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Although not intended to be manoeuvred by the RMS on STS-2, the IECM was carried in the payload bay attached to the DFI pallet. Its task was to help scientists to better understand the effect of the Shuttle on its local environment, as part of continuing efforts to assess its atomic 'cleanliness' before committing sensitive telescopes and detectors to future missions. IECM was carried on the last three test flights and the first two Spacelab missions, and was either fixed in the payload bay or moved around using the RMS. Its STS-2 data revealed contamination levels to be ''within expected limits''.

Promisingly for future flights, it confirmed that exhaust byproducts from the main engines had not leaked into the payload bay - demonstrating the integrity of the door seals - and revealed that the vast majority (more than 90%) of particulate contaminants tended to 'boil off into space within the first day-and-a-half of the mission. In general, the payload bay environment was characterised as remarkably clean and debris-free. It did, however, show that after RCS thruster firings, small, short-lived 'clouds' of particles formed and hung over the bay.

In the meantime, the experiments in the payload bay were performing extremely well. SIR was in the process of gathering eight hours' worth of radar data, acquiring images with resolutions of just 40 metres of North America, southern Asia and Europe, Australia and the Pacific islands, North Africa and the northern part of South America. It enabled geologists to determine surface 'roughness' and picked out faults, drainage patterns and evidence of stratification, as well as making a truly unexpected discovery: ancient watercourses beneath the arid sands of Egypt, just as they might have appeared thousands of years ago.

Such data offered a promising preview of what might be achieved on operational research missions. The other instruments were also proving their worth. As Engle and Truly took photographs through Columbia's flight deck windows, SMIRR determined the best spectral resolution needed to identify and map rock or mineral deposits, while FILE evaluated new techniques to automatically classify surface features such as water, vegetation, bare land and snow, clouds or ice to better prioritise the timing of future Earth-resources missions. It complemented SIR and SMIRR by providing a means for them to be activated only when conditions were 'right' for data-acquisition.

Another experiment, MAPS, provided the first accurate indication of how severe the levels of atmospheric carbon monoxide really are. It surveyed the lower atmosphere - from the surface to an altitude of about 18 km - and began a series of missions that identified this uncomfortable trend. MAPS was flown again in 1984 and on two Space Radar Laboratory missions in 1994, revealing high pollution levels, particularly in the tropics, caused by seasonal biomass burning. Lastly, OCE evaluated a technique to map the colour patterns of plankton and chlorophyll as part of efforts to better identify schools of fish.

The last two OSTA-1 experiments were housed not on the pallet, but in Columbia's cabin. They were the Heflex Bioengineering Test (HBT) and the Night/ Day Optical Survey of Lightning (NOSL). The first investigated the effect of microgravity and soil composition on the growth of the dwarf sunflower (Helianthus annuus) to study the relationship between its height and moisture content. It was a precursor of an experiment slated for the first fully fledged Spacelab mission, planned for late 1983, which would analyse the sunflowers in greater detail. The STS-2 test, however, achieved only partial success because of the shortened mission.

NOSL was also affected by the halving of the flight. It required Engle and Truly to take daytime and nighttime photographs of lightning flashes over land and water, in the hope that it might lead to the development of new systems to give early warnings of particularly severe storms. The astronauts removed the hardware from a middeck locker soon after reaching orbit, assembled it and successfully acquired nighttime images and motion-picture sequences of six large daytime thunderstorm systems. In recognition of the 'lost' data from STS-2, more tests of both HBT and NOSL were planned for STS-3.

After a jam-packed two days in space, on 14 November Engle and Truly began preparing their ship for the return to Earth. Columbia's re-entry profile as she hurtled through the atmosphere differed significantly to pre-flight plans, due to the shortened mission. The RCS thrusters at the rear of the spacecraft were commanded to fire over 1,000 times, consuming 815 kg of propellant - far more than planned - because the Shuttle's fuel-consumption rate at the end of two days in space differed from pre-flight estimates, which were based on the demands expected of a five-day mission.

Shortly before hitting the uppermost traces of the atmosphere, a large quantity of propellant was dumped out of the forward RCS unit to provide precise control of Columbia's centre-of-gravity during the descent. A series of flight tests were conducted during re-entry, the most important of which was a so-called 'push-over/ pull-up' (POPU) exercise performed by Truly. As the Shuttle plummeted Earthwards at 22,500 km/h, he pushed her nose down from a 40-degree angle-of-attack to 35 degrees, lifted it to 45 degrees then returned it to 40 degrees. This provided extra data on the vehicle's aerodynamic performance during re-entry.

Engle took manual control of Columbia at an altitude of 94 m, pointing the Shuttle into a 37 km/h crosswind and Truly deployed the landing gear 18 seconds before touchdown. The first used spacecraft landed safely on Edwards' Runway 23 at 9:23:12 pm, wrapping up ajourney that lasted just seven minutes short of STS-1. Although nosewheel steering was not yet available, differential braking was applied to maintain a straight course down the runway. Engle would later say that a fluctuating indicator in the cockpit made it difficult for him to maintain a constant deceleration rate.

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