Space life sciences research is critical to preparing for the eventuality of long-term space exploration. Along the way, this same research increases our knowledge of basic biological processes and provides insight into the mechanisms and treatment of various Earthly medical conditions. However, these scientific results are not achieved easily. The study of living organisms in space poses many challenges that may be negligible or nonexistent in ground-based research.
Determining the most appropriate research subject for studying a particular biological question is not a simple matter. At the broadest level, basic research questions may offer more latitude in approach than questions of applied research, but in space life sciences, the two are often linked. In plant research, basic questions result from the need to maximize food production while minimizing the required onboard spatial volume or from the need to raise plants in an entirely closed environment. By the same token, much basic animal research derives from the need to maximize the health and safety of the astronaut crew. When it comes to the actual species selection, many issues must be taken into account (Fig. 1). Space flight imposes several unique operational constraints that must be addressed in addition to scientific selection criteria. The size, weight, and ease of maintenance of an organism, and the availability of flight-qualified support hardware are issues that become more central when conducting life sciences research in space rather than on the ground.
Species are often selected on the basis of their capacity to undergo some physiological adaptation process or life-cycle stage within a short period of time. For example, Japanese red-bellied newts were selected for experiments on the International Microgravity Laboratory 2 (IML-2) payload because their vestibular systems would undergo most of their development within the planned duration of the Shuttle flight.
An overarching question for much space life sciences research is how gravity, and its effective absence, influences the development and function of living systems. The selection of many research subjects is driven directly by this focus. For instance, the effect of gravity on growth is often studied in plant species because the growth patterns of roots and shoots differ in response to gravity. Jellyfish serve as excellent subjects for research on gravity-sensing mechanisms because their specialized gravity-sensing organs, statoliths, have been well characterized by biologists. In fact, throughout the history of space life sciences, the combination of research priorities and practical constraints has led to a veritable menagerie of organisms orbiting the Earth. Some of the more exotic include African claw-toed frogs, Japanese quail, tobacco hornworm pupae, flour beetles, sea urchin eggs, parasitic wasps, and pepper plants.
When investigations address human adaptation to space flight and its health implications, the use of mammalian species often becomes necessary when humans are not appropriate subjects. The rat is the mammal employed most frequently for space flight research. Its well-demonstrated biochemical and structural similarity with humans makes the rat an appropriate subject with which to test new drugs and investigate many disorders experienced by astronauts during and after space flight. Because of their phylogenetic proximity to humans, nonhuman primates, such as rhesus monkeys, have occasionally served as research subjects in space biology, but only when the need has been clearly demonstrated.
When working with higher organisms, such as mammals, stress caused by unfamiliar conditions can impact science results. To prevent this, the animals must be habituated to their flight habitat, life support hardware, and biosensors. Some animals, such as rats and rhesus monkeys, must be trained to use inflight feeding and watering devices. When performance and behavior is studied, as is sometimes the case with rhesus monkeys, the animals must be trained to perform particular tasks in response to automated stimuli.
Manned and unmanned space flight pose different challenges for conducting life sciences experiments. On manned missions, the primary consideration is the safety of the crew. When mammals are used as research subjects, microbiological testing of the animals is mandatory to ensure that they are free of pathogens that could be transmitted to crew members. Organisms that are part of the science payloads must be isolated from the humans onboard so that possible contaminants and odors do not affect crew health, comfort, or performance. Hardware for housing the experiment subjects is typically custom-built for this purpose and kept sealed or filtered for the duration of the mission.
Although crew members typically have busy inflight schedules, they may support experiments by monitoring research subjects visually on a periodic basis or performing contingency procedures made necessary by hardware malfunction or unexpected experiment performance. The crew may also replenish water and food supplies, substantially reducing the need for automation. On some missions, particularly those dedicated to life sciences, crew members conduct inflight experiment procedures directly on research subjects. Direct access to subjects is accomplished using a glovebox apparatus that maintains biological isolation of the organisms. On the STS-58 mission dedicated to the Spacelab Life Sciences 2 payload, Shuttle astronauts performed rat dissections and tissue sampling procedures. Such procedures will likely become commonplace on the International Space Station.
The costs of research on manned missions can be attributed largely to the extensive testing of experiment hardware and the need to meet crew safety requirements. Unmanned missions are generally much less expensive, with most of the cost going for hardware automation. Experiments on these missions must accommodate the lack of crew to conduct support procedures or intervene in the event of an equipment malfunction.
Space life sciences experiments often require that research subjects be installed in the spacecraft in a precisely timed manner. For instance, if germination of plant seeds is to occur in space, or embryos are to undergo a particular stage of cell division, they must be in a specific stage of development at the time of launch. If the launch is delayed because of inclement weather or a system malfunction, research subjects frequently must be unloaded from the spacecraft and a fresh group of subjects installed once a new launch time is set. To accommodate such an eventuality, researchers must have several backup subject groups, in varying stages of development, prepared for flight.
In order to prepare the spacecraft itself for launch, all payloads, including those accommodating live research subjects, must be integrated into the spacecraft as early as several months before launch. Only critical items, such as the subjects themselves, can be loaded up to several hours prior to launch. Installation of habitats with living organisms may require special handling, depending on the structure and orientation of the spacecraft. Installation of research subjects into the Space Shuttle, which is oriented vertically during the prelaunch period, can involve lowering the organisms in their hardware units through a tunnel into the holding racks in the Spacelab or SPACEHAB.
Because organisms begin to readapt to Earth gravity immediately upon landing, dissection and tissue preservation in orbit or quick access postflight is critical to the value of the science. Organisms can be removed from manned spacecraft such as the Space Shuttle within a few hours after touching down. Removal from the unmanned Cosmos biosatellite occurs several hours postflight because mission personnel must first locate, and then travel to, the landing site. Transport from the spacecraft to ground laboratories may be time-consuming when the biosatellite lands some distance away from Moscow. In such instances, a temporary field laboratory is set up at the landing site to allow scientists to examine the subjects before readaptation occurs. The issue of postflight readaptation highlights the value of inflight data and tissue collection.
Suitable habitats and adequate life support systems for research subjects are essential for experiment success. Hardware to support living organisms is designed to accommodate the conditions of space flight, but microgravity poses special engineering challenges. Fluids behave differently in microgravity. The relative importance of physical properties such as surface tension increases, and convective air currents are absent or reduced. Plants are usually flown attached to a substrate so that nutrients and water can be provided through the root system. Cultured cells are flown in suspensions of renewable media contained within specialized hardware units. Nonhuman primates are often flown in comfortable confinement systems to prevent them from endangering themselves during launch and reentry or damaging sensors or instrumentation during the flight. Other organisms such as rodents are typically flown without confinement so they can float freely within their habitats while in the microgravity environment. With the use of implanted biotelemetry hardware, as with squirrel monkeys on the Spacelab 3 payload in 1985, small primates can be flown unconfined.
The comfort and safety of research subjects is a high priority. Because trauma or stress can compromise experiment results, humane care and good science go hand in hand. Animals may be singly or group-housed, but group-housed animals tend to remain healthier and exhibit fewer signs of stress. When singly housed rhesus monkeys were flown within the Russian Primate Bios units on the Cosmos missions, the animals were oriented so that they could see each other throughout the flight. For nonhuman primates, environmental enrichment is provided in the form of behavioral tasks or "computer games," which can double as measures of behavior and performance. Such enrichment helps to prevent stress and boredom, a possible result of confinement and isolation.
Light within habitats is usually regulated so as to provide a day/night cycle similar to that on Earth. Air circulation and heating or cooling ensures that temperature and humidity are maintained at comfortable levels. Food is provided according to the needs of the species in question and the requirements of the experiments. Generally, a continuous water supply is available. Waste material, which includes not only excreta, but also particulate matter shed from the skin and debris generated during feeding activities, is eliminated using air flow systems engineered for the purpose. Within plant habitats, gaseous waste is similarly eliminated. Separation of liquid waste from solid is desirable for certain experiments, and systems to carry out such separations have been developed.
Frequently, researchers employ surgically implanted biosensors or sensors mounted within habitats to monitor animal subjects. These sensors provide important scientific data, and, with inflight downlinks of physiological parameters, researchers are able to remotely monitor the health and welfare of their subjects. Primates are often implanted with sensors that measure such vital signs as heart rate, ECG, EEG, and body temperature. Activity sensors are often mounted in the cages of rats so that researchers can assess animal activity while in space. Automatic gas sampling can provide a measure of the metabolic activities of plants. Still photographs taken by preprogrammed cameras allow researchers to obtain valuable information, particularly about plant growth and the development of embryos inflight. Video monitoring provides behavioral information on animals such as primates, rats, frogs, and jellyfish. On manned missions, crew members can directly observe subjects, keep records of their observations for later use, discuss their observations with researchers inflight, and, if necessary, intervene to assist a subject.
Investigators can also obtain data in the form of biosamples such as excreta, blood, tissue biopsies, and serial sections. When an experiment protocol requires the collection of biosamples, they are first obtained preflight to provide a baseline measure of organismal function. Inflight collection of biosamples other than urine and feces is possible only on manned missions. The first-ever inflight biopsies and dissections of animal subjects were conducted on Spacelab Life Sciences 2, the STS-58 Shuttle mission. Because of the extensive commitment of resources such as facilities, space, and crew time to payload operations, inflight biosampling procedures are rarely performed on the Space Shuttle. Inflight fixation, through hardware automation or simple crew procedures, is a more common way to collect biosamples. Biosample fixation and storage techniques are effectively used with small animal subjects, plants, and cultured cells.
Postflight collections of biosamples are carried out for many life sciences experiments. Because readaptation to Earth gravity reverses many of the changes that occur in tissues in space, it is imperative that biosamples be obtained as soon as possible postflight. To facilitate this, ground laboratories and personnel are usually prepared to implement such experiment procedures at the time of landing. In the case of the Cosmos biosatellite, biosample collections are carried out in mobile field laboratories set up at the landing site. Unused tissues from the organisms flown in space may be fixed or frozen and stored in archives for later use by scientists.
Space life sciences experiments make extensive use of control groups in part because limited flight opportunities may not allow for replication of a given experiment. Employing control groups is essential to increase the statistical validity of the results of an experiment with a relatively small number of subjects in the experiment group. Control groups help researchers isolate the effects of microgravity and the vibration, acceleration, and noise of spacecraft launch and landing from the effects of other conditions that research subjects may encounter inflight, such as altered environmental conditions, and the stress that can be associated with confinement, isolation, implantation of sensors, and biosampling procedures.
Three types of control groups are often employed in space life sciences experiments. The synchronous control consists of organisms that are identical in type and number to those flown onboard the spacecraft. They are housed in identical habitats and kept within a simulated spacecraft environment in a ground laboratory. Conditions within the simulated spacecraft environment, such as humidity and temperature, are set to levels expected to occur within the actual spacecraft during flight. The synchronous control procedures begin at the time of launch and end upon landing. The purpose is to isolate the effects on the research subjects of extraneous conditions experienced during space flight.
An asynchronous control (or delayed synchronous control) is similar to the synchronous control except that procedures begin several hours or days after the flight. For the asynchronous and delayed synchronous controls, conditions within the simulated spacecraft environment are identical to those that prevailed within the actual spacecraft throughout the flight. Asynchronous and delayed synchronous control procedures last for a duration identical to that of the flight. This control is used to determine whether the effects that may be seen in the flight organisms are the result of anomalous environmental conditions, such as increased temperature, that may have occurred during the flight.
A vivarium control is usually conducted to determine whether effects that may be seen in the flight organisms could be due to the stress of being confined or isolated or of being housed in flight hardware units. In this control, a group of organisms similar to the flight group is housed in standard laboratory conditions for a duration identical to the length of the flight.
Scientists sometimes carry out studies in simulated microgravity conditions on Earth in order to obtain pilot data for flight experiments or to verify the results of flight experiments. The effects of microgravity may be simulated by removing the gravitational load on a particular portion of the body or by effectively canceling out or minimizing the force of gravity. Bed rest is the most commonly used method for simulating microgravity when the research subjects are humans or nonhuman primates. Studies of muscle and bone atrophy are sometimes conducted using this method. Tail suspension is used to simulate microgravity in rats. The gravitational load to the hindlimbs is eliminated by suspending rats by their tails, leaving them free to move about on their forelimbs. Horizontal, rotating clinostats that apply a constantly changing vector acceleration force canceling out the vector force of gravity are often used to simulate microgravity in plants.
Space life sciences research, like that in other fields, is subjected to outside peer review both at the proposal stage and the publication stage. However, because of the unique nature of space flight, research conducted in space is subjected to additional scrutiny. Researchers must meet not only the guidelines and regulations prescribed by mission managers and safety panels, but also ensure that their experiments comply with the requirements of crew members and other researchers participating in that mission. Furthermore, all NASA-sponsored research using animal subjects, whether conducted in space or on the ground, must meet the rigorous review of the Institutional Animal Care and Use Committee (IACUC). Each research institution must convene an IACUC whose mandate is to ensure that all animal use is necessary and that all experiment protocols and animal care procedures meet federal animal welfare guidelines. Lastly, research carried out in space, because of its importance and high visibility, must bear the scrutiny of the public eye.
Ballard, Rodney W. and Richard C. Mains. Animal Experiments in Space: A Brief Overview. In: Fundamentals of Space Biology, edited by Makoto Asashima and George M. Malacinski. Tokyo: Japan Scientific Societies Press and Berlin: Springer-Verlag, 1990, pp. 2141.
The Ethical Use of Animals in Space Life Sciences Research
an interview with Joseph Bielitzki