Luckily, it was on the second day of the mission, when Fullerton again felt ''great'', that he uncradled the RMS for the first of what would turn out to be a marathon 48 hours of tests. Soon after 3:00 pm on 23 March, he flexed the arm's robotic muscles, before returning it to its berth on the left-hand sill of the payload bay four hours later. One problem that cropped up during these tests was the failure of the wrist-mounted television camera which, crucially, would enable Fullerton to view the grapple fixture on the IECM and pick it up satisfactorily.
The IECM tests were a vitally important part of the mission because of the unit's relatively large size and weight of 385 kg, which would help to demonstrate for the first time the handling and manoeuvring capabilities of the RMS, before the Shuttle could be dedicated to carrying out more ambitious satellite deployments and retrievals. Already, a risky recovery and repair of NASA's Solar Max spacecraft, which would involve a pair of lengthy spacewalks and intensive RMS usage, had been provisionally pencilled into the Shuttle's calendar for 1984.
With the wrist camera effectively out of action, it was feared that faults in other cameras on the arm or in the payload bay itself would not provide the astronauts with acceptable views of the IECM on their monitors on the aft flight deck. This, in turn, might then have prevented them from safely reberthing it onto the DFI pallet after the tests, perhaps forcing them to discard the valuable payload. Nervous NASA managers decided, therefore, to defer the IECM deployment until the fourth and final Shuttle test flight, STS-4, in July 1982.
Instead, the IECM was substituted on this occasion for the smaller PDP, although the latter was less than half the mass. This obviously meant that the handling characteristics of the RMS with the PDP in its grasp would differ quite significantly from carrying the bigger IECM, but, in the event, the tests proved satisfactory. Original plans called for Fullerton to unberth the PDP for a series of eight-hour tests on both 24 and 26 March, allowing it to examine the electromagnetic and particle environment within a range of about 14 metres from Columbia.
In the event, the astronauts completed three PDP deployments and a good day-and-a-half of additional information was gathered while the canister remained attached to the OSS-1 pallet. Its data provided, for the first time, detailed insights into the strange ionospheric plasma 'wake' generated as the Shuttle passed, boatlike, through the electromagnetic environment of low-Earth orbit. This wake might, it was theorised, complicate the measurements of very sensitive scientific instruments and the STS-3 results proved beneficial for the planning and development of state-of-the-art space plasma detectors due to be carried on the Spacelab-2 mission.
The PDP was also used in conjunction with several other experiments affixed to the OSS-1 pallet. One of these was the Vehicle Charging and Potential (VCAP), provided by Utah State University to examine Columbia's electrical characteristics and her effect on surrounding ionospheric plasma. The experiment comprised a fastpulse 'gun', which fired 100-volt bursts of electrons for durations ranging from 500 nanoseconds to several minutes. It investigated the extent to which electrical charges accumulated on the Shuttle's insulated surfaces and how 'return currents' could be established through a limited area of surface-conducting materials to neutralise active electron emissions.
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