Work on activating SLS-2 - which, like its predecessor, utilised the European-built Spacelab module - began almost immediately after the seven astronauts reached orbit. Even before the hatch to the module had been opened, Lucid and Fettman were busy taking blood draws from each other's arms, while Wolf took their blood pressure in order to gather new data on the astronauts' early adaptation to weightlessness. The findings correlated SLS-1 data by showing a slightly lower central venous pressure than predicted in terrestrial studies, coupled with a slightly larger volume in the heart's left ventricle than would be expected with the lower pressure.
This shed new light on the basic physiology of the heart in microgravity. Immediately after the Spacelab module had been activated, Seddon took ultrasound measurements of Lucid's heart with a state-of-the-art echocardiograph imaging unit. Both Lucid and Fettman had, like Drew Gaffney on SLS-1, travelled into orbit with catheters threaded into veins in their arms, which ran to the tips of their hearts to provide these central venous pressure measurements. Lucid's catheter was removed late on 18 October and Fettman's was taken out the following day.
Data dropouts on the echocardiograph led to the astronauts resorting to the use of a portable unit, known as the American Flight Echocardiograph (AFE). This was a piece of equipment very close to Wolf's heart in more ways than one, because he had helped to design it before becoming an astronaut. ''As the person responsible for turning the AFE into flight equipment, you should be very pleased to have it work so well,'' Alternate Payload Specialist Larry Young told Wolf.
The 9,900-kg SLS-2 payload continued the research begun on SLS-1 two-and-a-half years earlier. Unlike the consistent repeatability of experiments in the physical and chemical sciences, it had long been recognised that physiological studies involved looking at variations in responses from one individual to the next. To reduce the influence of biological variability and improve the quality of the scientific results, it was preferable to collect data on numerous individuals and compare it through statistical analysis. When added to the SLS-1 data, the new results would provide the most detailed and interrelated physiological measurements since Skylab.
This second mission, in which NASA had invested $175 million, in addition to its overhead costs for staging a Shuttle flight, carried 14 major experiments, eight of which used the astronauts themselves as test subjects, while the other six utilised the rats. Body tissues from the latter would be preserved and distributed after Columbia's landing to US, French, Russian and Japanese medical scientists as part of an extensive biospecimen-sharing project. Each of the rats underwent radioisotope and fluorescent bone-marker injections before launch to measure their blood parameters and bone formation.
The science crew then took frequent blood draws from the rodents' tails during the first part of the mission and performed additional radioisotope and hormone or placebo injections to measure plasma volumes and track their protein metabolism. This was part of a study into how red blood cell mass changes in space. ''The mechanisms controlling red blood cell production that are affected in space are the same as those affected in people on the ground with illnesses like leukaemia, chronic renal disease and auto-immune diseases affecting red blood cell production,'' Fettman told an interviewer during the flight. ''If we can show that the mechanisms are the same in the rats as they are in the people in the process of space anaemia, and erythropoietin is an effective counter-measure, this may have long-ranging effects in benefiting both people and animals back on Earth.''
Ultimately, the six unlucky rats destined to meet their maker while the Shuttle was in orbit were decapitated by Fettman and Seddon on 30 October using a modified $100 laboratory dispatcher. Pre-flight studies had already opted to decapitate, rather than anaesthetise, the rats because the latter procedure would have caused their neural tissues to deteriorate.
''Things went pretty well,'' said Fettman after the six-hour dissection procedure. ''We're happy to accomplish this. It was a big day for us.'' Each of the rodent tissues was chemically preserved, refrigerated or frozen in preparation for landing. The mood on board Columbia was one of solemnity, with only Fettman and Seddon working quietly at one stage in the Spacelab module. Even Blaha, who typically would always check up on his crew to see how they were doing and offer his help, only popped his head once or twice over Fettman's shoulder before returning to his other work.
The decapitated rats were part of a series of neurovestibular and musculoskeletal investigations, which examined changes in their gravity-sensing organs and the effect of microgravity exposure on their limb muscles and bones. It was already known that a person's awareness of his or her body orientation, on Earth, is partly due to the detection of gravity by the otolith organ deep within the inner ear. Combined with touch sensors in the skin and, of course, our eyes, this provides a mechanism to sense our relationship with other objects. In space, however, this balanced state is upset somewhat.
Information sent to the brain from the otolith and other sensory organs no longer corresponds to the cues experienced on Earth and some medical scientists have found links between this conflict and instances of disorientation or space motion sickness in astronauts. ''Gravity'', said Larry Young of Massachusetts Institute of Technology, ''is as profound a factor on the evolution and development of biology on Earth as oxygen and water. Yet we know so little about its influence because, until the Space Age, we simply couldn't get away from it!"
Research conducted during SLS-1 had led researchers to conclude that gravity sensors in the adult rats adapted to their new environment by changing the number, type and groups of 'synapses' (gaps between their nerve cells across which chemical transmitters propagate). The second mission attempted to uncover the precise nature of this adaptation, as well as structural changes within the rats' inner ears as a response to microgravity exposure. Investigators also used the rats who remained alive throughout the mission to measure the speed of readaptation to terrestrial gravity after Columbia's landing.
It was hoped that such experiments could have clinical applications for motion sickness sufferers or patients with vestibular disorders, which often lead to falls or bouts of dizziness. In addition to the studies of the rats, a series of joint US/ Canadian
studies, which were originally carried on Spacelab-1 as well as SLS-1, were reflown as part of investigations into space motion sickness and vestibular changes in humans. For SLS-2, the apparatus included a rotating chair mounted in the Spacelab module's centre aisle, which tested changes in the astronauts' reflexive eye motions.
Seddon, the head of the science crew and a veteran of SLS-1, was first to use the rotating chair on 21 October, as part of studies of the 'vestibulo-ocular reflex' in the eye, ''which is what allows us to see while we're moving'', according to co-investigator Daniel Merfield of Massachusetts Institute of Technology. ''Without this reflex, objects around us would appear blurry as they move.''
Lucid manually spun the chair, with Seddon in it, rapidly, stopped it suddenly and then asked her to pitch her head forward. When a person spins around on Earth, his or her eyes tend to move in the opposite direction to stabilise their 'picture' and then snap quickly ahead in a repeating pattern. After about 20 seconds of constant speed, however, both the eye motions and the sense of movement halt. Then, when the actual rotation ceases, the person's motion-sensing organs signal the brain that he or she is moving in the opposite direction and eye motions reflect that change.
However, if the person suddenly leans forward, as Seddon did, gravity sensors overrule the false perception of motion. The astronaut reported that she continued to feel a 'rolling' sensation after tilting her head forward. She would later tell colleagues that her ''sense of 'down' had completely gone away''.
Other experiments featured a rotating dome, which was placed over the astronaut's head; on the interior was a pattern of dots that appeared to 'rotate' in a direction opposite that of the moving dome itself. The astronaut then used a joystick to indicate their perceived direction and velocity of rotation. Crew members also viewed targets and pointed at them with their eyes closed as part of a study of differences in their perceived relationship between their body and the surroundings.
Seddon was first to try out the dome, on 20 October, and as it began to rotate, her eyes told her that she was spinning. The experiment carefully measured her perception of how quickly she was moving and what kind of eye, head and neck motions she exhibited in response to her perceived movement. ''The difference between the data on Earth and the data in space is absolutely amazing,'' Blaha told Mission Control after Fettman participated in one experiment using the dome.
On several occasions, starting just a few hours after launch on 18 October, Wolf donned a special skull 'cap' fitted with motion sensors. Known as the Acceleration Recording Unit, the device included a pocket tape recorder on which the astronaut reported the time and severity of any symptoms of space motion sickness. Investigators hoped that having crew members wear the skull cap throughout their working day might enable them to correlate instances of sickness with periods of provocative head movement. The science crew also performed vigorous exercise routines and head movements to assess their subjective levels of discomfort.
It was already known that the musculoskeletal systems of both humans and rats are not used as extensively in space as they are on Earth; changes in load-bearing tissues, caused by the absence of gravitational force, therefore lead to reductions in bone and muscle mass, as well as the obvious use of a 'floating', rather than 'walking', motion for locomotion. Furthermore, the system's metabolic state can be altered by dietary intake, exercise levels and space motion sickness. Each of these factors could potentially have a detrimental effect on an astronaut's physical fitness.
Human muscular atrophy in space is characterised by a loss of lean body mass, decreased muscle mass in the calves and decreased muscular strength. During SLS-2, the astronauts ingested amino acids labelled with a non-radioactive isotope of nitrogen which allowed them to track protein metabolism in their bodies. Urine, saliva and blood samples were then periodically taken to determine the rates of protein synthesis and catabolism. Other musculoskeletal experiments looked at the performance of the rats' hindlimbs in microgravity, which showed an almost 40% reduction of their muscle fibres after Columbia's two-week-long mission.
The rats tended to rely more heavily on their forelimbs for bipedal locomotion in space, using their hindlimbs only as grasping aids, and upon their return to Earth exhibited slow movement and an abnormally low body posture. All of these observed characteristics pointed to a weakened muscular state, fatigue and problems with bodily coordination. Similar muscular shrinkage and weakness was also anticipated in humans on long-duration missions on board the International Space Station or on trips to the Moon or Mars.
''It is not surprising that it takes astronauts a few days to recover their pre-flight strength and coordination after flight,'' said Kenneth Baldwin of the University of California at Irvine, ''since their muscles are remodelled by microgravity.'' Moreover, since muscle protein 'turnover' in rats is much more rapid than in humans, two weeks of microgravity exposure in them was roughly equivalent to two months in us, which was why they were essential to the study.
In addition to the neurovestibular and musculoskeletal investigations, SLS-2 experiments studied the cardiovascular and regulatory systems of the astronauts' bodies. Already, data from SLS-1 had pointed to increases in heart rate, size and output, which many medical researchers had attributed to the initial increase in central blood volume caused by fluid shifts within the body. Three SLS-2 studies assessed the functional capabilities of this system by monitoring the astronauts' cardiac output, heart rates, arterial and venous blood pressure, blood volume and the amount and distribution of blood and gases in their lungs.
Since the early days of manned spaceflight, cardiovascular 'deconditioning' had been frequently reported by astronauts upon their return to Earth. This was evidenced by a reduction of 'orthostatic tolerance' - light-headedness - and was frequently accompanied by an increased heart rate and decreased pulse pressure. Additionally, measurements of blood and body water highlighted fluid shifts towards the head, which in turn 'fooled' the body into thinking that too much fluid was present and resulted in a reduction of fluid volume. This shifting pattern then influenced cardiac output and arterial and venous pressures.
Moreover, many medical scientists believe that microgravity may affect lung functioning and on SLS-2 sought to investigate the effects of weightlessness on the pulmonary system and particularly on respiration, blood flow and gas exchange. During experiments on SLS-1, cardiac output from all seven astronauts stayed elevated, but total 'peripheral resistance' - the resistance of blood flow throughout their bodies - adapted fairly quickly. Central venous pressures produced unexpected results by decreasing, which refuted earlier hypotheses that microgravity-induced fluid shifts should cause it to increase. Despite this, the astronauts' heart sizes increased, as did overall cardiac output.
This finding could point to a general 'opening' of the blood vessels in orbit. In addition to general cardiovascular measurements, central venous pressures were measured using the catheters in Lucid and Fettman's arms. Echocardiograph data was also taken and leg cuffs measured blood flow and volume. Other experiments looked at the unexpected finding from SLS-1 that, far from being more even in space, lung ventilation actually improved only by about half as much as on Earth. The science crew inhaled oxygen, nitrogen and other trace gases, as part of tests to measure blood and gas motions in their pulmonary systems.
To do this, they used the Gas Analyser Mass Spectrometer (GAMS), which measured the composition of both their inhaled and exhaled air. As part of these cardiovascular investigations, the astronauts also employed a bicycle ergometer in the Spacelab aisle to exercise vigorously - so vigorously, in fact, that on 25 October the bike came away from its floor attachment point and had to be hurriedly fixed by Blaha and McArthur.
''One key issue is the ability to maintain blood pressure when standing up after spaceflight,'' said Alternative Payload Specialist Jay Buckey. ''Gravity will have a profound effect on them after they come back from space. Secondly, we want to look at the early adaptation in space. SLS-2 is changing our understanding about how the
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