by Viktor SINYAVSKY, Dr. Sc. (Technology), and Vladimir YUDITSKY, Cand. Sc. (Technology), Applied R&D Company Ltd. (Korolyov, Moscow Region)
It is well over forty years since the launching of the first artificial satellite of the earth. Today even a dyed-in-the wool skeptic won't query the benefits of space research for TV and computer networks, geological surveying, cartography and all. Most sophisticated electronic hardware and lots of other things are inconceivable without materials obtained in orbit. But what about another challenge and menace-the global energy pinch? Here, too, outer space may help us over the hump.
The available reserves of the most effective fuels, gas and oil, are limited enough and can satisfy mankind's demands for just a few decades. Broader use of coal, another common fuel, is fraught with ecological hazards. Hydraulic power stations cannot solve the outstanding problems either.
Nuclear energy has seemed to be a way out: atomic power stations are capable of supplying any amount of heat and electricity. But they are not sure-fire either, what with a radioactive background exceeding the cosmic radiation level, the risk of grave accidents (recall the Chernobyl disaster of 1986!). And last but not least, the radioactive waste disposal.(*)
In the opinion of many experts, thermonuclear power can be the best option. True, we are still on the initial leg of our work toward controlled nuclear fusion. Today we are examining in good earnest the possibility of building a reactor capable of fusing deuterium and tritium (D/T) nuclei, a process resulting in a release of considerable energy (17.6 MeV). But even this version does not save us from tritium-induced radioactivity and, which is still worse, from neutrons as a byproduct of the reaction. However, we can minimize that in a thermonuclear reaction involving deuterium and a helium isotope with an atomic weight equal to 3 (e). Then we shall obtain an alpha- particle and a proton with a total release of energy equal to 18.3 MeV. Since the number of
* See: V. Subbotin, "Nuclear Power: From the Past into the Future", Science in Russia, No. 6, 1996 and No. 1, \997.-Ed.
Mining/transportation space complex for e recovery on asteroids: 1-thermal emission reactor; 2-radiation shield;
3-refrigerator-emitter; 4-voltage transformer; 5-electrorocket engine; 6-first docking unit; 7-technological module;
8-refrigerating machine; 9-liquid helium storage; 10-third docking unit; 11- separator of helium isotopes;
12-helium extraction module; 13-rock-processing module; 14-second docking unit; 15-approach and landing module.
neutrons and radioactive tritium isotopes (what is generated in the course of reaction and thus unavoidable) is significantly less in this case, we had better build thermonuclear reactors of the future according to this very principle.
But one essential stumblingblock is this: our planet has too little of the helium needed for the desired isotope. According to tentative estimates, the total mass of He on the globe is about 50 tons scattered abroad here and there. And so nuclear physicists have turned their sights to ET space.
The idea of mining this thermonuclear fuel was first suggested by Russian and American experts in 1987. Studying the samples of lunar rock, the authors of this idea have estimated the total reserves of helium on the moon at 1 million tons. And considering that the calorific value of one ton of the isotope 3He is equivalent to 10 million tons of coal, there could be no question about the prospects of this lunar fuel. A mere 25 tons of helium will be enough to generate as much power as is produced by the entire nuclear power industry of the globe within a year.
However, the conceptual design of lunar helium mining proceeded from terrestrial technologies-with due consideration, of course, of the gravitational difference and air- free space. What we needed was a special-purpose mining complex, including transportation and delivery vehicles, a unit for the recovery ofe and its liquefaction. Containers (tanks) with liquefied helium were then to be loaded into the lunar craft. To keep all that hardware on-stream, a high-capacity nuclear power plant was necessary to give a non-stop supply of energy during a few longish lunar days. Therefore a manned lunar base had to be set up too-with the required life-support systems and the like. A real mining-and-industrial complex! A white-elephant project downright impossible in the foreseeable future...
So, the lunar helium idea reached an impasse and had to be scrapped. But then, in 1994, one of the authors of the present article came up with a brain wave: Why not extract helium on asteroids? That was no idle dream either: at that time the aerospace corporation ENERGIYA had a nuclear electrorocket engine on the drawing boards, i.e. what was needed for an appropriate space vehicle.
This line of research received support from the Ministry of Science and Technology of the Russian Federation. Another plank was added to the federal program for basic space research in 1994-1997, and that concerned R&D relative to the nuclear electrorocket engine for recovery of the ecologically clean thermonuclear fuel e in outer space and its delivery to earth.
This job was assigned to our R&D Company which has a significant scientific potential and experience in this field. First, we had to attack several essential problems-namely find out the reserves of helium on asteroids. Here researchers of the V. I. Vernadsky Institute of Geochemistry (RAS) were of great help. Studying the meteorite substance and using other techniques, they assayed the e content in- asteroid rock to be within 25 kg/km2.
Another important problem was this: how to take a heliummining complex to asteroids? It was tackled by a research team of the Moscow Aviation College
Shorthand of Complex Z operation.
A. Approach and landing.
B. Translation of the complex and isolation of helium-3.
C. Detachment of liquid helium-containing capsule from the technological complex and homeward journey.
1-approach and landing module; 2-helium extraction module; 3-rock- processing module;
4-liquid helium storage; 5-refrigerating machine.
(Institute) that studied the flight mechanics of low-thrust spacecraft on their way to bodies of the solar system (with the thrust equal to 1/10,000 of the terrestrial weight of the craft); the flight trajectory toward the asteroid Fortuna was taken by way of example, which was no accidental choice of course: the most suitable asteroids for helium extraction are those with a size of 50 to 200 km across.
The low gravitation of asteroids allows to expend much less energy on approach and landing; and then the mining complex is much simpler in design, it could be assembled right in circumterrestrial orbit and taken to the target in a vehicle equipped with a nuclear electrorocket engine powered by a single generator in the system.
Next, we examined the technological side: the design of the transportation/mining complex as well as the main technological processes and respective equipment.
Here's our scenario: conventional booster rockets carry separate parts of the complex into a radiation-safe circumterrestrial orbit about 800 km away from the ground surface. Once the complex has been assembled in orbit, it executes instructions in the automatic mode or by command from the ground control center.
The Earth/Fortuna spacecraft is composed of two major parts: the nuclear power unit (NPU)- which is to feed power to the helium-mining complex on the asteroid- and the electrorocket engine with a supply of fuel. For NPU we suggest using thermal emission nuclear units (space atomic power stations) in the blueprint stage now; their forerunner was the NPU Topaz tested aboard the orbital satellites Cosmos-1818 and Cosmos-1867. But the new NPU will be significantly higher in generating capacity.
The heart of this complex is a thermal emission converter reactor operating on fast neutrons; in it the heat energy liberated through the fission of uranium-235 nuclei is converted to electricity. This power unit does without conventional turbines and generator which are replaced by thermal emission converters. Manufactured from refractory metals (tungsten, niobium), they make it possible to actuate a high- temperature thermonuclear cycle, with the working temperatures in the 1,700-1,900C range ensuring high efficiency in ET conditions.
Maximum parameters of the unit: diameter-6.92 m; length- 26 m; mass-16.5 tons; capacity-2.5 MW. Its rated service life is from 3 to 5 years.
As to our interorbital electrorocket engine, it operates this way: the fuel (in our case, the
inert gas argon or the alkali metal lithium) is ionized by electric current fed by NPU (220 to 1,540 V D.C.); the plasma thus formed is accelerated in an electromagnetic field generated by special devices of the engine. Since jet propulsion from one kilo of the working medium (fuel) is dozens of times as high as that provided by an ordinary jet engine, the fuel takes up only 30 to 40 percent of the total mass of the electrorocket unit-the rest is its weight proper and payload (i.e. the mining complex). Similar engines are already employed on space vehicles for flight correction, but they are getting their power from solar batteries of much lower capacity.
When our mining/transportation complex has approached the Fortuna asteroid, it should descend on a suitable site for work. Flat relief features will be best, or else it might be a shallow basin 6 to 7 km2 large, with the elevation differential not higher than 3 or 4 meters and good visibility of the central part. Landing on asteroids involves techniques other than those used for terrestrial planets-the velocities of the asteroid and spacecraft should be equalized and then, on touch-down, the descent module should moor on the surface with the aid of a penetrator anchor entering to a depth of one or two meters.
Once we are on target, the service module unloads the mining equipment and gets it ready for operation. Choosing the most acceptable mode of helium mining, we have considered the available data on the structure of the asteroid rock and on the concentration of the product wanted. It turns out that the helium-rich rock lies in the surface layer not deeper than 2 meters. Yet the bulk of the substance is just 1 mm below or else absorbed on the surface composed of micrograins or larger particles. In its characteristics the surface layer of the asteroid is a material similar to the lunar regolite; the density of this rock is around 3,000 kg/m3, and the average dimensions of its particles, 1 mm. Accordingly, we have suggested the following procedure: heat the helium-containing rock to 700-800 C whereby the gaseous mixture of e and e is desorbed readily and then is subject to isotope separation.
This technology can be realized in a variety of ways; however, the limited generating capacity of our power plant imposes certain constraints. All things considered, we have calculated an optimum output of the complex to be 400 to 450 kg/s of the processed rock. The total stripped area on the asteroid would be about 6 km2 a year to a depth of 1 meter. Since the mass of the rock refined per second is sufficiently large, the mining unit should be subdivided into autonomous modules, say 12 in number, with a productivity of up to 40 kg/s each. The subsequent steps of the process (heating, collection of the gas mixture and isotope separation) would be performed at one and the same set within NPU.
So, we get the following design of the extraction module: a closed lifting chain or metal belt carries scraper bowls that dig into the rock and feed it into a receptable. The chain or belt is fixed to a traveling rod capable of rotating about a vertical axis so as to work all of the surface around the moored complex.
The mining module is in the form of a mobile platform with a unit where the extracted rock is heated. The gas mixture obtained thereby is fed via a bellows-type pipeline(*) to the isotope separation module.
A flexible cable supplies power to the extracting module which is equipped with a walker enabling it to move about and holding it tight in the process of work.
So: the gaseous mixture from the operating extraction units is pumped into the module where it is separated into the desired e ions and the associated He (the latter, incidentally, can be utilized as a fuel on a return voyage). Looking into all the methods known to date, we have opted for the gas diffraction technique: it does not depend on the gas pressure and temperature, and its efficiency correlates well with the atomic masses of the components separated. Furthermore, our polymer "nuclear filters" are remarkable for high permeability and resistance in the helium atmosphere, a factor that allows to reduce the mass of the units and cut down the losses during the pumping of the gas mixture. And last, the gas diffraction process ensures a high degree of e separation given a small number of dressing and impoverishment steps.
Finally, the isolated helium is liquefied, pumped into capsules and ferried to earth. In a year of work (such is the rated time of the station's operation) it would be possible to extract about 130 kg of the product, an amount that could keep a 1 GW electric power station at work for a year.
Almost all the components of this system have been manufactured as prototypes and put to practical use. Now it all depends on the headway in controlled nuclear fusion and in the rates of space hardware development. Good progress in these fields will make the helium project a practical possibility.
* Here, a thin-wall (usually all-metal) cylindrical hose with a transverse corrugated surface.-Ed.
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