Although previous Shuttle crews had been assisted by an RMS officer in Mission Control, it was decided that the new RMU would also supervise the hydraulic systems, payload bay doors, the troublesome APUs and also the upper stages that would help to boost two communications satellites into geosynchronous orbits on STS-5. Mattingly and Hartsfield's primary work with the robotic arm was the deployment and manoeuvring of the large IECM unit, which had not been demonstrated on the previous mission because of a failed television wrist camera.
The IECM itself was somewhat different on STS-4 in that it had been fitted with an extra instrument to monitor bursts from Columbia's forward RCS thrusters. In total, Mattingly and Hartsfield completed two successful deployment and berthing runs with the device, lasting around nine hours in total, and commented that the handling characteristics of the RMS were ''crisp and precise'' with the IECM in its grasp. They also evaluated a new berthing device in the payload bay, known as a
Retention Engagement Mechanism (REM), and acquired good images of the payload in their monitors on the aft flight deck.
Images of the payload bay during the flight were minimised to keep the Department of Defense's secret package under wraps. It seems that, in addition to the CIRRIS infrared detector, there were six other experiments on board Columbia, all of which were attached to a cross-bay 'bridge' known as the Experiment Support Structure (ESS). These were the Horizon Ultraviolet Programme (HUP), the Autonomous Navigation and Attitude Reference System (ANARS), the Shuttle Effects on Plasma in Space (SEPS), the Sheath and Wake Charging (SWC), a set of passive cosmic-ray-collection panels and a pallet alignment modelling experiment.
Each of these payloads apparently functioned autonomously and without incident throughout the flight, except for CIRRIS itself, which refused to work when its lens cap would not open. Mission controllers in Houston discussed the possibility of knocking the cap off with the RMS or sending Mattingly out into the payload bay on the first-ever Shuttle spacewalk to open it manually, but it was ultimately decided not to complicate what was, after all, a test flight. Payloads were considered a bonus, rather than a necessity, and CIRRIS would have to wait for another chance to display its ability in space.
Mattingly, on the other hand, was eager to make a spacewalk, having already performed one during his Apollo 16 mission 10 years previously. Owing to the curtailed STS-2 mission, for which he and Hartsfield had served as the backup crew, the new-specification Shuttle spacesuit had not yet been used in orbit. Before a spacewalk could be tried, the procedures and timeline for donning and doffing the suit needed to be fine-tuned. That honour was completed admirably by Mattingly on STS-4, although the only problem, he said, was that ''I didn't get to open the door''.
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