to the column ends. All parts of the column will expand outward against the enclosing rock. Because the enclosing colder rock could be a bit stronger, the heated column of rock will tend to bow outward from the surface. This produces a tensile (pulling force) between the center of the rock column and the surrounding rock.

The compression of the rock tends to be ineffective since competent rock in compression is usually pretty strong. The shearing forces produced by lateral movement tend to be the most effective. The hotter you can heat the rock the more effective those shearing and tensile forces will be.

Thermal structures are used in systematic combinations. By varying the shape and orientation of thermal structures you can rapidly vary the direction of stress fields within the rock. Heated rock expands and cooling rock contracts and by artfully combining these two effects they can produce an oscillating or sawing stress field within the rock. The rock’s brittleness and strength are being used to rip it apart.

Thermal structures can be compared to accelerated thermal aging of rock. But instead of centuries it might only take an hour to fracture and separate 10 to 15 centimeters of rock surface. In space much of the rock outwards of Earth’s orbit is colder than -100 C. This makes it a sink for heat which means that fracturing operations of long duration are not likely to cause permanent local heating. The greater the thermal differential the greater the stress.

There is a book W Maurer “Novel Drilling Techniques” 1968 gives examples of how to break rocks with microwaves, jet burners, electrical currents, etc.

Another book that had some interesting ideas is called J.A. McGeough Advanced Methods of Machining 1988. Electrochemical and spark types of machining. If you have the chemicals (and can seal the environment) these might represent other ways of doing mining or manufacturing in space.

(Part 2)​
 
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Methods of Producing Heat Within a Rock Surface​

Rocks and minerals can be modified by drilling and other process to change their responses to electrical forms of heating. This section will deal with unmodified rock.

The Chondritic class of asteroids may have a fictional geological origin. You cannot automatically assume that all chondrites are ancient. The Chondritic class appears to be a useful type of asteroid since individual chondrules could be base metals or distinct minerals. Crushing and separation of chondrites might produce a host of ores from a single source asteroid.

Microwaves​

Microwaves are only useful on rocks which do not totally reflect or completely allow microwaves to pass through them. Microwaves are reflected by iron asteroids so it doesn’t work well on iron asteroids. On Earth microwaves are nearly useless on rocks with a lot of quartz. Quartz is transparent to microwaves so it doesn’t absorb microwaves and heat up. The microwaves pass right through them without producing significant heating. On asteroids’ quartz or high silica rocks will be largely absent since they are products of crustal differentiation on planets.

To effectively use microwaves on rock you need to form a narrow beam. To get that beam to enter the rock you will need to match the electrical impedance of the rock. The electrical impedance of rock is usually several times that of vacuum for this example we will assume the target rock has an impedance of 3 at the microwave frequency that we are using.

Physically an impedance of 3 means microwaves have 1/3 the physical dimensions that it has in vacuum. The simplest way to get an impedance match is within a waveguide. If you impedance match from vacuum to a solid dielectric with a value of 3, the dimensions of the dielectric filled wave guide will be 1/3 that for the air waveguide (I think).

The longer the wavelength the more power a single magnetron can generate. 120 kW output power at 915 MHz and 3 kW output power at 2450 MHz (common household microwave frequency but more powerful).

At 120 kW the dielectric in the waveguide and horn has to have incredibly low losses (heating) or it has to be actively cooled, or a liquid dielectric could be circulated through a heat exchanger. If nothing blows up you can use a bag full of dielectric material (or circulating dielectric fluid) to bridge the gap between the end of the dielectric filled horn and the rock surface.

The microwave applicator can be a lot more sophisticated than this. They can be designed to generate and sweep multiple independent beams across the rock simultaneously. To take full advantage of the thermal structure technique you need a much more capable design which can create multiple thermal structures simultaneously. A detailed discussion of this design is way beyond the scope of this blog. (Plus, I left the plans on Scarif).

This is where the actual conditions on asteroids become a problem. With multiple high power microwave beams you might get them to intersect as much as a meter below the surface. At the intersection of these beams’ interference can create a hot volume of rock (thermal inversion of normal gradient). This could cause a detonation especially if volatiles are present. Both rock and equipment could be blown off the surface of the asteroid if this hot spot ruptures.

There are practical limits on the amount of microwave power which can used for asteroid mining. A big single beam is not as useful as more numerous less powerful beams.

Another problem is the power source for the microwaves. Out to about Mars you can still use photovoltaics. Using photovoltaics with concentrating mirrors can improve on that some. Either way your power generating capacity limits your mining capabilities. You can charge batteries and then use them to fracture material from the rock face. While you are collecting or processing the fractured material you can let the system recharge.

Producing photovoltaics, batteries, and mirrors will easily be in the top 10 priorities for space manufacturing.

You will need to have the knowledge and techniques down for manufacturing these staples in space before you leave Earth. At the very least you have to minimize the duration of experimentation. An experiment in space must help select between a predetermined set of options.

Checking out the alien archaeology comes after you have made sure that the colony doesn’t die horribly in a thousand different ways. Most of the alien artifacts won’t get up and walk off while you are not looking.
 

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