The Spacelab Life Sciences Missions: SLS-1 and SLS-2Prior to 1991, the last major opportunity to study human physiology in an integrated way was during the Skylab era (1973). The scientific results from Skylab led to the development of preliminary hypotheses (predictions) to explain the physiological changes found to occur in space. The SLS-1 (1991) and SLS-2 (1993) missions represent two of the most significant life sciences space research missions that have taken place since the space shuttle began flying in 1981. This pair of missions was specially designed to test the hypotheses generated from the results of Skylab.
From SLS-1 and SLS-2, scientists have obtained a greater understanding of how the body adapts to space flight and how the body readjusts to Earth's gravity by examining the effects of space flight on six interdependent body systems:
The SLS - 1 Mission
The SLS-1 mission can boast a very impressive record of "firsts" in the history of the space program. It was the first mission to include three women out of the total seven crew members. This is a very important fact because of the need to collect physiological data on both women and men. The SLS-1 mission was also the first to fly five medical and research doctors, three of whom were the women. Interestingly enough, the SLS-1 mission carried some extra passengers: more than 2000 jellyfish and 29 laboratory rats. The SLS-1 mission provided scientists the first opportunity to study the acute (short-term) effects of weightlessness - their mechanisms (what causes a change), time course (when and how fast the changes take place), and magnitude (the extent of the changes) - in a comprehensive, interrelated fashion using males, females, and "other animals."
A major component of the SLS-1 mission involved the extensive participation of the crew as subjects for all the experiments. Besides all the inflight data collection, the scientists ran their experiments on the ground before the flight, collecting preflight data on the whole crew, and again after the flight, collecting postflight data. The experiments could not be completed without the preflight data because each astronaut's condition in space must be compared with his or her preflight "Earth-normal" condition. The postflight data were important to track the readaptation of the body back to the "Earth-normal" condition. The scientists measured the same body functions for each crew member starting sometimes as early as 6 months before the mission flew. The experiments were then repeated after the flight for about a month.
The crew of the SLS-1 mission was extremely busy during each day of the mission. Most crews who fly onboard the space shuttle carry out a split-shift operation, where half of the crew (Team A) work while the other half sleep (Team B). Then, while the members of Team B work, those on Team A will sleep. This can upset the normal body "rhythms" of the crew. SLS-1 followed a single-shift operation where all crew members followed the same work/sleep cycles that were matched as closely as possible to normal, everyday life cycles here on Earth. The mission timeline of activities (Figure 13) was very busy and included daily work cycles of about 10 hours in duration, sleep cycles of about 8 hours, and a presleep period and postsleep period (each about 2-3 hours) that the crew used for personal time. The maintenance of a stable schedule, such as this one, is of extreme importance in order to preserve the normal rhythms of the body and minimize the disturbances that might affect the scientific physiological data that will be collected on the astronauts.
The highly successful SLS-1 mission provided the science community with much needed data to extend and refine the observations on Skylab that took place nearly two decades earlier. It established confidence in carrying out such an ambitious mission by demonstrating that complex studies of living things could actually be done in space. Large quantities of preflight, inflight, and postflight data were collected on all three of the species that flew together. Finally, the results of SLS-1 helped researchers fine-tune their plans for the next big effort: SLS-2.
The SLS-2 Mission
The SLS-2 mission was launched on October 18,1993, and after 14 days in space, it returned to Earth on November 1. This was a longer mission than SLS-1 and it provided further data about the physiological changes that occur later in a mission. By the time the 2 weeks were over, the SLS-2 mission set a duration record for shuttle flights and could boast the most extensive set of physiological data collected to date. Again, rats were onboard the mission and a more extensive set of procedures were performed on these animals than was ever attempted before.
As stated earlier, research studies using animals are essential to obtaining an understanding of complicated human physiology. This is true on Earth and in space. On the SLS-1 and SLS-2 missions, many of the same tests that were performed on humans were performed on the laboratory rats so that scientists could determine whether the animals' internal systems function in a similar way to human systems as far as space flight is concerned. If the animals function in a similar way to humans (and this is true in many cases), then much needed and detailed research, inappropriate for healthy humans, can be done with the laboratory rats. The animals used for space research are always treated in a humane and appropriate way. Studies using these animals have provided us with very important and otherwise inaccessible information to help us understand how the body works.
Both the SLS-1 and SLS-2 missions utilized the laboratory facilities of Spacelab. Similar experiment equipment was flown on both missions and the investigators coordinated and shared the use of the equipment, minimizing hardware development. Most of them used NASA Life Sciences Laboratory Equipment, an inventory of multipurpose, reusable medical and biological instruments that have been developed or modified for use in microgravity. This equipment includes animal holding facilities, refrigerator/freezers, a urine sampling device, small and large mass measurement devices, and a special laboratory work station. There are also unique pieces of equipment designed for particular investigations, such as a rotating chair for vestibular or balance studies, and a specialized exercise bicycle for cardiovascular and cardiopulmonary studies.
The results from both the SLS-1 and SLS-2 missions will be put together to provide a more complete picture of how the changes in the body relate to one another. Look at yourself in a mirror. All your body parts are physically connected, but the connections go much deeper than you realize. Your nervous system and blood offer avenues of communication among distant parts of your body, and these avenues are active all the time. As one system in our body changes, this change is communicated to other systems throughout the body, resulting in further changes "downstream." Figure 15 illustrates some of the connections between different physiological responses and changes that occur as a result of the space environment. This "flow chart" is color coded to represent the different body systems, and it may appear complicated. In fact, it is complicated because it represents some of the space flight changes that occur to the most complex system in the world: the human body! In order to simplify this confusing display of physiological relationships, it is best to break it down into simpler parts. This is what we are going to do. We will cover small parts of this flow chart in each of the Focus sections (or chapters) that follow. You will see this flow chart again at the end of each Focus, but it will look somewhat different because certain individual boxes will be identified to show which ones were covered in that particular Focus. Before you know it, you will understand this flow chart better than you ever dreamed possible! Finally, Table 1 is a list of all the major experiments on SLS-1 and SLS-2.
As stated earlier, the remainder of this book will concentrate on individual body systems and how they are studied in space. Each Focus will highlight one or more space experiments that have already flown on the SLS-1 and SLS-2 missions. Each experiment has discovered some interesting phenomena that are helping to define the next generation of space experiments. That is what science is all about: learning enough to go on to the next set of questions.
So let's move on to our investigations into how space flight affects the human body.