Ice has been discovered on Mars and is present in the permanently shadowed craters on the Moon and on many asteroids. The ice is usually buried beneath an overburden of regolith. Evidence indicates this overburden may be a meter deep in some locations for the Moon; for Mars, it varies with latitude and may be as deep as or deeper than two meters in many locations. To obtain this ice as a resource in usable quantities, existing technology will require that it be strip-mined.
The overburden of regolith must be mechanically removed so that a rover can drive down to the ice and chip off quantities from its upper surface. Chipping frozen regolith is extremely difficult because it has the mechanical hardness of granite. Using lightweight rovers appropriate to spaceflight, the removal of so much overburden will be a long and cumbersome process, making the use of this resource much less feasible. Chipping ice in low gravity will be energetically expensive and mechanically difficult. Excavation of regolith in low gravity with vanishingly small traction forces is very difficult. Operating in cryogenically cold conditions where the ice exists is difficult because metals become brittle and machinery wears out or fails. A better, more efficient method to extract ice from the regolith of bodies in space is needed.
The purpose of this innovation is to drill into the regolith to a depth that is both rich with ice and beneath the surface, sublimate ice around the bottom of the bore shaft, and capture it in vapor form at the cap at the top of the bore shaft, where it is liquefied or re-frozen for transportation and storage. The icy regolith therefore need not be excavated via strip-mining, but instead acts as a relatively impermeable bottle to channel the vapors back to the equipment at the surface. The technology performs this sublimation in an energetically efficient way so that a sufficiently large quantity of ice is extracted from each borehole without wasting energy.
The innovation consists of a drill that is extracted from the regolith after each borehole is drilled. One of several different devices is then lowered into the borehole to heat the regolith around the shaft of the drill, especially toward the bottom of the shaft. A cap on the top of the bore shaft keeps sublimating ice from escaping. The relatively impermeable regolith ensures the vapors will travel up the bore shaft where it will be captured, rather than sublimating away to the vacuum of space.
For regolith conditions where the borehole might collapse as ice is removed from the surrounding soil, the technology includes three different methods to keep the borehole from collapsing. One method is to insert a sleeve or pipe into the shaft before ice extraction begins. The second method is to sinter the soil around the bore shaft so that it melts locally and becomes a mechanically competent pipe. The third method is to pour cement (made from local regolith at the surface with an added binder) into the bore shaft similar to the method used for oil drilling on Earth. However, in many, if not most cases, there will be no need to stabilize the shaft because the regolith is already sufficiently competent to maintain a smalldiameter shaft without collapse.
The technology also includes a system to cap the top of the bore shaft to capture and cool the vapors after they reach the cap, and to pressurize them in pipes or tanks to bring them to the liquid state for transportation and storage or to freeze them back to the solid state for transportation and storage.
Heating can be supplied in the bore shaft by several methods. It can be by simple electrical heating coils, which then radiate and/or directly conduct thermal energy into the ice. It can also be by microwave or lasers that can be tuned to the maximum absorption frequency of the icy regolith. The microwave or laser hardware could be inserted into the shaft or it could remain at the surface, pointing down the shaft. Passive reflectors or re-radiators could exist at the bottom of the shaft to help couple the radiated microwave or laser energy into the surrounding regolith. For example, a material that absorbs microwave energy to become hot could be inserted into the bottom of the shaft to couple the microwave energy into the regolith far more efficiently.
For sintering the surrounding soil, microwave, laser, or electrical heating coils could be used. The device can be lowered slowly into the shaft, sintering the walls as it goes. The sintering hardware could be attached directly to the drill bit so that the shaft is drilled and sintered in one operation, or it could be lowered into the shaft after the drill is extracted. It is optimal to perform the sintering as closely behind the drilling operation as possible, because the drilling will have already heated the walls of the bore shaft so sintering will be more energetically efficient. The sintering can be accomplished with the same device or a different device than the one that performs heating for ice extraction. However, for sintering, the energy levels need to be much higher so that not just ice melts/vaporizes, but the mineral grains of the soil also melt. Partial melting creates a sinter instead of a glass, and is desirable so that molten material does not fall into the bore shaft.
Peripheral equipment includes one or more transportation devices such as a rover, a flying platform, or any other method of moving equipment around on the body in space; hardware to liquefy or freeze the vapors after they reach the well’s cap; and pipelines, tanks, or vehicles to move the liquid or frozen volatiles around after they have been either liquefied or re-frozen at the surface.