Linux at the University
The Linux operating system (OS) coupled with powerful and efficient available software development tools, has been the platform of choice for several research and commercial projects at the University of Colorado located in Boulder. The projects include several payloads that have flown seventeen sortie missions on the NASA Space Shuttle. The payloads were built by BioServe Space Technologies, which is a NASA Center for the Commercialization of Space (CSC), and is associated with the Aerospace Department within the College of Engineering. BioServe's payloads have logged over a full year of combined operation in micro-gravity, including two extended stays on the Russian Space Station Mir. Student projects include an unmanned ground vehicle (UGV), which navigates autonomously based upon its assessment of dynamic environmental conditions.
Linux has proven to be an invaluable teaching instrument from a pedagogical standpoint at our university. Several classes are taught in the Aerospace Engineering Sciences and Computer Science departments that take advantage of the sophisticated and robust features of the Linux OS, not only as a development platform but as a method to teach and implement advanced topics in hardware/software integration, control theory, operating systems research and systems administration. This article details several of the projects from a scientific and engineering perspective and provides an overview of common software methodologies used in the development of the complex control schemes necessary for reliable and robust operation.
BioServe Space Technologies has built and flown several payloads that use the Linux operating system to interface with the control hardware, provide support for operator input and output devices, and provide the software development environment. The workhorse of the BioServe payload fleet is the Commercial Generic Bioprocessing Apparatus, CGBA, shown in its version 1 configuration in Figure 1. The operator data entry and visual feedback devices are located on the front panel. The keypad and mini-VGA screen are attached to an embedded control computer running Linux. The CGBA payload operates as an Isothermal Containment Module (ICM) with the capability to precisely control chamber temperature from 4 to 40 degrees C. Inside are the biological and crystal growth experiments loaded before launch. CGBA is a “single locker” payload (approximately 21"x17"x10") for insertion into either the Shuttle mid-deck or into large payload modules that fly in the Shuttle cargo bay.
The Fluid Processing Apparatus (FPA) shown in Figure 2 is the fundamental experimental unit of the CGBA payload. It is essentially a “micro-gravity test tube” which allows controlled, sequential on-orbit mixing of 2 or 3 fluids. The fluids are contained inside a glass barrel with an internal diameter of 13.5mm. Up to eight of the FPAs are loaded into a motorized Group Activation Pack (GAP), shown in Figure 3. Control software initiates the mixing of liquids containing different cell culture mediums at specified times once the payload has attained micro-gravity. A second motor initiation, which depresses a plunger in a manner similar to a syringe, combines the cell culture mixture with a fixative reagent, which squelches further biological reaction. The flight samples are then analyzed in comparison with identical samples in ground-based experiments to determine the effects of micro-gravity upon fundamental biological processes. Pharmaceutical and crystallization investigations are common candidates for these types of experiments; a small increase in the efficiency of a reaction can lead to a significant increase in the desired products, and thus to a company's profit margin.
A second version of the CGBA payload highlights a configurable internal palette, upon which several different habitat chambers can be built. This payload contains multiple video cameras to provide time-sequenced image recording of the habitat contents. BioServe has used this payload configuration, in conjunction with SpaceHab, Inc., to provide exciting space-based educational outreach to primary and secondary school students across the world. SpaceHab's Space Technology and Research for Students (S*T*A*R*S) program supports a wide array of potential experiments for investigation.
In July of 1999, two experiments flown in the payload onboard Shuttle mission STS-93 involved students from high schools in the United States and students from a secondary school in Chile. As shown in Figure 4, this payload had three center habitats which contained ladybugs and aphids while the larger chamber in the front left housed butterflies. The first experiment, of interest to students in the U.S., involved the incubation and development of pupae larvae from a crystalith (cocoon) to the maturation stage of a butterfly. The second experiment, designed entirely by a group of teenage girls in Chile, investigated the effects of micro-gravity upon the predatory relationship of ladybugs and their favorite prey, the aphid, which is notorious for its devastation of food crops. Figure 5 shows aphids, the small black objects on immature wheat plants before the introduction of predatory ladybugs into the habitat chamber. The same habitat after the release of the ladybugs is pictured in Figure 6, showing a (expected) decrease in aphid population. Note the ladybug in the middle right of the image as it attempts to catch an aphid in microgravity. The results of this experiment hope to gage the potential for using natural predator/prey relationships as a means to minimize detrimental insect destruction of both terrestrial and space-based food crops.
The second S*T*A*R*S flight, to be flown aboard Shuttle flight STS-107 in 2001, will support an international array of educational science experiments. Participation in this program is literally worldwide. The educational departments of numerous countries submitted proposals for selection, from which six experiments have been chosen. Australia will fly an AstroSpider experiment, which is a collaboration between Glen Waverely Secondary College and researchers from the Royal Melbourne Institute of Technology. They will investigate how the mechanical properties of the spider's silk, which can be 100 times stronger than steel, differ when it is produced in microgravity. Jingshan High School in China has proposed an experiment which will investigate one of their country's icons, the silkworm. Their experiment will attempt to reveal the effects of microgravity on silk production and the metamorphosis process as the silkworms transform from worm-like larvae, spin cocoons and then emerge as adults. Two of the new S*T*A*R*S experiments will be a joint effort involving investigators in Japan and the United States. Both groups will each fly an enclosed, aquatic ecosystem that contains plants and tiny aquatic animals. They hope to investigate the navigation and locomotive processes of these animals as they swim in microgravity. Israeli science students from Motzkin Ort Junior High School are designing their experiment to investigate the formation of crystals in microgravity. They hope to see differences in the shape, structure, strength and size of space grown crystals over similarly grown, earth-based crystals. The final participant in this flight is Fowler High School from Syracuse, New York. In conjunction with Syracuse University, they are designing a zero gravity ant farm. They hope to chart differences in the species' well-characterized social activities brought on by the disorientation of space flight. Recently, a third S*T*A*R*S mission has been scheduled for the year 2002 onboard the International Space Station, which will allow longer-term scientific investigations to be conducted.
The CGBA payload, in its version 3 configuration (see Figure 7), allows for individual temperature control of each GAP container by coupling it to a heat sink (water loop) with thermoelectric modules and insulating each GAP from one another. The water loop is distributed on the top and bottom of this locker to virtually eliminate thermal gradients and to provide efficient temperature control of the individual sample container. These individual temperature controlled containers allow for time sequenced experiment initiation and termination. The National Institute of Health will fly an experiment investigating the development of fruit fly maturation, from larvae to adults, using this configuration. The fruit fly maturation process can be effectively controlled by warming Petri dishes contained in the individually temperature controlled GAPs at different times using pre-programmed temperature profiles. The fruit fly has a scientifically well-documented development process. Scientists have been able to genetically splice fruit fly neurons with fluorescent markers and are interested in the development of neural based muscular activity. The research is aimed at investigating the difference in neural pathway development between space-based samples as compared to similar ground-based control samples. The scientific goal is an increased understanding of how gravity affects neural development.
The Plant Generic BioProcessing Apparatus (PGBA) (see Figures 8 and 9) is twice the volume of the “single locker” CGBA payloads described previously. It is designed to precisely control the environmental conditions of temperature, humidity and oxygen/carbon dioxide levels within the plant growth chamber for fundamental space-based research in the plant sciences arena. Plant science is fundamental for many large commercial entities including the pharmaceutical and timber industries. Both of these commercially important industries are, in part, based upon plant-produced compounds. For example, the timber industry funds significant research programs investigating the production of lignum which is responsible for providing structural strength for both plants and trees. From a commercial perspective, lignum is undesirable for the formation of paper products but a desirable constituent for strong timber products. In micro-gravity, a plant's need for structural strength is greatly diminished. Current research focuses upon the change in lignum production in micro-gravity based plants and trees to determine potential methods for regulating the compound's terrestrial in situ production.
The Fluid Generic Bio-Processing Apparatus (FGBA) (see Figure 10) is interesting from both the research and commercial perspectives. FGBA was developed in conjunction with the Coca-Cola Company and was flown onboard STS-77 in 1996. FGBA was used to provide basic research about human taste perception, which changes in micro-gravity. The payload also investigated fundamental relationships pertaining to two-phase fluid flow and dispensing in microgravity, as the beverages were mixed and dispensed using pressurized carbon dioxide in a manner similar to earth-based soda fountains. From the commercial perspective, this payload demonstrated the potential for the funding of space-borne projects based not only upon their fundamental research potential, but also on their capability to bring commercial advertising dollars to the cash-strapped International Space Station.
In all of the payloads, an accelerometer-based system is used to detect launch, thus allowing experiment initiation (motor-activation, temperature change, lighting conditions) immediately upon entering orbit. Additionally, automatic experiment termination can be programmed to occur at any time during the mission, including just prior to reentry, based on pre-planned (or updated) Shuttle end-of-mission time. These two capabilities combined allow an early-as-possible experiment initiation and a late-as-possible termination, since those are periods when astronaut crew availability for payload tasks is typically at a minimum.