although the astronauts averaged 14-hour workdays during their time aloft. Their 'circadian rhythms' - sleep/wake cycles - were strictly followed in this way to provide for a more uniform set of biomedical experiment results from the whole crew. This mission was the first time that a Spacelab fully dedicated to medical research had been sent aloft, together with a team of medical professionals; as the launch commentator had remarked as Columbia speared for the heavens, it was ''the first dedicated medical research flight''.
SLS-1 consisted of 18 major experiments to investigate the fundamental problems affecting the biology of humans, animals and fish in microgravity. Ten of the experiments required human subjects, seven used rats and one used almost 2,500 jellyfish. During the mission, Columbia's crew explored how the heart, blood vessels, lungs, kidneys and hormone-secreting glands responded to a microgravity environment, studied the causes of space sickness in greater depth and examined minute changes in muscles and bones while in orbit.
Since this mission would include the most detailed medical studies since Skylab, scientists were particularly interested in the astronauts' physiological responses during their first few hours in microgravity. In support of these objectives, the science crew - Bagian, Seddon, Gaffney and Hughes-Fulford - underwent an extensive battery of demanding medical tests both before launch and after landing. They went through a full 24 hours of 'head-down' bedrest to evaluate whether or not this ground-based analogue to spaceflight produced similar cardiovascular responses and a series of vestibular tests were also conducted to assess their sensitivity to linear acceleration.
The mission also had a pronounced international flavour, with researchers from France, Russia, Germany and Canada participating in a biospecimen-sharing programme. This kind of cooperation would feature with increasing prominence in subsequent Spacelab missions, helping to lay the groundwork for today's International Space Station. Like ASTRO-1, the SLS project got underway in 1978 when NASA issued an Announcement of Opportunity for medical and biological experiments to fly on future Spacelab missions. Testing of SLS equipment began on Spacelab-3 in April 1985 and highlighted several operational flaws, notably contamination, leaks and odour problems from animal-housing facilities in the Spacelab module.
Originally assigned to fly on STS-61D some time in early 1986, by the time of the Challenger disaster SLS-1 had slipped until at least the spring of 1987. The almost-three-year post-Challenger downtime provided SLS-1 investigators with ample opportunity to modify their experiments and facilities, in particular the Research Animal Holding Facility (RAHF), which would accommodate the rats. It was, therefore, five years overdue and 13 years after the initial plans were laid, when Jim Bagian finally entered the Spacelab module on the afternoon of 5 June 1991, turned on the lights and began activating the experiments for nine days of intensive research.
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