Integration of Plant Growth into a Mars Habitat

Paul D. Campbell
Lockheed Engineering & Sciences Co., 2400 NASA 1, Houston, TX 77058

Nathan Moore
NASA Johnson Space Center, 2101 NASA 1, Houston, TX 77058

INTRODUCTION

Human exploration of Mars is part of the long-term vision of the world's space community. To accomplish this exploration with current and foreseeable technologies requires a high degree of mass and volume efficiency in the design of crew living and working areas. It also necessitates the consideration of air, water, and food systems that recycle the essentials of human life. This study was performed in an effort to integrate mass-efficient habitation and life support systems in the context of a human Mars mission.

Background

In 1993, NASA conducted a study of a potential Mars exploration program to determine its feasibility (Weaver and Duke, 1993). The study defines a program consisting of three human missions to the same site on Mars. Part of that study was focused on definition of human habitation elements and operations on the Martian surface. Mission-specific objectives and constraints were combined to develop Mars habitation concepts that could then be subjected to feasibility and cost analyses. One of these concepts is the subject of this study.

Purpose

This study was performed to develop and evaluate a concept for increasing human self-sufficiency on Mars. To enhance a Mars crew's self-sufficiency, it is desirable to incorporate closed-loop life support systems into their surface habitat. Increasing closure of the food system may only be achieved with a life support system that includes growth of edible biomass; thereby reducing the size of the food supply needed from Earth. A system-level solution was thus sought for integrating crop growth into the Mars mission.

Approach

Most previous studies of plant growth on Mars have focused on large greenhouse structures that are attached to the crew habitat elements. This study, however, changes the focus to one of integration of the plant growth function with other crew habitation functions. There are several driving reasons for this change: the mission under study involves a large amount of habitable volume resulting from the addition of a new habitat element on every crew mission; these habitat elements are connected on Mars to form an integral outpost; and the food growth volume requirement is minimized by restricting it to vegetable crops.

RESULTS

Assumptions

A concept has been developed for the integration of a plant growth subsystem into a Mars habitat under the following assumptions:

each habitable element of the Mars surface outpost is sized to contain 250 cubic meters of pressurized volume;
each element is a 7.5 meter diameter, vertical-axis cylinder with two habitable levels;
an initial laboratory element is sent prior to the first crew;
each crew rides in a single habitat element from Earth to Mars, including Mars entry and landing;
each crew habitat contains consumables for the crew for the entire outbound journey and Mars surface stay;
the crew habitat is connected to the other surface habitable elements and the crew lives in them for the entire surface mission.

Figure 1 illustrate the habitat element configuration concept.

Under these conditions, it was found that after three crew missions, the total pressurized volume is 1000 cubic meters, including the initial laboratory element and three crew habitat elements. Much of this volume, initially filled with consumables such as food and clothing, is emptied after the crews use their consumables. It may then be used as plant growth volume if it is configured for this purpose.

Figure 1. Mars Surface Habitat Concept

Crew consumables for a Mars mission include food, clothing, hygiene and cleaning supplies, work supplies, and other items. Taken together these supplies make up a volume of approximately 0.016 cubic meters per person-day. For a 680 day mission (180 days from Earth to Mars and 500 days on Mars) in the habitat element, with six crewmembers, a total crew consumables volume of 66 cubic meters is expected.

Mass constraints on the Mars habitat element drive the use of these crew consumables as radiation shielding during the outbound trip from Earth to Mars. Configuration of the consumables around the perimeter of the habitat pressure shell is expected to provide 15 gm/cm2 shielding with a shielding effectiveness between those of aluminum and water. Figure 2 illustrates the interior configuration of the habitat including crew consumables anticipated for the outbound trip/surface habitat.

Crew access to the stowed consumables is necessary for efficient mission operations. The crewmembers must be able to reach any required supplies as the need arises. The configuration of Figure 2 allows this crew access in weightlessness during the Earth-Mars trip and in partial gravity on the Martian surface.

Figure 2. Mars Habitat Interior Configuration

Requirements

The consumables stowage subsystem is designed to meet the following requirements:

support of contents under launch and landing accelerations: launch vehicle thrust produces up to four times Earth-normal gravity accelerations on the habitat and its contents;
crew access to stored consumables: an astronaut must be able to reach any needed consumable item within a short period of time after the need arises;
radiation shielding of the habitat crew: mass restrictions on the habitat and the severe radiation environment of deep space make it necessary to configure the crew consumables as shielding during the outbound trip to Mars.

The plant growth subsystem is designed to meet the following requirements:

minimum crew time for setup and operation: preflight integration of the plant growth components ensures that crew time can be used for more productive tasks. This also allows plant growth system testing and verification prior to habitat launch;
crew access to crops: for planting, tending, and harvesting, the plants must be easily accessible by the crewmembers;
demonstration of plant growth on mission one: the first crew will show the capability to grow crops for life support purposes so that subsequent crews can depend on this capability;
supply of a significant fraction of crew food on missions two and three: it is expected that no more than 25 percent of the crew's diet would consist of fresh, uncooked vegetables. The balance of the food system can be brought from Earth as long-shelf-life packaged items.

Analysis

Merging the requirements of the consumables stowage and plant growth systems, it was found that a stowage system consisting of modules with a 1 meter by 1 meter plan-form could also be used as plant growth chambers when the consumables contents are emptied. If, after allowing for lighting and nutrient systems, each module provides 0.67 meters of plant growth height, then a 1 meter module height would be appropriate. Such a module would provide consumables stowage volume equal to 67 percent of its total volume. Figure 3 illustrates this concept.

Based on (Drysdale, et al, 1993), it is expected that 40 square meters of plant growth area will be required to grow 25 percent of crew food mass. Approximately 40 modules are therefore required, occupying 40 cubic meters of total pressurized volume, not including tankage, ventilation, and support equipment. These 40 modules would provide stowage for 27 cubic meters of crew consumables (67 percent of 40 cubic meters). Additional dedicated stowage is therefore needed for the other 39 cubic meters of consumables (66 cubic meters minus 27 cubic meters).

The initial mass of the plant growth function is estimated to be 3 metric tons, and the potential payback from food savings is estimated to be 1.5 metric tons per crew mission based on food mass requirements in (NASA, 1989). After three crew missions, it is estimated that the plant growth subsystem mass could be repaid through reductions in food launched from Earth.

Figure 3. Plant Growth Module Concept

Table 1 shows the breakdown of the plant growth system mass estimate, based on data from (Schwartzkopf, 1990) and (Drysdale, et al, 1993). This does not include the structural enclosures and supports that are required for consumables stowage regardless of the addition of a plant growth function. Only those components that would be added to meet the plant growth function are included in the table.

Based on (Drysdale, et al, 1993), at least 1 kW per square meter plant growth area total power level is expected for this system. This means a total plant growth power of no less than 40 kW for a 40 square meter food growth system, with most of this power being supplied to artificial lighting. Natural lighting might be useful in some cases, if it can be distributed to crops inside the habitable element without a large mass penalty.

Table 1. Mass Estimation for Plant Growth Subsystem Integrated into Crew Consumables Stowage Racks
	Specific Mass (kg/m2)	Plant Growth Area (m2)	Plant Growth System Mass (kg)
Lighting	5	40	200
Nutrient Delivery	11	40	440
Nutrient Resupply	8	40	320
Air Circulation	6	40	240
Dehumidification	3	40	120
CO2 Buffer			240
O2 Buffer			240
Heat Rejection			600
Control System			466
Total			2866

Based on (Schwartzkopf, 1990) it is expected that 5 to 6 person-hours of crew time will be required per day to operate and maintain the food growth system. For the crew size of six people, this translates to 5.5 percent of total crew awake time and does not appear to present an undue burden on crew science and exploration activities. A larger crop growth area, providing on the order of 50 to 100 percent of crew food needs would require two to four times as much crew effort, necessitating the dedication of at least one full-time crewmember to plant growth.

CONCLUSIONS

Plant growth for human life support is a volume-intensive function that can be a valuable reuse of empty consumables volume in a long-duration Mars surface habitat. This study has found that integration of a bioregenerative life support system into a Mars crew habitat can result in rapid return on the initial mass investment, while meeting a significant fraction of total food requirements with food produced on Mars. Crewmembers can tend the crops and maintain the plant growth system without a major time impact to the basic science and exploration mission.

ACKNOWLEDGEMENTS

The authors thank the following persons for their valuable assistance and advice in this study: C. Bourland, J. Connolly, M. Evert, A. Gonzalez, D. Gutierrez, T. Polette, D. Weaver, L. Weaver, and H. E. Winkler.

REFERENCES

Drysdale, A., et al, "CELSS Engineering Parameters", SAE 932130, 23rd International Conference on Environmental Systems, July 1993.

NASA, Man-Systems Integration Standards, NASA STD-3000, Volume 1 Rev. A, NASA Johnson Space Center, October 1989.

Schwartzkopf, S., "Lunar Base Controlled Ecological Life Support System (LCELSS) Preliminary Conceptual Design Study Interim Report", LMSC/F369717, Lockheed Missiles and Space Company, October 1990.

Schwartzkopf, S., "Lunar Base Controlled Ecological Life Support System (LCELSS) Preliminary Conceptual Design Study Final Report", LMSC/F280196, Lockheed Missiles and Space Company, April 1991.

Weaver, D. and Duke, M., "Mars Exploration Strategies: A Reference Program and Comparison of Alternative Architectures", AIAA 93-4212, September 1993.