Chapter 5 Growing Crops for Space Explorers on the Moon, Mars, or in Space

Chapter 5

CROWING CROPS FOR SPACE EXPLORERS ON THE MOON, MARS, OR IN SPACE

Frank B. Salisbury I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . A. Choice of Plants. . . . . . . . . . B. Economics of CELSS . . 11. Design of a CELSS . . . . . . . . A. Component . . . . . . . . . . . . . . . . . . . . . . . B. Problems to Be Overcome with Any CELSS . . . . . . . . . . . . . . . . . . . . . . . 111. CELSS for Different Locations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....... B. The Moon . . . ... .......................... C. A Microgravity CELSS in a Space S h p . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Cultivation of Plants in Space. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . EarlierExperiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Experiments with S .................................... A.

Experience with Biosphere-2, Arizona . .

Advances in Space Biology and Medicine, Volume 7, pages 131-162. Copyright 0 1999 by JAI Press Inc. A11 rights of reproduction in any form reserved. ISBN: 0-7623-0393-X

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B. Experience with Bios-3, Siberia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,150 C. BIO-Plex under Construction at Johnson Space Center, Houston . . . . . . . ,156 ,157 VI. Conclusions: Lessons Learned . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Summary.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 Acknowledgments. . . . . . . . . . . . . . . . . . . . 160 References and Notes . . . . . .

1.

INTRODUCTION

The human exploration of space has required solving many challenging problems: ( I ) designing and constructing vehicles with enough propelling force to break their ties with Earth's gravity; ( 2 ) protecting the astronauts from the radiation and vacuum of space and providing them with reasonably comfortable living quarters; (3) providing an atmosphere containing sufficient oxygen and as little as possible of any toxic gas (humans can tolerate carbon dioxide concentrations of about 1%-compared to 0.036% in the Earth atmosphere-but a slightly higher concentration adversely affects their physiological responses, producing minor to very serious symptoms); (4) providing drinking water (about 2 liters per day), free of toxins and pathogens at harmful levels; (5) providing sufficient nutrients and chemical-bond energy to keep the astronauts alive and functional; Schwartzkopf, in a review published in 1997 in this series, estimates that an average human requires 408 kg.y-' of food, 304 kg.y-' of oxygen, and 2926 kg.y-' of water including 676 kg.y-' for drinking and the rest for food preparation and body hygiene, a total of nearly four metric tons of consumables per year!' On short-term missions, sufficient oxygen, water, food, and waste storage have been carried to last throughout the mission. CO, accumulating in the cabin atmosphere is removed by physicochemical means, for which supplies are carried. The Russian Space Station Mir, which has been in orbit for over a decade, has been regularly resupplied from the ground with these items, and waste has mostly been returned to Earth. An alternative to resupply and waste return is to recycle waste on board, which to a certain extent can be accomplished with physicochemical techniques (e.g., much effort has been expended on studies of physicochemical water I ecycling). Nevertheless, it has long been recognized that an alternative approach might be a biological technique based on green plants. In theory, this would convert C 0 2 into oxygen, recycle water, and provide food. Around the turn of the century, long before space travel was achieved, Konstatin Edwardovich Tsiolkovsky (considered to be the father of Russian space science), already suggested the use of green plants in future space exploration. Plants, through photosynthesis, remove CO, from the atmosphere and release oxygen and water vapor that can be condensed in nearly pure form. The photosynthetic process is driven by light energy with a maximum efficiency of 10 to 13% (chemical-bond energy/light energy).3 With a suitable choice of plants, part of this fixed chemical-bond energy can be used as

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food by space explorers. The edible portion of the total biomass, including roots, can vary from less than 30% (e.g.. for soybeans) to 80% or more (e.g., lettuce, potatoes); a typical value is 45% for wheat.4 This percentage is referred to as the hancst index. An additional distinct advantage is that green plants provide a pleasant environment for earthlings who are adapted to their presence on Earth. Green plants can also aid in waste disposal, e.g., they can utilize urine. Such a system using green plants is properly referred to as a bioregenerutive life-support system, for which I shall use the acronym CELSS, which stands for Controlled (or Closed) Environment (or Ecological) Life-Support System. A.

Choice of Plants

Which plants r i g h t be used in a CELSS? When the CELSS concept was developing during the 196Os, there was much discussion and considerable research about the possibility of using green algae, especially Chlorella ~ u l g a r i s .Green ~-~ algae are certainly efficient at supplying oxygen and removing carbon dioxide. Chlorella, however, is not easy to eat" and has caused nutrient deficiencies and illness in both test animals and in humans." The strains that have been studied mostly contain large quantities of proteins and nucleic acids but no carbohydrates, which is not a very suitable composition for human nutrition. Chlorella has been successfully incorporated into various baked products, but so far nobody has been able to develop a satisfactory process to make it suitable for human consumption in significant quantities. In contrast to most green algae, however, Chlorella 241 80 is a strain that produces maltose, which can be used in human nutrition. The strain has been employed in a prototype photo-bioreactor that can regenerate the air and provide some food in space, as has been described in this series by Luzian Wolf? He also reviewed earlier work on algal bioreactors and describes the technical problems of making such systems function in microgravity. All components of this bioreactor have been designed to function in microgravity, and some have been tested in space experiments. Bioengineering might further refine the nutrient contents of various algae, including Chlorella. Some cyanobacteria (blue-green algae, e.g., Spirolina and Nostoc) can be consumed directly and have been proposed as an organism in bioreactors for space exploration. Although these organisms are edible, astronauts would soon tire of a diet that included large quantities of Spirolina or its relatives. Nevertheless, an important feature of the cyanobacteria is that they can fix atmospheric nitrogen into forms that higher plants can use,12 and fixed nitrogen is likely to be lost in a CELSS as a result of waste processing and the activities of certain microorganisms. In any case, when air purification is the only goal for a bioregenerative system, algal systems are certainly viable alternatives that have been tested in laboratories all over the world.

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Higher plants are more difficult to grow, but a great advantage i s the edible food they can produce, as well as the psychological effect of having these familiar l i f e forms on board. A disadvantage i s that the harvest index is seldom, i f ever, 100%. Thus, part of the higher plants must be consigned to the recycling or waste dis-

Food production with waste processing (CELSS) 0

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Mission Duration (e.g., years)

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Figure 1. An illustration of principles involved in determining the mission duration at which recycling is economically beneficial (after Myers,’ cited also by Schwartzkopf.’) The lines, which show the mass required for life support (ignoring the mass of the launch vehicle) as a function of mission duration, are based on the initial launch mass plus the mass that must be resupplied as the mission continues. The curve labeled resupply assumes that all water, atmosphere, and food is either taken in the initial launch or resupplied at intervals; hence, the longer the mission, the greater the amount of food and other supplies that must be launched. Since much of the required mass consists of water, the slope of the line is much less when water is recycled, but obviously the equipment required for water recycling adds to the launch mass. Recycling the atmosphere requires even more launch mass, and a bioregenerative life support system (CELSS) requires much more mass. Yet in each case, with additional recycling, less resupply is needed,so the curves are not as steep. When closure approaches loo%, the slope of the curve approaches zero, but 100% closure is unlikely ever to be achieved. The arrows represent break-even points where the duration of the mission justifies recycling of increasing complexity.

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posal system. The first question that comes to mind is whether a functional and reliable CELSS can be built for longterm use in space. Considerable research has hien devoted to this question, especially at the Institute of Biomedical Problems in Moscow and at the Institute of Biophysics in Krasnoyarsk, Siberian Russia. Active efforts to construct a functional ground-based CELSS, are currently under way in Japan and at the Johnson Space Center; the latter project is called B10-Plex (see section V.C). Building a CELSS is not simple, but most of us who are familiar with past and current projects think that the challenge can be met.

5. Economics of CELSS If a CELSS can be built, then the second question appears; namely, what is the cost of a CELSS versus resupply from earth? CELSS equipment will be rather massive, costing much to launch into space. Furthermore, it will require large amounts of energy to operate. The expense of launch, maintenance, and energy must be considered against the cost of resupply. It is also extremely costly to launch, preserve, and deliver food, oxygen, and other necessary items from earth to space explorers. The actual expenditure will depend upon whether these explorers are in near-earth orbit, on a base on the Moon or Mars, or in a spacecraft on a mission to prospect for asteroid material. Figure 1 illustrates the concept as proposed by Jack Meyers in 1963.13The graph plots launch mass of food and/or materials to produce water, air, and food for each person as a function of the mission duration. Obviously, the goal is to minimize the launch mass. If everything, including water, air, and food, is brought up and resupplied, and waste products are simply stored or jettisoned, the mass begins at zero for a mission of zero duration and increases with mission duration according to a steep curve. Because humans require large amounts of water, if water can be recycled, the curve will be much less steep. It will not begin at zero because the launch mass will have to include the mass of the water-recycling equipment (whatever it happens to be). Regeneration of the atmosphere requires still more equipment, but doing so leads to a curve that is still flatter. If equipment for food production and waste processing (recycling, when possible) is added, the initial launch mass is very high but the curve becomes almost flat. It is not completely horizontal because some materials, so-called deadlock substances, cannot be recycled in any practical way. Some resupply will be needed. The break-even times are when the curves for recycling of water, air, or food cross the steep line for completely expendable supplies. The break-even points have been calculated for various scenarios. Steven Schwartzkopf, for example, has calculated break-even for a lunar colony with a CELSS.'*14If the colony supports four crew members, Schwartzkopf calculates break even at about 2.6 years; if there are 100 crew members, break-even will come as soon as 1.7 years. Of course, many more data are needed before such a calculation can be carried out with real confidence. Nevertheless, it seems almost

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obvious that a CELSS would soon pay for itself as part of a Mars station where resupply would be very expensive.

II. DESIGN OF A CELSS A.

Components

Considering the functions required of a CELSS, the equipment will consist of four essential components:15 biomass production unit-where the plants are grown; this unit must include equipment to control the environment, particularly light, temperature, atmosphere (controlled to a great extent by the plants themselves), pressure, humidity, and cultivation medium for growing the plants. 2. food preparation area-most crops need processing before they can go to the kitchen. Furthermore, special food technologies will have to be developed to make use of as many normally inedible plant parts as possible, thereby increasing the harvest index. 3. waste recycling system-this would be aimed at recycling as much waste as possible. Inevitably, some materials, the so-called deadlock substances, cannot be recycled without incorporating far more equipment and supplies (e.g., strong acids for liquid oxidation), so there must be storage facilities for such substances. Extra supplies are needed to compensate for the deadlock substances that are eliminated from the system or else they will have to be resupplied during the mission. 4. computerized monitoring and control system-such a system also requires spare parts, (e.g., lamps, sufficient for the planned life time of the CELSS), stored or resupplied.

1.

The vast resources of Earth allow for long durations and inefficiencies in production and recycling. Natural processes suffice, even when they are quite inefficient. Earth has such a huge buffering capacity that human activities have only in recent times begun to have an impact on such global phenomena as the climate. If farmland produced well below its potential, the farmer only needed to increase the growing area, but cultivatable land areas are now beginning to be limited. Slow microbial action or even geological processes are permissible means of recycling on Earth." However, since a CELSS facility is costly to build and to launch into space, it is essential to achieve optimal productivity and efficiency from the start. This will require the use of appropriately programmed computers. In a CELSS facility with its small buffering capacity, the processes are in a much more rapid state of flux than is the case with Earth's biosphere. For example, it may take eight years for the CO, in the Earth's atmosphere to be completely recycled through

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living organisms, but in a CELSS facility all CO, must be recycled in just a few days or less. The problems encountered in designing a functional CELSS facility strongly depend on where the CELSS is to be located. In the foreseeable future, three situations are apparent: the surface of Mars, the surface of the Moon, and a microgravity spacecraft in orbit or in free-fall transit to Mars, some other planet, or the asteroids. Some problems, however, are a function of the space environment and are, with very minor exceptions, common to all three locations once the Earth’s protective atmospheric mantle has been breeched. l 6 B.

Problems to Be Overcome with Any CELSS

Any CELSS facility must provide sufficient oxygen for the astronauts and sufficient carbon dioxide for the plants. In some cases, it will be necessary to supply all of the required CO, from Earth. The plants will also need water and minerals, including fixed forms of nitrogen. Nitrogen gas will probably make up about 80% of the CELSS atmosphere as it does in the Earth’s atmosphere. Since fixed nitrogen used by the plants may be converted to gaseous nitrogen, some means of nitrogen fixation will be required or enough fixed nitrogen for the entire voyage must be taken along. Cyanobacteria might serve this function or physicochemical methods might be used. Leaks from the space craft or planetary habitat to the vacuum of space or the low atmospheric pressure on Mars will be a serious hazard, as was demonstrated when the supply vehicle caused a small leak in the Spektr module of Mir on June 25, 1997. Enough food will have to be carried as backup in case of unexpected difficulties in growing the plants. Difficult decisions will have to be made about what to take and what to resupply. It will be necessary to take spare parts for everything that could wear out. Lights eventually bum out, lubricants evaporate or wear out, motors burn out. Virtually everything in the mechanical environment of a CELSS facility is subject to wear and tear and eventually complete failure. This is certainly a serious limitation, one that has now been encountered in the Mir after so many years in orbit. It is important to note that the plants are much more dependable than the machinery that creates and maintains a suitable environment for their growth. Energy to run a CELSS facility is a serious problem. It is especially serious if energy is required to irradiate the plants with visible light of suitable wavelengths. A logical energy source would be a nuclear power plant on Mars or the Moon or possibly even in a spacecraft. Solar cells have provided much of the energy for space exploration so far, but very large arrays would be required to produce enough power to irradiate the plants even with the most efficient lamps. The best solution would be to use direct sunlight for the plants, which would be possible on Mars. Formidable engineering problems would have to be overcome, however, to make this possible on the Moon or in a spacecraft.

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Many authors have suggested that, in the absence of a protecting atmosphere, hard cosmic radiation and occasional solar flares could be damaging or fatal to both plants and humans virtually anywhere in space. This point is somewhat controversial, however. Schwartzkopf, for example, defends an inflatable greenhouse on the surface of the Moon by saying that "dangers posed to plants from galactic cosmic radiation. solar flares, and meteorite strikes are statistically very low, even over an assumed 20-year lifetime of the facility".', l7

111.

CELSS FOR DIFFERENT LOCATIONS A.

Mars

Once we get there, Mars is the easiest location to build an extraterrestrial CELSS facility. Although the atmosphere is only about 0.1 % that of Earth, it is enough to make parachute landings possible and to provide slight protection from radiation. The atmosphere consists mostly of carbon dioxide, which is a big advantage because it would not be necessary to transport C 0 2 for the plants. There is actually 25 to 50 times as much CO, in the Martian atmosphere as in the atmosphere of Earth, depending on elevation on the Martian surface. Water is available in the form of ice, but it will be difficult to obtain. Nevertheless, this is another great advantage compared to a microgravity CELSS facility or probably one on the Moon, where water will be more difficult to obtain. The Martian regolith could perhaps be used as a plant substrate that is watered with a solution of minerals essential for plants, but it has still to be determined whether it is nontoxic. Silicate rocks can be used as a source of oxygen as well as mineral nutrients for the plants." The gravitational force at the surface of Mars is about one-third that of Earth, which is sufficient to facilitate convective cooling and circulation of nutrient solutions to plants. The fact that the Martian day has a length of 24.7 hours, which is close to a day on Earth, is a great advantage for plants that flower and that initiate and stop their developmental processes in response to the relative length of day and night (phot~period).~, 19, 2o Because Mars has a highly elliptical orbit around the sun, irradiance at its surface will vary from 37 to 52% of the light that reaches Earth's upper atmosphere. This is sufficient for excellent growth of plants, and is probably much more than many crops receive in cloudy regions on Earth. Indeed, it is seldom cloudy on Mars, although occasionally dust storms seem to obliterate all of the surface features on the planet as viewed from Earth. These storms might significantly decrease the light reaching the surface. Mars has seasons because it's equator is inclined 25" to the ecliptic, as compared with 23.5" for Earth and only 1.5" for the Moon. The Martian year is equal to 687 Earth days. However, Mars also presents some serious problems. The atmosphere is so thin that leaks are essentially as dangerous as on the Moon or in space. Explorers on

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Mars would need space suits when they move out of their artificial habitats. Mars also experiences extremely low temperatures. It appears that the surface (regolith) temperature at noon near the equator and in Martian summer may reach 20 "C, but at night, even in summer, heat radiates into space so much that the temperature typically drops to at least -75 "C. There is water on Mars, but as noted above, it will be difficult to obtain. Nitrogen could also be a problem since there is no evidence that the Mars atmosphere contains any nitrogen. Certainly the greatest problem with a Martian CELSS facility is to get there and to resupply it with whatever cannot be recycled or brought on the initial voyage.

B. TheMoon The Moon has a gravitational force of 0.165 G, which is about 1/6 of that encountered at the surface of the Earth, a sufficient amount to facilitate the circulation of nutrient solutions to plants and convective cooling. It should be possible to obtain oxygen and perhaps also hydrogen (although the latter remains to be determined) from silicate rocks. If both hydrogen and oxygen can be obtained, water could be produced and perhaps the energy of combustion of hydrogen and oxygen could be used in the system. The lunar regolith could also provide mineral nutrients. If lunar regolith is nontoxic and inert, it might be used as a solid substrate that is watered with a solution of minerals essential for plants, although hydroponic growth of plants is relatively simple and is probably more reliable. Several possibilities have been considered for energy production on the Moon. Solar cells could relatively easily be deployed on the lunar surface. However, such cells are expensive, relatively inefficient, and quite massive to transport to the Moon. Furthermore, it is dark for half of the lunar day, which is about 29.5 Earth days long. Another possibility would be to install a conventional nuclear fission reactor on the Moon. Finally, nuclear fusion, if ever developed to a practical level on Earth, might be an energy source, since it has been speculated that tritium exists on the lunar surface, adsorbed onto regolith particles. l 8 This tritium could serve as the energy source for a fusion reactor. This might also be a reason to go to the Moon in the first place-namely, to collect tritium for use in fusion reactors on Earth. In the absence of any atmosphere, the problem of leaks is very serious on the Moon. Furthermore, the Moon has no known source of carbon, which thus would have to be brought from Earth in order to be burned (with oxygen produced from silicates) to produce CO, for the plants. Solar flares are relatively common and, without a protective atmosphere, potentially lethal at the lunar surface. It has been recommended that any living being on the Moon should be protected by at least three meters of lunar regolith. If this is true, the lunar CELSS will have to be underground (but note the comment of Schwartzkopf' l 7 mentioned above). This means that no direct sunlight would be available for plants, although during the lunar day light could be 'piped' in through fibre optics or a similar system. The 9

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size of the collectors, however, would probably have to be twice as large as the area illuminated because of the inefficiency of moving light this way. The long lunar night would, however, remain a problem. Because the Moon turns once on its axis for each revolution around the Earth, the same side of the Moon is always facing the Earth. This means that if the lunar colony with its CELSS facility is located in the Sea of Serenity on the Moon, for example, the Earth will appear about 60” above the horizon, just west of south, and it will always be in that position. Because there is no atmosphere, the sky is always dark and the stars will be visible to the dark-adapted eye. Although the Apollo astronauts, who visited the Moon during lunar day, could not see them, the stars in the black sky along with the Earth should be visible for anyone looking out of a porthole in a dark or poorly illuminated room, even if the room would be beneath three meters of regolith. S ~ h w a r t z k o p fhas l ~ ~presented ~ details of a proposed lunar CELSS facility built on the surface of the Moon. Earth-watching from the Moon would certainly be a wonderful pastime. The diameter of the Earth as seen from the lunar surface would appear 3.7 times that of the Moon as seen from the Earth. The Sun, however, will appear essentially the same size as it does from Earth, and is almost exactly the same size as the Moon appears from the Earth, varying slightly with the Moon’s distance from Earth. The Earth will go through a similar series of phases as the Moon appears to do from Earth. When the Sun is nearly behind the Earth, only a small crescent of Earth will be illuminated (i.e., the “new Earth”). Once or twice a year, the Earth will come between the Sun and the Moon causing a solar eclipse. The Earth atmosphere will then refract red light all around the Earth-a circular, celestial sunset! When the Moon is located between the Sun and Earth, the Earth will be fully illuminated a (i.e., “full Earth”). This will be at lunar midnight, when the Moon is seen from the Earth as a “new Moon”. When the Moon is directly between the Sun and Earth, the Moon’s shadow with its umbra (the dark center) and penumbra (the lighter, graded, surrounding shadow) will be visible as it moves across the Earth’s surface, a phenomenon that has already been photographed from satellites. Seen from the Earth, this is called a solar eclipse, but seen from the Moon it is an eclipse of the Earth, even though only a small portion of the Earth will be darkened. Such an eclipse will occur once or twice a year. C.

A Microgravity CELSS in a Space Ship

Building a CELSS facility to orbit the Earth that would thus operate in a microgravity environment will surely present two challenges. Firstly, engineering the movement of liquids and gases in microgravity will be a technical challenge. Water or nutrient solutions will not drain through a porous substrate as they do on Earth, nor will heated gases rise. Liquids must be pumped and gases must be moved by fans or other means. Toilets in microgravity have had serious problems in the past; they only work when properly engineered with rapid air movement.

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The second problem is how microgravity might affect the crop plants that are to be used in a CELSS facility. Plants on Earth are highly sensitive to g r a ~ i t y .21-23 ~. When a young bean seedling is placed on its side, the tip of its stem will within hours have bent so that it will be growing upright. Even if such a stem is turned only a few degrees from the vertical, it will soon bend just enough to be vertical again. Even when a tree is tipped to its side, it will bend as compression wood forms on the bottom of the trunk and or tension wood forms on the top of the trunk. Gradually, the upper part of the tree will approach the vertical. Thus, it would certainly not be surprising if plants could not complete a life cycle in microgravity. Even if they appear to grow normally at some time in their life cycle, at some stage of their development an accelerating force would be required. So this stage cannot be completed in the absence of such an accelerating force, which on Earth is provided by gravity. Could this prevent the use of plants in a microgravity CELSS facility?

IV.

CULTIVATION OF PLANTS IN SPACE A.

Earlier Experiments

To determine whether plants can be grown for a full life cycle in microgravity, a few dozen experiments about plant growth have been carried out in orbiting space vehicle^.^^-^^ In some cases, the plants seemed to grow quite normally for the short duration of a shuttle flight,28but in other cases, even such short-duration flights led to serious a b n ~ r m a l i t i e s . ~Russian ~ . ~ ~ scientists have carried out several longterm plant-growth experiments in space, and in most of those instances, plant growth was quite abnormal. The paramount achievement was the growth of Arabidopsis thaliana through a complete life cycle, from seed to seed.27,32This experiment was carried out in the Phyton-3 device in Salyut-7 in 1982.The plants were grown under continuous light for 69 days from sowing until return to Earth. Five plants produced 22 fertile seed pods, while two plants produced 11 sterile pods. There were about 200 seeds, half of which were immature. Only 42% germinated to produce normal plants. Compared with plants grown in the same devices on Earth, growth in space was retarded and generally very poor; plants aged prematurely. This may have been due to the accumulation of ethylene in the cabin atmosphere (see discussion in section IV. C). It is premature to conclude that microgravity per se causes poor growth of plants in space. These findings clearly necessitate further and more extensive experiments in which all other factors have been optimized. This is discussed in section IV. B.

B.

Experiments with Super-Dwarf Wheat in Mir

In 1990, Russian and Bulgarian scientists sent a plant growth chamber called Svet (Russian for “light”) to the Russian Space Station Mir. The first plant exper-

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iment in Svet produced wilted plants that probably suffered from lack of water in the ~ u b s t r a t e . ” In ~ ~1992, ~ I was asked to form a United States team35 to work with the Russian and Bulgarian scientists, and this combined team has recently completed three experiments with a cultivar of super-dwarf wheat (Triticum aestivum L.) in Svet on Mir.36337Wheat is a good candidate for growth in a CELSS facility for the following four reasons: (1) it constitutes a significant portion of the diet of many peoples, (2) it can easily be stored, (3) it can simply be converted into edible and nutritious food by soaking or grinding and boiling to make a mush, or even by making bread, and (4) its yield in controlled environments can be surprisingly high. Bugbee and Salisbury4 were able to produce 60g m-2d-1,five times the world record yield in the field, thanks to continuous irradiation at the level of noan, summer sunlight (2000 ymol.m-2K1 PPF, which is photosynthetic photon flux in moles of photons between 400 and 700 nm in micromoles per square meter second), optimum temperature (23 “C) and humidity, enriched C 0 2 (1200 pmol.mol-’), and a nearly ideal hydroponic nutrient solution. At that rate, only about 15 m2 plant growth area would be required to provide adequate nutrition for a single crew member if that crew member were willing to eat nothing but wheat! With addition of other crops plus a safety factor, 50 m2 should suffice.15 Although super-dwarf wheat gives poor yields and would never be used in a CELSS facility, it is short enough (about 30 cm tall) to fit in Svet and otherwise provides a good model for higher-yielding wheat cultivars. Svet has about 0.1 m2 of growing area. The so-called root module has two compartments filled with a nutrient-enriched zeolite called balkanine. It is covered tightly with a metal lid that is perforated with small holes and with four open channels where plants can be grown. Our wheat seeds were attached to plastic strips in proper orientation, so shoots would go “up” into the air and roots “down” into the balkanine. The seed strips were placed between two layers of wick material that absorbed water from a so-called hydroaccumulator in the center of the balkanine in each compartment. Water diffused from the wicks into the balkanine. Originally, there were six pairs of fluorescent lamps, a system manufactured in the Soviet Union before 1990. The irradiation from the lamps was just high enough to permit growth of plants (-120 p n ~ l . m - ~ . sPPF), - ’ but not high enough for optimal growth. The lamps were cooled by a fan that pulled cabin air into Svet, past the plants, and over the lamps. When our Utah State University team began to cooperate with our colleagues in Moscow, it was agreed that our team would build additional equipment to attach to Svet. This equipment included four infrared gas analyzers for monitoring CO, and water vapor in air entering and leaving two plastic cuvettes, one attached above each compartment of the root module. This allowed measurements of photosynthesis, respiration, and transpiration of the plants. Sixteen moisture probes operating on a thermal principle for controlling moisture levels in the balkanine were provided and together with two probes already part of the original root module made a total of I8 probes. Two infrared sensors detected temperatures of the

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Figure 2. Schematic drawing of Svet as it is installed on the wall of the Krystal module of Mir. The hatch, to which the U.S. Shuttle attaches, is just to the right on the drawing. When the hatch door is open, it misses the Svet equipment by only 1 cm. The root module consists of two compartments, and two plastic cuvettes cover the plants above each compartment, although this is not illustrated in the drawing. There are two gas-exchange systems, one for each of the cuvettes. The cuvettes were used only in the second planting of the 1996197 experiment.

growing surface or plant canopy, two thermistors measured air temperature, and oxygen level and cabin pressure were also monitored. The instrumentation was controlled with a notebook computer. Figure 2 shows Svet mounted on a wall in

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Mir with the Utah equipment attached (sent to Mir with the Spektr module in June 1995). Our experimental plan was to grow the plants from seed to seed by allowing at least 90 days for the plants to mature and produce seed after planting. Plants were to be sampled into chemical fixative at five stages during their growth, and photographs and video recordings were to be made to document the course of the experiments. After the mature plants were harvested, a second crop was to be planted, partially to see if the balkanine would continue to supply nutrients. With proper timing, the second crop would be harvested after 30 to 40 days, coinciding with the arrival of the U S . Shuttle with a liquid nitrogen freezer. The frozen plants were to be tested on Earth for several plant hormones and other metabolites as well as mineral nutrients. In 1995, super-dwarf wheat was grown in Mir for 90 days (August 12 to November 9), but a series of equipment failures led to extremely poor growth and almost totally vegetative plants.37 Four of the six lamp pairs failed early in the experiment, leading to light levels (-80 pmol.m-2.s-') only slightly more than the photon flux needed to reach the compensation point at which photosynthetic CO, uptake just equals CO, loss in respiration. Although the plants remained alive for most of the 90 days, they were spindly and somewhat disoriented in the microgravity environment. However, much was learned about the equipment and about the difficulty of maintaining a suitable moisture level in the balkanine. Concerning the latter point, particle size proved to be extremely important. If the particles are too large, water will not move by capillary force; if they are too small, roots suffer from oxygen deficiency. This knowledge, which is critical for any microgravity CELSS to be built in future, was applied in our next experiment. In 1996, we were able to repeat the experiment with new equipment including a new lamp bank that utilized more modem fluorescent lamps (very low mercury content) that produced about 400 p m ~ l . m - ~ . sPPF, - ' about one fifth of the photonflux of full sunlight. The equipment functioned well (faulty equipment had been replaced), and plants were grown for 123 days from seed planting until final harvest (August 5 to December 6). Plant samples were taken at five stages and chemically fixed, and photographs and video recordings were made several times during the experiment. Unfortunately, new plastic cuvettes arrived too late to be installed (plants were already too large), so the gas-exchange measurements could not be carried out. On the day of harvest the second planting was completed, and these plants grew for 41 days until the arrival of the Shuttle Atlantis at Mir on January 14, 1997. During this period, the cuvettes were installed and successful measurements of photosynthesis, respiration, and transpiration were completed-the first time such measurements had been achieved in a space experiment with plants. These young plants, which were just forming heads, were harvested and frozen in the liquid nitrogen freezer. The samples and other returned materials are being analyzed, and the results will be reported elsewhere. The most encouraging observation was that the plants

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Figure 3. Two photographs, made from video recordings transmitted to Earth by photographing the screen, of Super-Dwarf wheat plants growing in Mir. The top image shows the dense canopy 34 days after planting, and the bottom image shows two large and healthy appearing wheat heads 86 days after planting.

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grew extremely well (Figure 3); the high irradiance and carefully controlled substrate-moisture levels produced much more biomass than had been produced in any other plant experiment in space. About 280 heads appeared on the plants growing in the 0.1 m2 area. The environment was monitored carefully throughout the experiment, including PPF, air and leaf temperatures, CO, and O2 concentrations, humidity, and moisture in the balkanine. Substrate moisture was measured at 18 locations at two different depths and two distances from the wicks. Thus we were able to show that healthy plants can be grown in microgravity at essentially the same rate as on Earth. This is encouraging for the construction of a microgravity CELSS facility. The frozen plants arrived back on Earth in excellent condition. They have been analyzed for several plant hormones and other factors, a measure that is providing insight into the stress levels encountered by plants growing in space. A surprising and, at the time, disappointing aspect of the experiment was that the harvested and sampled heads were all sterile. Not a single seed could be found in any of the heads! Ground studies (not yet published) have been and are being carried out by William Campbell at Utah State University, David Bubenheim at NASA Ames Research Center, and our Russian colleagues at the Institute of Biomedical Problems and the University of Moscow. Studies of the dry heads brought back from Mir invariably indicate the same three phenomena: development of the flowers stopped just at the stage where the stamen filaments began to elongate, pollen was not released, and the pollen found in the anthers often had only one or two nuclei instead of the normal three. Some anthers contained no pollen at all. It is clear that seeds failed to form because pollination did not occur, that is, the plants suffered from male sterility. The fact that this was the case for all heads, although they developed at different times during about two months, suggests that the arrested development did not occur in response to a single, temporary stress such as an interval of high temperature. Rather, development was stopped by some prevailing factor that was always present in Mir.

C . Discussion Is microgravity the direct cause of poor plant growth in previous experiments and the lack of pollination in the Mir studies? Not necessarily, because plant experiments in space have always been plagued with several conditions that are stressful for plants even on Earth: 1. Photon flux has been low in nearly all experiments, on the order of 150 pmol.m-2.s-1PPF. Although healthy plants have been grown at this low level on Earth in equipment closely similar to that used in space, this is a lower limit for acceptable plant growth and has surely contributed to poor growth in previous experiments.

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Carbon dioxide has always been high in previous experiments, and this factor has probably not always been duplicated for the ground controls. Optimum CO, levels for many plants are around 1200 pmol.mol-'. A cabin atmosphere of 5000 pmol.mol-'(O.S%) is common in spacecraft because maintaining the CO, below this level requires too frequent a change of the lithium hydroxide in the CO, scrubbers and this relatively high COz level is well tolerated by humans. 3. Temperature in spacecraft fluctuates much more than in ground situations, sometimes reaching levels as high as 37 "C, which can be harmful to plants. 4. Root environment is probably the most important factor. Poor drainage may lead to water logging around the roots, and attempts to compensate for this may overcompensate and lead to lack of water, as apparently happened in the first plant experiment in Mir in 1990.33,34Clearly we cannot draw conclusions about plant responses to microgravity when so many other stress factors may be present. 2.

However, we have reason to believe that an entirely different factor was responsible for the arrested development and sterility of the super-dwarf wheat. Samples of cabin atmosphere returned from Mir showed that ethylene was present in levels of 300 to 1200 pmol.mol-' air. Ethylene is a gaseous plant hormone that is responsible for many plant r e ~ p o n s e sIt. ~is produced by ripening fruit, for example, and through positive feedback is responsible for continued ripening. Low concentrations of ethylene can induce male sterility in wheat and other cereal^.^*-^' Most studies with cereals were carried out with the soluble compound ethaphon (Ethrel), which releases ethylene after application, so it is difficult to relate the concentration of applied ethaphon to the concentration of ethylene in the atmosphere. Nevertheless, in one study with oats (Avena sativa), only 150 nmol.mol-' of gaseous ethylene produced almost total ~terility.~' My colleagues at Utah State University (William Campbell and Bruce Bugbee) and at NASA Ames Research Center (David Bubenheim) have performed studies in which super-dwarf wheat was grown in atmospheres that contained various levels of ethylene.37 Their results show that the heads that formed were not only sterile but exhibited the exact symptoms of the Mir wheat: inhibited filament elongation, failure to release pollen, and defective pollen within the anthers. Other symptoms noted in the Mir wheat, namely, shortened internodes and excessive tillering (branching at the ground level), also appeared in wheat grown in an ethylene-containing atmosphere. The conclusion of these studies is that ethylene was almost certainly responsible for the sterility of the Mir wheat. Thus, in future experiments with plants in space, as well as in any future CELSS facility on the Moon, Mars, or in microgravity, it will be critical to monitor ethylene levels and to scrub it from the atmosphere used. The ethylene in Mir could have been generated in several ways, particularly by fungi that are known to grow in damp places in the relatively large Russian space

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Figure 4. Aerial photograph of Biosphere-2, located in the desert of Arizona, U.S.A. The photograph was kindly supplied by John Allen.

station!3 Cooled walls cau3e condensation, and fungi take up residence. It is virtually impossible to eliminate them. Indeed fungi were growing at the base of the plants in our experiment, apparently without any direct harm to the vegetative growth of the plants. It should be noted that the atmosphere in Mir has not been completely changed in the 1 1 years of its operation, and the activated charcoal in the air purification system does not remove ethylene. We feel confident that viable seeds of wheat as well as Arubidopsis can be produced in microgravity if the atmosphere and all other factors are properly controlled.

V. A.

G R O U N D EXPERIMENTS

Experience with Biosphere-2, Arizona

The $1 50-million Biosphere 2 facility in Oracle, Arizona, has often been likened to a future CELSS on the Moon or Mars. It is a large greenhouse (Figure 4) covering 1.2 hectares of Arizona d e ~ e r t . Seven ~ ~ . ~ so-called ~ biomes (ocean, fresh and salt marshes, tropical rain forest, savanna, desert, intensive agriculture, and human habitat) stocked with 3800 species of plants and animals attempt to duplicate similar biomes on Earth. In the first Biosphere-2 project, four men and

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four women were sealed i n the structure for two years from September 26, 1991 to September 26, 1993. Although the designers and operators were willing to apply extensive and necessary technological intervention like the provision of large amounts of energy in the form of electricity and natural gas in order to achieve the desired level of environmental control and stability, a primary goal was to see if such a complex assemblage of organisms might arrive at a natural balance of some kind. Ecologists were of a divided opinion: some suggested that complexity might inevitably lead to stability of an ecosystem since, if one component fails, others will take over its function; others suggested the opposite, namely, that complex systems have more points of vulnerability. Certainly, once a complex ecosystem (e.g., a rainforest) has been destroyed, it is very difficult to restore it. The Biosphere-2 project provided an opportunity to test these ideas in a controlled environment. In the publicity about Biosphere-2, it was often stated that it was a prototype for future structures on the Moon or Mars. Clearly, such a relatively flimsy, pressurized structure could not exist on the airless or nearly airless surfaces of the Moon or Mars, nor could a stronger structure of such complexity, including so many species, be built on Moon or Mars even in the distant future. The actual design of Biosphere-2 suggests that it was aimed at a better understanding of the biomes on Earth, and it is now being used for that purpose. Hence, many if not most CELSS scientists tended to ignore the project. Nevertheless, a number of the results of the two-year trial and of subsequent studies should be of interest to scientists who concern themselves with the design and operation of CELSS facilities. Indeed, Biosphere 2 is a bioregenerative life-support system, a controlled environment life-support facility; as such, it qualifies as a CELSS facility. John Allen, the prime mover in the Biosphere-2 project, has published a review that describes the results in much As expected, there was an inverse relationship between light and CO, removal: the more light, the more CO, was removed by photosynthesis. During the summer of 1992, CO, levels were between 800 and 2000 pmol.mol.', ideal for photosynthesis. Since both winters in the two-year trial were unusually cloudy and the superstructure of the facility blocked about half of the light, decay and respiration of organisms led to much higher CO, levels during the winters, reaching 4500 pmol.mol-' during January, 1993. When levels exceeded 5000 pmol~mol~', CO, was removed by a scrubber in order to protect the pH of the aqeous biomes. The high CO, level was accompanied by an unexpected, gradual drop in oxygen level, which reached 14.2% before fresh oxygen was added to the atmosphere, because the human inhabitants experienced difficulty in working. Clearly, much more oxygen was leaving the system than could be accounted for by the increase in CO,; the drop in oxygen was about 6% of the total atmosphere, while C 0 2 increased by less than 1%. The discrepancy between 0, loss and CO, gain was a mystery for some time. As it turned out, 0, was being used in respiration and decay of the large amount of organic matter taken in before the structure was

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sealed, but much of the C 0 2 that was produced by this respiration combined with the unsealed structural concrete. Because there was opportunity for biological waste management, Biosphere-2 was able to recycle all human and animal wastes for the first time in a closed system with human inhabitants. Fresh and salt water were also recycled. The inhabitants were able to produce 8 1 % of their diet in a sustainable agriculture system is an area comprising 0.2 hectare. Allen notes that 100% of caloric requirement could have been supplied with more light (10 mol.m-2.d-'). This capability was demonstrated when artificial lights were installed above the agriculture system after completion of the two-year Biosphere-2 e ~ p e r i m e n tNitrous .~~ oxide (N20) increased continuously during the experiment. This gas would probably have to be removed by physicochemical means in a functioning CELSS facility. Species diversity remained high during the two-year trial as more species survived than had been expected. The unbalanced C 0 2 and 0, levels and the increase in N 2 0 demonstrated that enclosing even a large volume with thousands of species is not necessarily sufficient for providing a balanced turnover of matter. This will therefore be even more difficult to achieve in a necessarily much smaller CELSS facility. Much of the computer and other regulatory technology at our disposal will be required to reach this goal. However, if we are willing to settle for only partial closure with resupply, the challenge becomes much simpler. We would almost certainly have to settle for an incomplete closure because of the impossibility of recycling true deadlock substances. However, it is estimated that it might be possible to achieve closures as high as 95%. B.

Experience with BIOS-3, Siberia

When the Russian space program began to develop around 1960, scientists in the Institute of Physics in Krasnoyarsk, central Siberia, became intrigued with the CELSS concept. Details are summarized in an article by Salisbury, Gitelson, and L i ~ o v s k y In . ~ 1965, ~ the Siberian scientists constructed a system, called Bios-1 , that could regenerate the atmosphere and produce sufficient oxygen to support one human. A sealed 12-m3 chamber was connected through air ducts with an 1 %liter algal cultivator containing Chlorella vulgaris. About 8 m2 of the algal culture were irradiated with three, 6-kW xenon lamps. In 1968, the structure was attached to a 2.5 x 2.0 x 1.7 m chamber for higher plants that the builders called a phytotron. (This term was first applied in the late 1940s by James Bonner and Samuel Wildman to the Earhart Plant Research Laboratory at the California Institute of Technology in Pasadena, in jest, to indicate that botanists could have something as imposing as the cyclotron being constructed then at the University of California, Berkeley. The term was soon being applied to controlled-environment facilities all over the world).

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In a later version of the Siberian facility called Bios-2, oxygen was regenerated not only by the algal culture but also by food plants, which additionnally provided a small amount of food for the individual living inside. The test subject could go through a sealable hatch from the chamber into the phytotron to tend the plants and to harvest the crop. The algae still provided about 75 % of the air purification. A third version, Bios-3, constructed in 1972, has since then been used almost continuously and in various ways, including three full-scale experiments with crew members sealed inside. The total time of closure for the three Bios facilities now exceeds two years. Bios-3 is completely underground, reached by a passageway from the main building of the institute.47 It is constructed of welded plates of stainless steel to provide a hermetical seal. The structure is divided into four compartments of nearly equal size (ca. 7 x 4.5 x 2.5 m). Each compartment has three doors that are sealed tightly with rubber gaskets, and one of these doors leads to the outside. Occupants of the system can escape through the outside door within 20 s in case of an emergency, but such an escape has never occurred. Each compartment can

Figure 5.

Interior of one of the three phytotrons of Bios-3. Two xenon lamps were installed in each water jacket. To make the lamps visible, the upper part of the black-and-white print (from a color slide) was given three times the exposure time of the bottom part; that is, the lamps are much bri hter relative to the plants than they appear in this print (from Salisbury et al).45

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be sealed independently in combination with any other compartment. There are large round windows in some doors and other large portholes in the living compartments. The crew area occupies one compartment. It is subdivided into three individual sleeping rooms, a kitchen, a lavatory, a control room, and equipment for processing wheat and inedible biomass, for making repairs and measurements, and also for purifying water and air. During the early years of Bios-3, one compartment included algal cultivators that provided enough air revitalization to support at least three crew members, although the remaining two compartments did not provide enough space to grow the vegetable requirements of three humans. Eventually, the algal cultivators were removed, and a common configuration was to grow wheat, chufa nut sedge, and a set of vegetable crops in each of the three compartments (Figure 5). The total growing area equals 63 m2, which provides ample air regeneration capacity. Each phytotron initially had 20 vertical, 6-kW xenon lamps, each surrounded by a vertical glass cylinder through which water circulated for cooling. The cylinders are inserted through a hole cut in the ceiling, allowing the lamps to be changed from the outside. By 1991, the number of lamps had been doubled in one phytotron by inserting two lamps into each water jacket. Light levels varied from 900 to 1600 p m ~ l . m - ~ . sunder -' single lamps (depending upon voltage applied) and from 1,300to 2,450 ymol.m-2.s-' under double lamps. The high irradiancesthe highest being well above summer sunlight--come at the expense of a relatively high air temperature (ca. 27 "C to 30 "C), which is too high for many crops including wheat. This required expanding the cooling system and providing additional energy above that needed for the lamps. In the experiments, so far, the lamps were operated continuously without a dark period, which excludes crops like tomato and potato that require a dark period. Maintaining inside pressures slightly above outside pressures to exclude entrance of pathogens causes calculated maximum leak rates of only 0.20-0.26 vol.% per day. Air is circulated to the crew quarters from the phytotrons and back. It is partially purified by the plants, but a thermocatalytic filter operating at 600-650 "C oxidizes all organic molecules to C 0 2 and H20 and completely eliminates ethylene from the atmosphere. Transpired water is condensed and reused for nutrient solution for the plants, for washing linen and dishes, and for general cleaning. Drinking water is additionally purified by ion exchange filters with the addition of small quantities of potassium iodide and fluoride for health and potassium chloride and some other salts to improve the taste. Samples are passed through small air locks to the outside for analysis, and the health of crew members is continually monitored, sometimes by attachment of various sensors to their bodies. Wires from these sensors are passed through the walls in sockets designed for the purpose. Occupants have privacy during their free time but are still monitored for medical parameters. There was no health deterioration after six months, although the microflora of skin, mucous membranes, and intestines changed significantly but without pathological consequences.

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Table 1.

Bios-3 Crops during the Third Experiment

Crop

Expected Crew Needs

(d4 I . Wheat. grain (dry mass) 2. Chufa, tubers (dry mass) 3. Pea, grain 4. Carrot, edible roots (fresh mass) 5. Radish, edible roots (fresh mass) 6. Beets, edible roots and leaves (fresh mass) 7. Kohlrabi, vteins and leaves (fresh mass) 8. Onion, leaves, bulbs (fresh mass) 9. Dill, greens (fresh mass) 10. Tomatoes (fresh mass) 11. Cucumbers (fresh mass)

12. Potatoes (fresh mass) Notes:

520 234 52 220 I10 130 180 120 30 150 100 250

Actual

Yield

Area

Area

(dm’di

(m21 40.0 9.0 4.0 1.4 0.9 0.9 1.1 0.7

fm’1

13 26 13 160 125 170 170 170 30 110

250 80

39.6 8.6 4.0 1.2 0.9 0.9 1.0 0.6

C

C

1.4 0.4 3.2

1.2 0.4 4.8

Harvest Harvest Indexh (dd) (%o) 496 34.7 120 48.1 26 25.4 236 54.9 266 59.8 132 67.5 164 37.1 110 90.1 16 93.0 88 33.1 276 54.6 22 5.9

‘Scientific names of crops are as follow^: 1 . Trificumurstivum; 2. Cyperus esculentus; 3 . Pisum sativunz; 4. Daucu,>carotu; 5 . Raphanus sativus; 6. Beta vulgaris; 7. Brassica olerucea gongylodes: 8. Allium sp.; 9. Arirthum graveolens: 10. Lycopersicon esculmtum; 1 I.Cucumis sativus; 12. Solanurn tuherosuni. ’Harvest index is calculated on a dry mass basis. ‘L. =crop was grown between other culture rows

All three of the closure experiments in Bios-3 were initiated during early winter to minimize invasion of pathogens from the outside. The first experiment lasted six months during the winter of 1972 to 1973 with three male crew members; some of them exchanged during the experiment. The second experiment during the winter of 1976 to 1977 lasted four months, again with three male crew members although one left during the experiment. The goal was to test the ability of the enclosure to supply food. In the third experiment, two male crew members were sealed in the facility for five months from November 1 1, 1983 to April 10, 1984.47 Table 1 lists the crops that were grown during this experiment. The plants were grown in artificial, solid substrates with hydroponic nutrient solutions. Each phytotron contained crops of three to seven different ages, allowing the continued availability of food during the experiment and contributing to stable oxygen levels. Chufa nut sedge (Cyperus esculentus), which has an oil-rich tuber, was an interesting addition to the crops grown in Bios-3. It was used as a delicacy by many peoples for millennia, but since its culture was never mechanized it has essentially dropped from modern use. However, it proved to be an excellent source of oil in the Bios-3 experiments. Chufa and some of its relatives are known in many parts of the world as the world’s worst weeds.48 The choice of plants4935”depends not only on taste, nutritional qualities, and caloric content, but also on their gas-exchange qualities. The average respiratory quotient for a human (RQ = C02/02) is about 0.89 to 0.90 rnol.mol-’, depending on the diet. Plants that store energy primarily as starch have an assimilation quo-

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tient (AQ = O,/CO,) close to 1.0. Thus if only starchy plants such as wheat and potatoes are grown, oxygen will tend to decrease in the atmosphere as C 0 2 builds up. To achieve a better balance and also for the human diet, it is necessary to have oil crops, which have a lower AQ, the exact quotient depending on the crop and the growth conditions. Introducing chufa into Bios-3 lowered the AQ from about 1.0 to 0.95, and the gas concentrations remained relatively stable. The CO, varied from about 0.5% when crops were growing especially well to slightly over 2% shortly after the beginning of the second experiment. The Bios-3 crew members consumed about 20% of their calories in the form of meat stored at the beginning of the experiment or passed in through the air locks. This was generally lyophilized meat to which water was added for reconstitution. None of the crew members were vegetarians, and none of them were desirous of becoming such. This raises the question: Is production of meat in a CELSS facility needed? It is quite possible to be a completely healthy vegetarian, but many potential crew members find that idea unappealing. Nevertheless, growing the usual meat animals in a CELSS facility would require at least 10 times as much area to grow feed for the animals as that required for vegetarians. Actually, a few animals could be part of a CELSS facility by feeding them plant parts that are inedible for humans. Chickens and fish have been mentioned in this context. Nevertheless, construction and operation of a CELSS facility become much simpler as the crew moves in the direction of vegetarianism.49750Another important aspect is the recycling of waste material. In Bios-3, the urine was added to the nutrient solutions for wheat (contact only with roots). Other human wastes were dried and stored, so the Bios experiments made only a beginning at the recycling of waste material. A rather large team of researchers was involved in the Bios-3 experiments. There were chemists who studied mineral balances including trace metals released from the air purification and other systems. About six researchers studied the microflora (i.e., bacteria, fungi, actinomyces, and yeasts) of nutrient solutions, plant root and shoot surfaces, solid media, and human skin and intestines (fecal samples). It was found that, although stability was never achieved, populations of various micro flora never exceeded the normal limits encountered outside of Bios-3. On the other hand, it appeared that staphylococci on the skin had the potential to endanger the very existence of humans in the system. The microbial population varied extensively in the first experiment, so measures were taken in the second experiment to reduce this fluctuation. Linen was no longer washed, but clean linen enough for the duration of the experiment was stored at the beginning. The catalytic converter was added. Crew members wore gauze masks when they worked with the plants. This led to higher stability in the microbiological communities, although they never became completely stable. Several theoretical analyses were carried out based on the Bios-3 experience. One practical conclusion emphasizes the reliability of plants over machinery. When the correct environment is provided, plants are highly reliable; most prob-

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lems resulted from failure of the equipment providing that environment. While an algal reactor may contain 1013 cells, any one cell may regenerate the system because the ability to do so is encoded in its genome. The same is true for higher planta, where a single seed, or even a single plant cell if tissue-culture techniques are used, may regenerate the plant culture. Engineered components, on the other hand, have no such capability for self-regeneration. Chemical studies showed that it would be essentially impossible to recycle some substances-the so-called deadlock substances. The Bios-3 experience made it clear that, although it is a desirable goal to reduce unrecycled waste substances to the barest minimum, this might prove to be more costly than resupply. To convert the minerals in ash to plant nutrients, for example, might require sophisticated equipment that itself requires substances such as strong acids that would have to be resupplied. Would it be better to grow the plants in solid substrates rather than hydroponically, so that waste products could be composted and returned to the substrate? Unfortunately, such biological waste disposal is slow and has its own problems, like the potential for plant and even animal diseases. There are many possibilities that await future study. In any case, thinking about Bios-3 helps us to appreciate the balances that have existed for $0 long on Earth. Clearly, there are also balances in our industrialized society. Such a complex society, with its manufacturing capability, could hardly be compressed into the confines of a practical CELSS facility. Rather than trying to duplicate the balances in our biosphere and industrialized society, it will be necessary to learn to achieve balances and thus stability in the confined volume of a CELSS facility. Achieving stability proved to be a serious problem not only in the Biosphere-2 project, but also in the Bios-3 experiments. In the latter experiments the instabilities were mostly in microelements and microflora, and the recognition and evaluation of these instabilities was a clear achievement of the Bios-3 experiments. Microfloral instability poses a potential threat, both to plants and to crew members, and microbial communities may exhibit new processes not recognized in the design of the system. Viruses and plasmids remain to be studied in such systems. Stability was much higher in Bios-3 than it was in Biosphere-2. As noted, in spite of expensive environmental control and other human intervention, the goal in Biosphere-2 was to let the diversity of species lead to stability. There were a few instances in which this seemed to be taking Cockroaches, for example, multiplied exponentially until they were visible almost everywhere. Lizards (geckos) that fed on the cockroaches then multiplied until the cockroach population was brought back under control. Some crops, however, failed to produce as had been predicted, while others seemed to flourish. Apparently, too much carbon had been taken into the system at the start because of the concern that growing plants would soon exhaust the carbon dioxide in the atmosphere. This overcompensation led to a drop in oxygen levels. In any case, the high level of diversity in Biosphere-2 did not seem to be much of an advantage in achieving stability. In spite of some shortcomings, the Bios-3 experiments were more suc-

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cessful in maintaining an acceptable level of stability through the application of advanced technology and human intervention in a relatively simple system, particularly in food production. It should perhaps not be surprising that it is easier to obtain stability in a simple system in which the parameters can be better understood and controlled. The CELSS experiments carried out so far, including Biosphere-2, strongly call our attention to the importance of size in a functioning ecosystem. The Earth, with its huge hydrosphere, atmosphere, lithosphere, and biosphere, has an immense capacity to buffer against change. Our industrialized society has been pumping CO, into the atmosphere at increasing rates for almost two centuries, and so far the changes have been relatively small, though significant. In a CELSS facility with its small buffering capacity, CO, levels fluctuate over hours instead of years. John Allen has called this phenomenon a time microscope.44 The clear conclusion is that a functioning CELSS facility must depend on technology that makes up for its tiny buffering capacity.

C.

BIO-Plex under Construction at Johnson Space Center, Houston

NASA is designing and building a facility called BIO-Plex at the Johnson Space Center (JSC) in Houston, The facility is essentially an updated version of Bios-3. It is hoped that lessons learned in the Bios experiments can be applied in B I O - P ~ ~The X.~ scientists ~ at JSC have consulted with their colleagues in Krasnoyarsk. Initially, the facility will consist of five cylindrical chambers, each 4.6 m in diameter and 11.3 m long, joined by an interconnecting transfer tunnel and accessed through an airlock, a configuration that has earlier been suggested for a lunar CELSS facility.I8 It will be possible to add two more chambers for a total of seven to meet future needs. Each chamber will have two decks and two hatches, one connecting with the tunnel and one for emergency entry or egress. The facility will be state-of-the-art with all the latest control systems, lighting systems for the plants, and so forth. Both physicochemical and bioregenerative life-support systems will be tested. Current plans are for a 120-day test in the year 2001 with three chambers providing 50% food production and 25% waste recycling (personal communication from Russ E. Fortson, Johnson Space Center, Houston, Texas). A 240-day test with five chambers is planned for 2003 when a laboratory chamber and a second biomass production chamber will be added. It is hoped to achieve 90% to 95% food production and SO% waste recycling. Present plans also include a 425-day test beginning in 2005 with 90% to 95% food production and 90% to 95% waste recycling. Of course, all these plans are subject to change. Actually, many preliminary tests have already been completed in smaller chambers at JSC, some including plants and others based on physicochemical systems. In one such test involving a single occupant, one crop of wheat was almost completely sterile. Although not proven, this may be the result of an ethylene concen-

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tration of about 200 nmol.mo1-' measured in the chamber. This observation is of interest in light of our experience with super-dwarf wheat in Mir. Additional CELSS facilities are being constructed in Japan53 and planned in Europe.

VI.

CONCLUSIONS: LESSONS LEARNED

Research in the field of bioregenerative systems, closed or nearly closed with respect to matter but open with respect to energy, has led to some important insights and generalizations, not only about the design and operation of a CELSS but also about earthly ecosystems. A number of the generalizations that can be concluded from this review are summarized here:

1. It is possible, at least over relatively short time intervals with the use of advanced technological control of the environment (temperature, light, air purity, etc.) and an outside energy source, to enclose in a relatively small volume a functioning ecosystem (i.e., CELSS), that accommodates humans who are dependent on green plants for recycling of the air (algae or higher plants) and for food production (mostly higher plants). 2. A CELSS will require a high input of energy to provide sufficient light for photosynthesis (if not obtainable from direct sunlight) and to maintain environmental control. 3. The challenge of creating and operating a CELSS facility is that its limited size leads to a highly limited buffering capacity against the changes in the environment that tend to occur as crops are grown and as humans interact with the system. The lack of buffering capacity must be compensated for by sophisticated control systems. 4. The time that such a CELSS facility can be maintained, even in semiclosed mode, is highly dependent upon the efficiency of waste management. Without resupply and removal of wastes, their accumulation will eventually limit the life of the facility. 5. Resupply of critical components will prolong the life of the CELSS facility. It seems clear that a practical facility will not achieve 100% closure with respect to matter but will depend on some resupply and waste removal. 6. The practicality of a CELSS facility for space exploration is determined by the break-even time when the extra mass required to operate the facility equals the mass of materials that would otherwise need to be resupplied (see Figure 1). Only if the break-even time is shorter than the duration of a proposed mission will a CELSS facility become practical (providing that costs also balance). 7. Biological recycling of organic wastes, as in Biosphere-2, is most efficient (i.e., produces products that can be used directly by plants), but is slow, requires relatively large mass and volume, and may harbor plant and ani-

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ma1 pathogens. Physicochemical recycling, although limited in other ways, may be needed. 8. An important conclusion is that large size and complexity of an ecosystem are no guarantee for ecological balance and stability. If we are to build such a small system as a CELSS facility must be, we will have to learn many things about how best to intervene in the functions of the system in order to keep it relatively stable and under control. Much headway has been made, but much remains to be done. 9. The weakest link in a CELSS facility is not the plants, but rather breakdown of the mechanical equipment. Plants can regenerate (reproduce) themselves after a crop failure (probably caused by failure of mechanical equipment), but machinery has no such ability. Broken machinery must usually be repaired by living organisms-the crew members. 10. There is good reason to believe that the absence of gravity will not limit crop production in a microgravity CELSS facility. Plants will grow well in microgravity if other stress factors are maintained at a minimum. 1 1. Inclusion of animals in a CELSS facility will greatly increase its complexity and size. The more vegetarian the diet, the simpler and smaller can the facility be. It might be possible, however, even in a relatively simple system, to include some fish and fowl that can feed mostly on food that humans cannot consume. Meat might sometimes be provided through resupply. 12. The success of a CELSS facility operated in a gravity environment or in microgravity is to be found in the details, and often those details are not evident until experimentation is carried out. In the Mir experiments, for example, the importance of balkanine particle size and of ethylene present in the atmosphere only became apparent after failuies were experienced in space experiments. Another example is the importance of balancing the respiratory quotients of the crew members with the assimilation quotients of the various crops. Such details may be known, but they are often overlooked.

VII.

SUMMARY

An option in the long-duration exploration of space, whether on the Moon or Mars or in a spacecraft on its way to Mars or the asteroids, is to utilize a bioregenerative life-support system in addition to the physicochemical systems that will always be necessary. Green plants can use the energy of light to remove carbon dioxide from the atmosphere and add oxygen to it while at the same time synthesizing food for the space travelers. The water that crop plants transpire can be condensed in pure form, contributing to the water purification system. An added bonus is that green plants provide a familiar environment for humans far from

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their home planet. The down side is that such a bioregenerative life-support system-called a controlled environment life-support system (CELSS) in this paper-must be highly complex and relatively massive to maintain a proper compwition of the atmosphere while also providing food. Thus, launch costs will be high. Except for resupply and removal of nonrecycleable substances, such a system is nearly closed with respect to matter but open with respect to energy. Although a CELSS facility is small compared to the Earth’s biosphere, it must be large enough to feed humans and provide a suitable atmosphere for them. A functioning CELSS can only be created with the help of today’s advanced technology, especially computerized controls. Needed are energy for light, possibly from a nuclear power plant, and equipment to provide a suitable environment for plant growth, including a way to supply plants with the necessary mineral nutrients. All this constitutes the biomass production unit. There must also be food preparation facilities and a means to recycle or dispose of waste materials and there must be control equipment to keep the facility running. Humans are part of the system as well a5 plants and possibly animals. Human brain power will often be needed to keep the system functional in spite of the best computer-driven controls. The particulars of a CELSS facility depend strongly on where it is to be located. The presence of gravity on the Moon and Mars simplifies the design for a facility on those bodies, but a spacecraft in microgravity is a much more challenging environment. One problem is that plants, which are very sensitive to gravity, might not grow and produce food in the virtual absence of gravity. However, the experience with growing super-dwarf wheat in the Russian space station Mir, while not entirely successful because of the sterile wheat heads, was highly encouraging. The plants grew well for 123 days, producing more biomass than had been produced in space before. This was due to the high photon flux available to the plants and the careful control of substrate moisture. The sterile heads were probably due to the failure to remove the gaseous plant hormone, ethylene, from the Mir atmosphere. Since ethylene can easily be removed, it should be possible to grow wheat and other crops in microgravity with the production of viable seeds. On the ground Biosphere-2 taught us several lessons about the design and construction of a CELSS facility, but Bios-3 came much closer to achieving the goals of such a facility. Although stability was never completely reached, Bios-3 was much more stable than Biosphere-2 apparently because every effort was made to keep the system simple and to use the best technology available to maintain control. Wastes were not recycled in Bios-3 except for urine, and inedible plant materials were incinerated to restore CO, to the atmosphere. Since much meat (about 20% of calories) was imported, closure in the Bios-3 experiments was well below 100%. But then, a practical CELSS on the Moon might also depend on regular resupply from Earth. Several important lessons have been learned from the CELSS research described in this review.

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ACKNOWLEDGMENTS I wish to thank Mary Ann Clark for help with the manuscript. Preparation o f the paper was partially supported by the Utah Agricultural Experiment Station (paper # 6015) and by Grant NCC-2831 from NASA.

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