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Spacelab Life Sciences I: Living with Microgravity N
early 30 years have passed since the first humans were launched into outer space. Yet the fundamental question of whether humans can adapt and work effectively for long periods of time in the strange world of microgravity remains unanswered (see Figure 1). With growing interest in a mission to Mars that may require a space transit time of two to three years, the United States and the Soviet Union are actively pursuing research on the effects of microgravity on humans. NASA has dedicated an upcoming Shuttle flight to investigations of the acute effects of weightlessness and of readapting to gravity upon return to Earth. A seven-member crew, including four scientific researchers, will perform experiments in the European Space Administration's Spacelab—a research module that fits snugly into the Shuttle's cargo bay. The nine-day program is entitled Spacelab Life Sciences 1 (SLS-1) and will run the gamut from cardiovascular and cardiopulmonary measurements to studies of human lymphocytes. This mission is a forerunner to life sciences missions planned for 1992 and 1993. Like so many Shuttle programs, this flight has a long history. The experiments were first approved in 1978. By then it was known that about 70% of astronauts experience some form of motion sickness or, in NASA-ese, space adaptation sickness after launch. Symptoms range from stomach queasiness to "provocative vomiting." After about two days the astronauts adapt to microgravity and the symptoms abate. They can then handle normal workloads. However, on Shuttle flights that last no more than 10 days astronauts must start work immediately even if they feel ill. Thus NASA would like to solve the space sickness problem and improve crew efficiency.
At the end of a mission, astronauts experience a new set of problems as they readjust to the Earth's surface gravity. After 10 days in space, astronauts have trouble walking just after landing. After 100 days in orbit, astronauts cannot stand upright upon return and are subject to fainting. Soviet cosmonauts, who have spent up to a year in Earth orbit, require as much as three days of bed rest to recover from their time in microgravity. Many of these problems stem from
state condition that lowers total body fluid volume. These changes can lead to dehydration and electrolyte imbalances, which in one case contributed to a heart arrhythmia during a lunar mission. Meals on board the Shuttle are planned to compensate for these nutrient losses. When the mission ends, the astronaut suddenly must again contend with full Earth gravity that drains fluids from the upper body. American astronauts prepare for return to Earth by
the realignment of body fluids in low gravity. Shortly after launch, astronauts' faces appear puffy because fluids that normally pool in the lower extremities collect in the upper body. The body senses these changes, and the renal and endocrine systems compensate by increasing urine volume and reducing fluid intake (e.g., the astronaut is less thirsty). Blood pressure also drops during space flights. In time, the body adjusts to microgravity and reestablishes a steady-
drinking as much as 32 oz of water and taking salt tablets prior to reentry. They also don lower-body negativepressure apparatus: "Basically," explains SLS-1 mission astronaut Tamara Jernigan, "to suck on the lower body to pull fluid downward." Long periods of low gravity promote muscle atrophy, skeletal loss, and possibly cellular disruptions. Measurements indicate that as much as 10% of bone calcium may be lost during space missions lasting more than 100 days,
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Clinical horizon
Neurovestibular system /-Fluids and electrolytes r Cardiovascular system
Time scale (months) Point of adaptation
Figure 1. The effect of microgravity on each physiological system. (Adapted from Space Lab Life Sciences I.)
and this loss may not be fully recovered after Earth return. Soviet cosmonauts on long flights spend 2 h every day in weight-bearing exercise, partly to fight calcium loss. However, as Jernigan points out: "No one wants to spend two hours a day exercising when you could be doing science." NASA also worries that when its Space Station becomes operational, astronauts spending longer periods in space might lose some of their flight skills. There are discussions about providing a flight simulator on board the planned U.S. Space Station for Shuttle pilots. To better understand the problems associated with weightlessness, SLS-1 will perform more than 20 studies to assess physiological changes before, during, and after flight. The astronauts will monitor such factors as blood pressure and volume; heart size and heartbeat rate; and changes in muscles, bones, and lung function. Animal experiments will also be undertaken. "NASA is trying to do something that is very difficult," explains T. Peter Stein of the University of Medicine and Dentistry of New Jersey in Camden. "They are providing an opportunity to produce research papers that will survive peer review." Early space life science experiments returned only anecdotal information. Cardiovascular and cardiopulmonary system experiments involve some of the most sophisticated analytical equipment on board Spacelab. An echocardiograph will use ultrasound to provide 2D pictures of astronauts' hearts during flight. Echocardiograms reveal heart diameter, ventricular wall
thickness, and the movements of the various components of the heart. An electrocardiogram (EKG) is also recorded, establishing at what point in the heart's cardiac rhythm the echocardiogram was taken. To determine changes in blood volume in the upper body, astronauts will monitor venous blood pressure near the heart. Prior to launch, a catheter will be inserted into an arm vein of each payload astronaut and snaked to a point near the heart. With the aid of a direct-pressure monitor worn outside the body, the astronauts will record the central venous pressure. As fluid moves to the upper body, central venous pressure increases. After 24 h pressures should stabilize and the catheter will be removed by the mission's cardiologist. The catheter will be reinserted after return to Earth for a new set of measurements. Lung function will also be tested during the flight. Gravity affects the distribution of air to the lungs and, because of the weight of the rib cage and the lungs, the anatomy of the lungs. Changes in gravity could lead to changes in pulmonary functions. To test blood-gas exchange in the lungs, astronauts will inhale various gas mixtures. The exhaled breath passes via a mouthpiece into a gas analyzer mass spectrometer that measures the concentration of O2, CO2, Ar, N2, C 1 8 0, and N 2 0 (see photo). At the same time, the astronauts' EKG will be recorded. "Because of the length of time," says Harold Guy of the University of California at San Diego, "we are flying what we would have flown on [the early U.S. space station] Skylab." The mass spec-
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trometer, a magnetic-sector instrument with a 1-amu resolution and a fast detection time (going from 10% to 90% sample detection in
