Preparing For Space Station Operations

As USMP-1 operated autonomously in the payload bay, Columbia's astronauts were busy with a myriad of other experiments in the middeck. MacLean, for example, had his hands full throughout the mission with the second set of Canadian Experiments (CANEX-2) - the first set had been conducted by Payload Specialist Marc Garneau on STS-41G in October 1984 - to such an extent that Wetherbee asked Mission Control to give him some free time. ''Steve's doing a great job,'' he told them on 27 October. ''He wants to do it all. It's my call to offload him a little bit.''

CANEX-2 consisted of 10 space science, technology, materials processing and medical experiments, provided by the Canadian Space Agency (CSA). Of these, one of the most exciting as the construction of the International Space Station loomed on the horizon was the Space Vision System (SVS), which helped to improve astronauts' perception of large structures under unfavourable viewing conditions. It was already known that, in space, there are frequent periods of extreme darkness and lighting and very few reliable points of spatial reference. This had led astronauts to comment that it was difficult to accurately judge distances and speeds.

During STS-52, MacLean evaluated a prototype vision system for space station proximity operations and assembly tasks, which provided him with data on the precise positions, orientations and motions of small 'target' objects. It employed a television camera on Columbia, which monitored a series of dots on what looked like a giant, 2-m-long 'domino', known as the Canadian Target Assembly (CTA). As the object moved, the computer in the SVS measured the ever-changing positions of its dots, providing MacLean with a real-time display of their locations and orientations.

Preparing for space station operations 175

Preparing for space station operations 175

The domino-like Canadian Target Assembly (CTA) is manoeuvred by the RMS.

''If our tests are successful, we could perhaps speed up the assembly of the space station,'' the Canadian astronaut had told journalists before Columbia lifted off. ''What we have tried to do is mirror the assembly of the station. The controllability of the [RMS] arm is very good now, but the position knowledge of the end of the arm is only good within two inches. That makes it difficult to do many of the operations that are planned for the Space Station.'' Veach added that ''this could be as revolutionary as 'instrument flying' was to aviation''.

The comments from both men, in hindsight, were interesting; one was never to see the construction of the International Space Station, while the other will be on board one of its pivotal missions. Veach, sadly, would die of cancer within three years of returning from STS-52, while MacLean would join NASA's astronaut corps in April 1996 to train as a fully fledged Mission Specialist. At the time of writing, he is set to fly on STS-115, some time early in 2006, which will attach a massive girder and new set of electricity-generating solar arrays to the station.

During that mission, expected to last around 10 days, MacLean will operate 'Canadarm2' - the International Space Station's 'own' RMS - and use a descendant of the space vision system he first tested on STS-52 to very precisely pick up, manoeuvre and install the girder and arrays onto the outpost. All that, of course, was far in the future when Veach uncradled the RMS on 24 October 1992 and used it to grapple, move and replace the 82-kg CTA in Columbia's payload bay.

Similar tests were performed over the next few days and, on 29 October, Veach lifted the domino and rolled it from side-to-side, then up-and-down, while MacLean closely monitored its movements with the SVS. These evaluations demonstrated the clearance-measuring abilities of the system, which was expected to be particularly useful during Hubble Space Telescope repair missions or station assembly tasks. Things went smoothly until Veach tried to lower the CTA back onto its berthing plate in the payload bay, when he met resistance.

After inspecting the target's berthing latches, he successfully lowered it into place on the second attempt. NASA spokeswoman Barbara Schwartz said, however, that the incident would not prevent further SVS runs planned for the end of the mission. Indeed, on 30 October, Jernigan joined Veach and MacLean in a set of tests to judge the amount of 'flex' in the RMS, guiding the CTA through a series of very precise manoeuvres. Then, at 11:05 am the following day, Veach finally jettisoned the domino and Wetherbee flew Columbia in formation at a distance of 42.6 m.

MacLean, who watched the departing target with the SVS, described the unit's performance as ''excellent'' and accurately measured the increasing distance between the Shuttle and the domino. As it drifted away, destined finally to burn up in the atmosphere on 1 November, Wetherbee 'blasted' the CTA with Columbia's RCS thrusters.

Other CANEX-2 experiments supervised by MacLean included a high-temperature furnace called the Queens University Experiment in Liquid Metal Diffusion (QUELD), capable of reaching 900 Celsius, which examined the 'diffusion' of bismuth and tin into one another. Knowledge of the rates at which atoms of different substances move around and between each other - their 'diffusion rates' -had long been considered important for industrial purposes. Such rates are difficult to examine in ground-based experiments, because convection masks the actual degree of diffusion taking place, but in microgravity is was possible to obtain more precise data.

Another intriguing experiment, the Material Exposure in Low-Earth Orbit (MELEO), took the form of a 'quilt' of about 350 different samples attached to the RMS. These materials were then analysed after Columbia's landing to assess the adverse impact of atomic oxygen exposure. It was already known from earlier materials-exposure experiments flown on the Shuttle that plastics and composites could become degraded. Materials flown on STS-52, which were exposed to space on three extended occasions, were candidates for use on the space station's robotic arm and also Canada's Radarsat remote-sensing platform.

The crew also tended a variety of other experiments. A Canadian-provided Spectrophotometer measured the light-absorption characteristics of Earth's atmosphere, while MacLean took photographs of the Shuttle's tail and flight surfaces during thruster firings in an effort to better understand the mysterious atomic oxygen-induced 'glow' around spacecraft. Heat pipes were tested as part of efforts to develop lighter, more efficient thermal-control systems for future spin-stabilised

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