An interesting new business has recently received considerable public attention. Planetary Resources is looking for ways to harvest mineral resources from space and turn them into commercial assets. Although they are employing a number of ingenious concepts and have hired a few extremely talented engineers, experts agree that it is a risky undertaking. Even Eric Anderson openly admits that in this venture there are no guarantees.
Rather than planets, dwarf planets, or moons, their strategy is to focus upon tiny, irregularly-shaped solar system bodies that are located near Earth’s orbit. Because these Near-Earth objects have minute gravitational fields, it should be possible to transport mineral resources back to Earth from some of these locations with minimum delta-V, using very low-thrust propulsion. The downside is that the transit time for these low-energy trajectories would be measured in years rather than days.
Although they have yet to reveal a comprehensive plan, the approach that they do take will probably be quite different from a recent study that proposed a billion-plus dollar NASA planetary defense exercise where a 5.5 metric ton solar-electric spacecraft fueled with 13 metric tons of Xenon would attempt to despin a 7 meter, 500 metric ton boulder and transport it to a lunar orbit in less than 10 years.
The primary resources of interest to PRI are ice and the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum. PGMs are very expensive, are required in the manufacture of many important products, and are becoming increasingly rare. Current estimates are that PGMs can constitute as much as 0.0002 of the mass of some NEOs. The abundance of PGMs in the Earth’s crust is much lower because most of them settled to the core in the early molten phase of the Earth’s formation. All known surface deposits of these elements on Earth are the result of subsequent asteroid impacts.
In space, water can be used for propellants, breathable oxygen, drinking water, and radiation shielding. What makes it so expensive is the launch cost, which comes to tens of thousands of dollars per gallon. The ice content of individual NEOs are believed to range anywhere between 1% and 60%, where the upper end of the scale constitutes the nuclei of “spent” comets. NEOs, because they are constantly warmed by sunlight, probably do not have any exterior surface ice.
Because most NEO mass will be neither ice nor PGMs, ideally, the most efficient approach could be to extract the desired resources at the NEO and only transport useful mass back towards Earth. Furthermore, if extraction can be performed before arriving in cis-lunar space, that will eliminate any chance of generating massive amounts of dangerous orbital space debris.
Extracting water from a small NEO could be simple: Encase small chunks in a balloon and warm them up. Then pump the generated vapor into another balloon, cool it down, and the result should be liquid water. Conversely, extraction of PGMs from NEOs will be a major challenge, so initial missions will probably target water.
This extracted water could be electrolyzed into gaseous hydrogen and oxygen, cryogenically cooled down towards absolute zero, and continuously refrigerated so that they do not boil off over the years required to reach Earth. That would require considerable amounts of inherently inefficient hardware and significant quantities of power. A simpler and more practical approach should be to not battle thermal equilibrium and just transport the water, as is.
However, there still remains the daunting task of hauling hundreds of metric tons of water back to Earth. Logically, it would make sense to use some of this water as reaction mass for propulsion, rather than paying hundreds of millions of dollars for propellant from Earth. The key questions are: exactly how could we utilize the water, and what power source could be used to make this possible?
A simple solution could be the steam rocket, where ultra-high-temperature steam would be expelled at high velocity. Although not as efficient as hydrogen, water could prove to be quite respectable. As noted by DARPA’s HiDVE program manager, if you flow water through a small heat exchanger and heat it up to somewhere between 2,000 and 3,000 degrees C, you should be able to generate a specific impulse of 400 meters per second or better. In comparison, the space shuttle main engines achieve 455 meters per second.
Clearly, a small nuclear reactor could be used to continuously generate high-temperature steam. Although this might be a viable deep space technology for NASA, Planetary Resources does not have the required nuclear technology. And if they did, the public might not be particularly enthusiastic about fallout from a launch failure.
A simpler and safer solution could be to apply the 1.361 kilowatts per square meter of thermal energy that is already freely supplied by the Sun, using a large parabolic solar concentrator. The space shuttle STS-77 IAE experiment successfully demonstrated the deployment of a 50 foot diameter inflatable parabolic dish that had a mass of only 60 kilograms. The pressure required to inflate such a structure in the vacuum of space is so minimal that gas losses from any micro-meteoroid punctures could be easily handled by a small reserve gas supply. This same solar concentrator could also be used to heat fragments of the NEO in order to liberate water vapor.
It is remarkable that the continuous thrust generated by a solar thermal rocket should be orders of magnitude greater than the minute thrust generated by an equally large solar sail. And unlike the solar sail, the steam rocket would be able to sail “into the wind.” (A solar sail can only approach the Sun by directing a breaking thrust against its direction of travel in order to lower its orbit.) What an enormous difference it can make if you “just add water.”
The Sorcerer’s Apprentice
The advantage of the solar thermal steam rocket is that in a single round trip a rocket should be able to return to Earth orbit with 3 times its initial mass in water. Some of this water could be used to fuel subsequent missions, theoretically delivering unlimited quantities of fuel to orbit from just one initial launch. Or, upon the return of each ship, fuel for two additional new ships would also be available, providing an initial growth rate that could be exponential. This would be most likely in the beginning, when a commercial market for water in Earth orbit is just being established.
In the paper “Logistical Implications of Water Extraction from Near-Earth Asteroids,” University of Arizona Astronomer Dr. John S. Lewis noted:
Round-trip missions between highly eccentric Earth orbit (HEEO) and the surfaces of near-Earth asteroids (NEAs) benefit greatly from the use of asteroid-derived water as a propellant. The most advantageous schemes utilize solar thermal or nuclear thermal propulsion with water as the working fluid (the "steam rocket"). Multiple round trips by steam rocket between HEEO and NEAs can achieve mass payback ratios of 50:1 to over 100:1…The MPBR advantage of NEA-derived propellants arises from the low delta V required for soft landings on many NEAs, the very low delta V for return from these asteroid surfaces to Earth orbit, and the near-optimal specific impulse of the steam rocket…
To minimize startup costs and hold down risks, a bootstrap approach can be taken with large numbers of tiny spacecraft flying out to several different NEOs. One always has to be careful about optimistic theoretical extrapolations, but should these missions prove successful, after many successive round trips enough water could be amassed to begin flying large unmanned (and eventually manned) exploration missions, possibly even opening up Mars. Solar thermal propulsion could possibly become as significant for interplanetary travel as the sail was for seafaring nations.
Because steam technology is centuries-old, these ideas almost sound like something from Jules Verne or H. G. Wells. One could imagine alternate histories with manned spaceships propelled by steam, no doubt communicating with each other in Morse code via brass telescopes and heliographs.
And when it comes to true planetary defense, imagine the day when an alien invasion fleet arrives. Confronting our spacecraft, they would see before them steam boilers, water balloons, and enormous parasols. Never mind the germs… They would all immediately die from laughter!