THE SPACE FLIGHT INVESTIGATION
The force of gravity on the Earth's surface has shaped the architecture of nearly all life. Our bodies look the way they do and function as they do partly because of the constant tug of this ever present gravitational force on all of our parts. We can only get away from gravity during brief moments in a very fast elevator or for an instant on an amusement park ride or when we venture into outer space. Another way to escape gravity that works for periods of up to about 30 seconds involves flying an up-and-down pattern in a jet airplane. Astronauts use this approach to develop and test equipment that will fly on the space shuttle and to verify that certain experimental techniques can actually be performed in the microgravity of space. A special airplane called the KC-135 is used to create these brief periods of microgravity by flying in a certain up-and- down pattern (called a parabolic pattern). Use of this airplane is important because it enables scientists to understand how to design experiments that will actually work in space. In fact, all of the space flight experiments that you have been or will be exposed to in this book have had some of their components tested on the KC-135 airplane; these even include the animal studies. In particular, for experiments using rats, it was necessary to actually take the rats for a ride on the KC-135 airplane to make sure that it was safe to work with them in microgravity, and that it was feasible to do the work planned without gravity. Let's see how gravity levels can change onboard the KC-135 airplane.
| Figure 12. |
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During different portions of the KC-135 parabolic flight pattern, the force that are experienced by everything and everybody in the plane vary from almost a complete absence of gravity to about two to three times the force of gravity. These different forces alternate as the plane goes up and down. Figure 12 shows the parabolic flight pattern for the KC-135 airplane. The forces during flight can best be understood by breaking the flight pattern into its four main parts:
- In the first part, as the plane flies upward, the force that is exerted on everything in the plane grows to a level greater than the force of gravity because of the plane's acceleration forces. Many of you have experienced similar forces during "take off" in a plane.
- The second part is a transition period where the plane begins to arc, pulling out of the upward path and into the downward path. It is during this period that everything in the plane experiences about 30 seconds of "free fall," similar to the briefer experience that one has when going over a hill in a roller coaster. This period of free fall is the microgravity phase. During this phase, the force of gravity is effectively cancelled out by the forces produced by the plane's movement, and everyone in the plane begins to float (Figure 13).
- The third part, as the plane travels downward, is where gravity again begins to take control slowly.
- The fourth part is also a transition period where the plane again begins to arc, pulling out of the downward path and into the upward path. During this portion of the maneuver, forces are produced that are equivalent to about two or three times the force of gravity! If you can imagine how difficult it would be to lift your body if it weighed two or three times its current weight, then you can imagine how this portion of the flight must feel to the passengers of the plane. In fact, you may have experienced this on a roller coaster as it came to the bottom of a hill and quickly began another upward climb. As that scooping arc begins, your neck muscles can barely lift your head!
The KC-135 has served as a test facility for various pieces of research equipment and for different techniques employed for the study of rats in space. In particular, the Research Animal Holding Facility (RAHF) was flown onboard the KC-135 to ensure that it would operate correctly in microgravity. The RAHF contains 12 rodent cages, each of which can house two laboratory rats (Figure 14). The facility contains all food, water, environmental, and sanitation arrangements for each of its inhabitants and permits access to the animals during flight for health checks and if any other need arises. A monitoring system gathers feeding, activity, and environmental data.
| Figure 14. |
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After the RAHF successfully operated in the KC135, it was certified for flight on the shuttle and plans were made to perform further tests of this equipment on an actual space flight mission. It was important to verify that the following subsystems of the RAHF functioned without any problems for longer periods in space:
- the environmental control system responsible for maintaining appropriate temperatures and humidity levels;
- the feeding and fluid delivery systems, which provide specially designed nutritional food bars for the rats to eat as well as a continuous water supply for the rats to drink ad libitum (at their will); and
- the ventilation system, which consists primarily of fans to circulate and filter the air within the RAHF as it moves from the space laboratory into the facility and back out again into the laboratory.
Proper operation of all three of these systems is obviously important for the health, safety, and wellbeing of the animals but one of these systems, in particular, is also important for the health and well-being of the astronauts as well. Can you guess which one?
Most of the laboratory rats flown to accomplish Dr. Baldwin's experiment were accommodated in the RAHF. His experiment was actually carried out on different space missions that involved different amounts of inflight animal handling. For their first flight, the animals were handled very little and the actual experimental procedures and data collection were carried out on the animals only After they returned from space. On a subsequent mission, some of the experimental procedures on the rats were carried out in space and others were done upon their return. In either case, the techniques that were carried out and the data that was collected from the flight animals were matched by the techniques and data taken from control animals. The control animals were those rats that did not get to fly in space but that were housed in a similar RAHF here on Earth. These animals were exposed to the same environmental, dietary, and waste processing conditions as the flight animals were. Theoretically, then, the only difference between the experimental conditions of the flight and control animals was the presence or absence of gravity. The experimental data collected on the control animals served as a point of comparison for the data that were collected on the flight animals. This comparison served to illuminate the effects of microgravity on muscular function. In the next section, we will review the results of Dr. Baldwin's experiment.
Dr. Baldwin's muscle experiment was designed to characterize the effects of microgravity on the structure and function of certain anti-gravity muscles in the hind limb of the rat. We will be looking at the results of only a subset of his various measurement sets, including:
- muscle mass and Type l/Type II muscle fiber distribution changes, and
- changes in the endurance level of rat muscle and a possible biochemical explanation for such changes.