On the next mission in April 1984, Challenger set off to repair SolarMax, which, in November 1980, a few months after its launch, had suffered a power spike that blew the fuses of its attitude control system. The satellite was to supplement the series of Orbiting Solar Observatory satellites by using seven instruments to monitor the Sun through the most active part of its 11-year cycle of sunspots.14 After the failure, the satellite had been put into a spin to enable the few instruments that did not need a steady view to undertake a restricted programme. If SolarMax could be repaired, it would hopefully survive long enough to follow the Sun through its minimum and the run up to the next maximum, and so carry out its original mission. In addition to restoring the attitude control system, the astronauts were to try to repair an instrument that had never met its specification. The rendezvous required Challenger to climb to 560 kilometres, which was much higher than the Shuttle had previously ventured. By the time the orbiter assumed station 100 metres away, the Goddard Space Flight Center had powered down most of SolarMax's systems, and almost cancelled its spin. Its fate rested with George Nelson and James van Hoften. As all the aspects of the repair that could be tested in space had been rehearsed on earlier missions, the recovery was expected to be straightforward. Nelson donned the MMU 'flying backpack' and collected a TPAD, which was a device built to mate with a trunnion pin on the satellite. At the base of SolarMax was the spacecraft bus, which was generic to several satellites. Projecting from each side of the interface ring to the science package was a pair of broad solar panels. As the trunnion pin was on the bus, Nelson had to station himself below the level of the solar panels, which passed over his head as the satellite slowly turned on its main axis at the rate of about one revolution per minute. When the trunnion pin came into view Nelson advanced to slide the collar of the TPAD over it, but when he activated the mechanism it failed to engage the pin.15 He tried again, imparting more force, but again without result, and this time he caused the satellite to nutate. In frustration, Nelson withdrew several metres to consider what to do. Almost as an afterthought, he reached and grabbed hold of the tip of one of the solar panels as it drifted by - if he could stabilise the satellite, Challenger's robotic manipulator might be able to grasp the pin to capture it directly. However, before the Goddard engineers could warn him off he had turned SolarMax's axial spin into a gyration that prevented it from holding its solar panels facing the Sun. Even with most of its systems powered down, it would soon exhaust its battery. Nelson's efforts to overcome the satellite's tumbling had made matters worse. As Challenger manoeuvred alongside, the efflux from its thrusters further upset the satellite. With his MMU running low on nitrogen propellant, Nelson beat a retreat. With shocking suddenness, it seemed that the satellite that he had set out to rescue had been written off! The Shuttle manoeuvred clear while the Goddard engineers endeavoured to restabilise the satellite and recharge its battery - it was a close call, but they succeeded.16
Challenger returned two days later. This time it was decided to attempt to snatch the satellite with the robotic arm, which was achieved without incident. To enable SolarMax to draw power from an umbilical, it was put on a structure at the rear of the bay that had been built to enable the Shuttle to deploy satellites using the same type of multipurpose bus, and had a U-shaped cradle with a tilt-table. If it proved impossible to fix SolarMax, it was to be returned to Earth in this cradle. The following day, van Hoften affixed a foot restraint to the arm and mounted it, and
Nelson tagged along when Terry Hart swung the arm to the rear of the bay. Nelson inserted another foot restraint into a socket to provide a fixed work platform. The tilt-table was rotated to enable the astronauts to access the satellite's attitude control system. The bus comprised three boxes about 1 metre square, set 120 degrees apart. The box with the electronics of the attitude control system was to be replaced. Fortunately, SolarMax was one of the first satellites designed to be serviced in space, and unbolting the box and fitting the replacement proved straightforward. This accomplished the primary objective by restoring the satellite's ability to aim its instruments at the Sun. If they had been running late, the astronauts might have stopped at this point, but as they were ahead of schedule they set out to repair the faulty coronagraph. This would be a more difficult task, because the instrument would have to be taken apart and repaired. First, however, they had to gain access. The satellite was rotated to present its instrument section, a heavy-duty cutter was used to slice through the thermal insulation, and the loose flaps were folded back and held in place by duct tape. Six screws had to be undone to release the hatch beneath. As the screws could damage the instrument, these had to be retrieved, which was an awkward procedure for gloved hands. Since it would not be practical to reuse the screws, another strip of tape was applied to serve as a hinge. The astronauts were making good progress, which was just as well because they had to undo a total of 22 screws to disconnect the instrument's controller! The task of wiring up the new controller fell to Nelson. Fast-action spring clips were used to re-establish the 11 circuits. This done, the hatch was closed, and the insulation was flattened and taped into position. Although this tape would alter the thermal properties of the instrument, the tape would not face the Sun when the spacecraft was operating. Having completed both their primary and secondary tasks, van Hoften and Nelson retreated to the airlock. When SolarMax was released the following day, the coronagraph proved to be fully functional.
A fortnight later, despite the Sun being well past its peak, SolarMax documented the largest flare since 1978. As the solar cycle neared its new peak it inflated the upper layer of the Earth's atmosphere, increasing the rate of decay of SolarMax's orbit. A propellant replenishment was tentatively assigned as a secondary task on a Shuttle mission in 1990, but the satellite re-entered in December 1989, having kept watch on the Sun to the end.
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