The concept of in-situ resource utilization (ISRU) has long been an important one for space settlement. The more one can “live-off-the-land”, the less one has to bring with them, the more affordable settlement becomes. If one can lower the cost of settlement far enough relative to the wealth distribution of mankind, offworld settlement may become feasible.
Each extraterrestrial environment has different characteristics, and lends itself to different types of ISRU:
- For the moon and NEOs there is practically no atmosphere, so most ISRU concepts focus on solid feedstocks–either bulk regolith, solid metals/ores, or possibly frozen volatiles mixed-in with regolith. Solid-state ISRU techniques typically require massive equipment for physically extracting the raw materials, moving them to a processing site, breaking them up into smaller pieces, and then whatever thermal, chemical, or electrical process is used to separate out useful ISRU products.
- For Mars, Venus, and other planetary environments that have atmospheres, the atmosphere itself can provide a feedstock for ISRU. Both atmospheres have a mix of gases and condenseable vapors that can be processed in the gas-phase. In the case of Mars, solid-feedstock ISRU is also feasible, but in the case of Venus, mining the surface is likely a much harder prospect due to the very high pressures and temperatures. But in the case of Venus, the upper atmosphere is, I think, mostly clear of dust, eliminating one of the key challenges to reliable atmospheric-feedstock ISRU.
It’s instructive to look at terrestrial resource extraction. Most people think that Liquid Oxygen for rockets must be very expensive, but it’s actually one of the cheapest bulk substances known to man. According to this list, about $175/tonne for rocket-grade LOX at KSC, compared to about $4800/tonne for bulk rocket-grade Kerosene (gasoline is a bit over $1200/tonne down the street for comparison). Liquid Oxygen is pretty much produced via what one could consider an atmospheric-feedstock ISRU process here on earth. Air gets sucked into a system that chills and compresses it until the liquid oxygen distills out, then the liquid nitrogen, and at various other temperatures other trace elements in the air. Energy costs end up being very important in the case of LOX and LN2 because they have to be chilled to cryogenic temperatures, but even then these are ridiculously cheap materials. Unprocessed dirt is cheaper–about $15/ton delivered in ~15ton chunks within the greater Atlanta area, and Portland Cement comes in at around $110/ton in Boston (according to this quote from ENR).
Isn’t it kind of crazy to realize that a rocket-grade cryogenic oxidizer, being sold by the government on a military base is only about 50% more expensive than bulk construction cement in Boston?
To me this illustrates why atmospheric-feedstock ISRU is so interesting economically, and why Venus’s thick atmosphere with a wide range of raw materials may be an ISRU bonanza.
Next up: What do we have to work with?
Jonathan Goff
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