Comet Mining -- An Overview

Copyright 1993, 1994 by Nick Szabo
distribute freely with attribution, without alteration

Introduction

A relatively unexplored area of space development, comet mining, may play a central role in cracking open the space frontier. Volatile extraction, considered by this author and others[1] to be an important near-term catalyst for large-scale space industrialization, involves the delivery and processing of volatile ice (water, methane, ammonia, etc.) delivered via ice rocket from Jupiter-family comets, which have elliptical orbits between Earth and Jupiter. A visual presentation and marketing analysis were given in [2], technical presentations can be found in [3-4], and various technical and marketing issues have been discussed in [1]. Government [4,10] and commercial [9] organizations are starting to sponsor research in this area. This paper attempts to unite this information into a comprehensive introduction to comet mining.

Comets, the Jupiter Family, and the Need for Volatiles

The only hard and fast rule to distinguish between comets and asteroids is that comets have been seen to outgas. Several asteroids have recently been reclassified as comets when outgassing was discovered, eg Charon. The outgassing indicates active sublimating volatiles. Asteroids either lack free volatiles or they never get warm enough to outgas. Different volatiles (H2O, CO2, CH4, NH3, etc.) outgas at different temperatures. Objects in circular orbits tend to reach an equilibrium temperature where outgassing stops; all known comets are in elliptical or chaotic orbits where temperatures change over the course of the orbit.

By far the largest proportion of materials used by most processing industries are volatiles and organics. This is true for Earth industry -- oil for energy, wood for structure, plants and animals for food, and vast amounts of water and air for nearly industrial processes, which we often take for granted. In space, SSF expendables consist overwhelmingly of volatiles: air, water, propellant, etc. The most advanced theoretical technology, worked out in detail by K. Eric Drexler[8] relies primarily on volatile and organic materials, especially carbon. The most promising microgravity industry, pharmaceuticals, would be dominated by volatiles and organics. By far the greatest bulk of near-raw materials launched from Earth into space are volatile propellants. Even metals extraction and refining industries rely on much larger amounts of air and water than they produce in metal product, and it is quite a leap to assume we can eliminate this dependency without costly R&D efforts and while maintaining reasonable levels of efficiency and thruput.

That's bad news, given that practically all native materials work today has, for historical and political reasons, focused on the dry Moon, and to a lesser extent dry asteroids. The good news is that volatile extraction from ice is much easier than trying to split oxidized metals into oxygen and metal (not to mention trying to capture solar wind particles for hydrogen, carbon, nitrogen, etc.). Comet ice, full of a rich diversity of water, nitrogen, and organic compounds, is readily available.

The best currently known targets for volatile extraction are the Jupiter-family comets. These have been tossed into the inner solar system by Jupiter, into highly elliptical orbits, with perihelions as low as Mercury's orbit (0.4 AU) and aphelions near Jupiter's. Influenced by the inner planets, the orbits slowly circularize. Most of the perihelions are near 1 AU, although to some extent that's observational selection; we're less likely to see objects with perihelions at 2 AU, and ice there sublimates much more slowly.

The comets live very short active lives on a geological timescale, 100,000's to millions of years, but Jupiter's gravity well is quite hungry and continually replenishes the supply when comets wander its way from the Kuiper Belt or Oort Cloud. Some of these visitors get a rought welcome; we recently saw a comet calve into dozens of pieces as it swung in too close to Jupiter. Caught in orbit around Jupiter, these pieces, the largest c. 10 km diameter, are projected to collide with the gas giant (on the far side, alas) in 1994[7].

Most such comets are whipped into heliocentric orbit (ie orbit around the sun), with aphelion at Jupiter and perihelion in the inner solar system. Over the comet's lifetime this orbit continues to circularize until one of the following happens: (1) all the volatiles exposed to the sun bake out, and the comet turns into an asteroid, or (2) perihelion increases, and volatiles remain frozen during the entire orbit, again turning the comet into an asteroid. For this, reason, many scientists believe that many earth-crossing asteroids, especially types C and D, may be old Jupiter-family comets, and some may still contain frozen volatiles. We know that many earth-crossing asteroids contain water, and perhaps ammonia, locked to the regolith by hydration. This is harder to extract. We should consider these closer objects as alternate targets for volatile extraction, comparing the tradeoff in transport and equipment costs.

Thousands of earth-crossing asteroids are believed to exist with round-trip delta-v's from Earth orbit lower than the lunar surface. Jupiter-family comets take somewhat more energy to get to; escape + 8 km/s one way for good windows, shaving off a few km/s on the way there and/or the way back if we can use Venus, Earth, or Mars for gravity assist [1,3,5]. With a slightly more sophisticated mission we can manufacture aerobrakes by sintering comet dust, eliminating most of the return delta-v [6]. A speculative method called "cometary aerobraking" has also been proposed [1].

Ice Rockets

The industrial flexibility of volatiles makes its first and most dramatic impact in deep space transportation technology. We can combine easily extracted native thermal propellants with a tankless rocket design, eliminating the need to launch either propellant, tanks, or heavy electric powerplants from Earth. The specific transport technology that embodies this principle I've dubbed the "ice rocket" [1]. The ice rocket design [1,2,3] consists of a long cylinder about the same size and shape as a Space Shuttle's solid rocket booster, but made out of ice and coated with a thin insulating paint. To this is attached a tiny thermal rocket, about the size of a fist, and a tiny nuclear reactor, or few square meters of mirror, which concentrates sunlight on the rocket engine. The engine slowly eats the ice, converting it into a high-velocity vapor exhaust. The rocket engine is designed for minitiarization and simplicity, so that dozens of them can be built and launched on a small, commercial budget at launch costs not much lower than today's. A larger nuclear-powered design is presented in [4].

To mass-produce the ice rockets we melt cometary ice and purify it with a centrifuge, in some designs combined with an inflatable still. We form the ice cylinder in two steps. First we freeze a thin shell by wetting a large, cold cylindrical form. As this ice gets thicker, it freezes further layers more slowly, so we start squirting small spheres across a shaded vacuum. These spheres freeze on the outside, then accumulate on the inside of the cylinder. Soon the cylinder is filled with partly frozen water, which will continue to freeze over several years while the rocket travels towards its destination.

The icemaking equipment is the most important part of the system. It must produce a very high ratio of ice mass to equipment mass (aka mass thruput ratio (MTR): output product per year divided by equipment mass launched from Earth). It must be automated and reliable; think of a tiny auto-maintained sewage treatment plant. Other parts of the comet (organics, dirt, etc.) can be gathered and attached as payload. The cylinder is then attached to the small rocket engine, whose tiny thrust over the course of two or three years delivers the payload to a variety of destinations: orbits around the Earth, Jupiter, or Mars, the surface of Earth's Moon, or to asteroids. To get to high Earth orbit we must exhaust about 90% of the ice, or 80% if we take a couple extra years to use a gravity assist. (See [5] for patched-conic math to compute such trajectories, and [6] for safety issues involved in using Earth for gravity assist and aerobraking of various varieties and sizes of payloads). We might also find hidden in some Earth-crossing asteroids, in Martian moons, or at the lunar poles, in which case more than 10% can be obtained.

If the output of the icemaking equipment is high, even 10% of the original mass can be orders of magnitude cheaper than launching stuff from Earth. This allows bootstrapping: the cheap ice can be used to propel more equipment out to the comets, which can return more ice to Earth orbit, etc. Today the cost of propellant in Clarke orbit, the most important commercial orbit, is fifty thousand dollars per kilogram. The first native ice mission might reduce this to a hundred dollars, and to a few cents after two or three bootstrapping cycles.

Furthermore, we can deliver volatiles not just to Earth orbit, but anywhere else in the inner solar system. A volatile dump around Mars can slice an order of magnitude off the propellant needed to launch a large-scale mission to Mars. Comet volatiles are synergistic with lunar operations, adding the missing elements needed to make lunar exploration or industry productive.

Once the volatiles and organics have been separated, they are fed to a series of chemical microreactors and converted to essential nutrients and construction materials for greenhouses. Greenhouses can be made in a very simple, automated fashion, for example by pumping air into liquid polymer spheres which are then solidified and filled with nutrients and trellises for the crop. The crops grow not only pharmaceuticals, but also fiber and resins to provide structural strength for further greenhouses, and genetically engineered enzymes are extracted and used in the chemical microreactors. The greenhouses contain a low pressure (c. 1/5 atm), CO2-rich atmosphere to facilitate the growth of genetically modified fiber plants while keeping the engineering task of building the pressure vessels minimal.

Methane, ethane, and several other hydrocarbons have been seen in varying abundance (<1% to 5% for methane) in comets. If you want to get rich in 2020, design a system to extract the methane from the water & ammonia ice and the gravel/muck of comets, perhaps manipulating a large gas/plasma interface (cf. comet tail dynamics).

A refinement of the ice rocket manufacturing process is a 3D printer to produce very large structures. Shoots droplets of several kinds of materials following a digital pattern. For example a high-temp-sublimating ice, a low-temp-sublimating ice, and a ceramic slurry. The target object forms on a very large, cold radiator in the shade. The goal is to have the particles mostly freeze before they impact the target, but nevertheless stick to and accumulate on precise points on the target (in 3-space, layer by layer) without too much splatter. If the target itself must freeze the droplets we run into heat conduction problems pretty soon, and the target object couldn't get very thick. The low-sublimating-point ice allows hidden surfaces to be "etched" by sublimation when the structure is rewarmed, provided there are escape holes.

Open Issues

Many engineering tasks need to be undertaken to make comet mining a reality:

* Simple processes to create 1/5 atm pressure vessels from cometary
ice & tar, including greenhouse windows
* elaborate/refine 3d printer designs
* variable gravity bolo
* minimize processes requiring gravity
* Gas/plasma separation processes
* plant automation (maintenence, etc.) -- this may be the
primary technological bottleneck
* Nitrogenous fertilizer from ammonia
* Carbon -> CaC -> acetylene vs. syngas (CO + H2) -> hydrocarbons
via Fischer-Tropsch
* Phenolic resins vs. urea-formaldehyde/fiberboard
* Polyethylene, alpha-olefins, polystyrene, PVC

Among cometary science to do:
* Cometary sources of phosphate, potassium, and halogens
* Detailed analysis of comet surfaces needed to optimize
choices and enable autonomous operations

Among biotechnology to do:
* Detailed design for "self-reproducing" greenhouses
* Fiber source that facilitates automated processing

Also of great interest are low pressure & plasma manufacturing processes, especially those that can be greatly scaled up in space. We would like to manufacture variety of products from cometary and asteroidal material, including but not limited to:
* paints (for spacecraft thermal control)
* polymer structures and coatings
* aluminization of polymer surfaces (very large reflectors/solar energy
)
* Inconel or silicon carbide frit (for rocket motor)
* radiation shielding
* propellant: thermal, mono- and bipropellant chemical, and ion/MPD
* pressure vessels
* pipes
* single-isotope diamond & fullerenes (C12, C13, C14)
* solar wind isotope separation (eg He3)
* etc. (your ideas go here)

Greenhouse Bootstrapping

The known Jupiter-family comets with lowest-eccentricity may get above 0C at their surface long enough to create an extensive plant growth. Don't need to move them. Moving cometary materials to a warm circular orbit via gravity assist and ice rocket is desirable for some markets, but it's not necessary to start out the operation if we have a sufficiently flexible biosystem that can colonize the comet. There's also the possibility of chemosynthetic life to live off the energy frozen in the comet as free radicals, in which case it doesn't matter how far from the sun we are, but that's speculative so I'll skip it for now.

A bigger problem is maintaining sufficient internal pressure to keep water above the triple point (otherwise it sublimates straight to gas, like dry ice). One way is to go back to the bag, except use it as a greenhouse instead of a still. (Or in addition to a still -- plants exhale pure O2 and H2O which can be extracted). A second way, with much higher MTR (mass thruput ratio), is to have the plants grow their own pressure vessel, some kind of strong cellulose fiber bound together with a resin secretion. It need only hold a CO2-rich atmosphere at < 1/4 atm.

This presents a chicken-and-egg (or more properly plant-and-seed :-) problem, so we probably would use bags for the first perihelion. The mass thruput ratio of this scheme becomes astronomical (pardon the Saganism :-), reaching millions/year by the second decade of operation, far outstripping the time cost of money, and all that from just one rocket payload full of plastic bags filled with seeds. This makes it highly economically attractive, even with only a minute fraction of the greenhouse products being separated (eg by electrophoresis) into pharmaceuticals, etc. and sent back groundside, or materials being processed and used as propellant, shielding, structure, life support, etc. in interplanetary or Clarke orbit, or on the Moon, etc.

Going from native comet goop to pure water, and resin/fiber for the pressure vessel, is an interesting problem in metabolic engineering, which combines genetic engineering with an in-depth knowledge of plant metabolism, in order to optimize certain growth features (in this case cellulose and resin output, and resistance to vacuum conditions). I'd probably started with a fast-growing fiber plant like hemp or jute or kudzu, and throw in some genes from deep-salt-lake creatures that maintain an active water gradient across their outer membrane, which should also be pretty good for vacuuum protection. Extensive testing/breeding in groundside vacuum chambers and micrograv testing in low Earth orbit would be in order.

Conclusion

Extraction and processing of volatiles from the Jupiter-family comets, combined with the crude but effective technology of ice rockets, present a wide variety of new possibilities along the path from our current small scale space operations to large-scale space industrialization. Native volatiles can be processed to supply current space operations, while making possible new industries with low up-front investment. Bootstrapping of transporation with native ice rockets and industry with chemical microreactors and self-reproducing greenhouses blazes a wide path along fertile territory, leading to the technological and economic resources for large-scale space industry and space colonization.

References

[1] Szabo, various articles posted to sci.space on native volatile extraction and processing, 1988-present. See also articles by Paul Dietz, Gary Coffman, Phil Fraering, and others.

[2] Szabo, "Comet Mining", a presentation to Seattle Lunar Group, Feb. 1992 (visual presentation & initial marketing analysis)

[3] Szabo, "Some Issues in Comet Mining", May 1992, unpublished (technical overview)

[4] Zuppero, et. al., in _10th Symposium on Space Nuclear Power and Propulsion_ (AIP Conf. Proc. 271, 1993)

[5] Sauer, "Optimization of Interplanetary Trajectories with Unpowered Planetary Swingbys", AAS 87-424, pg. 253

[6] Szabo, "Safety of flyby & aerobraking for large payloads at Earth", sci.space Message-ID: <1992Sep28.061517.1316@techbook.com> Mon, 28 Sep 1992.

[7] Baalke, "Comet Shoemaker-Levy, Possible Collision With Jupiter in 1994", sci.astro Message-ID: <25MAY199322260259@kelvin.jpl.nasa.gov>, and subsequent discussion.

[8] Drexler, _Nanosystems_, John Wiley & Sons 1992

[9] Brian Thill, Boeing Corporation, personal communications

[10] Tony Zuppero, U.S. Department of Energy, personal communications