(The following narrative is taken from "Mission Report: Repairing Solar Max," NASA, MR-41-C.)

REPAIRING SOLAR MAX

Chasing Solar Max

With LDEF successfully deployed, the crew turned their attention to catching up with the Solar Max satellite, hundreds of miles ahead and several miles higher in its orbit.

Solar Max was launched in February 1980, at a time of maximum activity in the Sun's 11-year sunspot cycle, to study solar flares over a wide range of wavelengths, from visible light to gamma rays. After ten months of productive science, however, three fuses in the satellite's attitude control box blew, leaving it unable to point itself accurately. This rendered four of its seven on-board instruments useless. Another instrument, the Coronagraph/Polarimeter, subsequently developed electrical problems and also failed.

Engineers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, kept Solar Max "alive" by placing it in a slow, stable spin of one degree per second with its wing-like solar panels constantly pointing at the Sun to keep the satellite's batteries charged. They accomplished this by using the spacecraft's backup attitude control system - magnetic torquer bars that stabilize the satellite by applying leverage against the Earth's own magnetic field.

Solar Max had been drifting in this way for more than three years before Challenger's flight. Because the satellite was the first to use a modular design that allows on-orbit replacement of parts, planning for a rescue mission began almost immediately after the satellite failed.

First Grapple Attempts Fail

On the morning of the mission's third day, Challenger made its final rendezvous approach to Solar Max, now orbiting at an altitude of 500 kilometers (268 nautical miles). With the orbiter "parked" about 200 feet away from the slowly turning satellite, Mission Specialists Nelson and van Hoften donned spacesuits and went out into the cargo bay.

The plan was for Nelson to fly untethered over to the Solar Max using a jet-powered Manned Maneuvering Unit (MMU). He was to stop the satellite from turning by docking himself to a pin protruding from Solar Max's midsection with a special Trunnion Pin Acquisition Device (TPAD) fixed to the arms of his MMU. The orbiter would then move in closer, pick up Solar Max with the manipulator arm, and place it in the cargo bay.

Nelson set out on his 10-minute commute to Solar Max on schedule, and was able to match the rotation of the satellite by firing his MMU jets as planned. When he tried to dock the TPAD to Solar Max's protruding trunnion pin, however, it did not clamp onto the pin as it was supposed to. Nelson backed away and tried moving in again, a bit faster this time, but the jaws inside the TPAD cylinder still did not trigger closed. He tried a third time, faster still, with no success.

At this point, Nelson and the Challenger crew could see that the docking attempts had begun to jostle the satellite out of its previous slow spin. Commander Crippen, watching from the orbiter, suggested that Nelson try to steady the satellite by holding onto one of its solar panels, but this made matters even worse. With nitrogen fuel in the MMU backpack running low, Crippen decided that Nelson should return to the orbiter.

Rescue Within a Rescue

The Solar Max, which up until this time had been in an orderly "coning" spin like a very slow top, was now tumbling unpredictably around all three of its axes. After Nelson had returned to Challenger, the crew tried four times to snare the satellite's grapple fixture with the manipulator arm, but had no success. Crippen then made the decision to back the orbiter away from Solar Max while the crew and Mission Control planned a new strategy.

The action now shifted to the Goddard Space Flight Center, Solar Max's command station, where engineers struggled to slow down the satellite's rotation rates so that the crew could try another grapple. The immediate problem, though, was power‹the solar panels were no longer pointing toward the Sun, meaning that battery power was draining without recharging. Goddard engineers turned off all the satellite systems they could, including the heaters on the scientific instruments, but the batteries still had only six to eight hours of lifetime left. The race to save Solar Max was on.

The first goal was to stop the satellite from tumbling. When it became apparent that Solar Max's magnetic torquer bars were not slowing the different motions (because on-board gyroscopes were overloaded by the high rotation rates) Goddard engineers devised a new plan. By telemetry command, they replaced the satellite's computer memory with another program that used a different method of sensing the spacecraft's position. This new program made the torquer bars more effective in pushing off against the Earth's magnetic field, and Solar Max immediately began to stabilize itself.

The satellite was now holding almost perfectly still, but its solar panels were still turned away from the Sun, and this particular computer program did not allow ground control to change its attitude. Luckily, just as time was running out on Solar Max's batteries, the spacecraft came around its orbit in such a way that the solar panels were facing sunlight, and the batteries began to recharge.

By the next morning the batteries were up to full power, the original attitude control program had been reloaded into Solar Max's computers, and the satellite was again facing the Sun and turning at an orderly one half-degree per second. Mission Control in Houston, meanwhile, had formulated a different plan for retrieving the newly revived spacecraft. Rather than have Nelson attempt another docking (orbiter fuel levels were now below the amount required to ensure a rescue if his MMU were to fail), Crippen would fly Challenger close enough for Hart to grab onto Solar Max's grapple fixture with the manipulator arm. The satellite was now turning at only half its original rate, slow enough for the arm to grab.

During the night Challenger had backed away from Solar Max, so Crippen and Scobee began a series of maneuvers in preparation for a re-rendezvous the following morning. Van Hoften and Nelson, meanwhile, used this fourth day of the mission to tend to the student bee experiment (the bees had begun to build a normal honeycomb after some early disorientation) and to continue their IMAX and Cinema-360 documentary photography. The footage shot on this and other missions will be assembled later into two feature films about the Space Shuttle program.

"We Got It"

Early on flight day five, Challenger successfully grabbed onto Solar Max and hauled it safely into the cargo bay. Crippen maneuvered slowly under the satellite so that Terry Hart could extend the manipulator arm upward and grab onto the slowly turning grapple fixture. Just at the critical moment, the orbiter passed out of communication range, leaving those on the ground in suspense. Minutes later, though, the word came from Crippen: "We got it," he said, and on the first try.

Solar Max was now securely attached to the end of the arm, and Hart gently set it down into the Flight Support System cradle. Once the satellite was latched onto the cradle, Hart let go with the arm. An umbilical line was connected to feed orbiter power to the satellite, and Solar Max was pivoted around so that Van Hoften and Nelson could more easily reach the electronics to be repaired.

A Quick Fix

Solar Max remained cradled throughout that day and night while engineers at Goddard performed a health check to verify that it had survived its ordeal intact. Flight day six was devoted to the mission's main goals - replacing the satellite's faulty Attitude Control System (ACS) and the main electronics box for the disabled Coronagraph/Polarimeter instrument. These repairs were originally scheduled for two separate days, but were condensed into one when the flight plan was restructured.

Nelson and van Hoften again donned spacesuits for their second trip into the cargo bay, and immediately set to work. The main task, changing out the attitude control box, took about 45 minutes. Van Hoften, standing on a work platform fixed to the manipulator arm, removed the old box by taking out two screws and plugged in the replacement unit that had been stored on the FSS station.

Replacing the Coronagraph/Polarimeter's electronics box was expected to take rnuch longer, since this instrument was not designed to be repaired in orbit and the task required that van Hoften pull back a panel covering the box, cut and tape back a layer of insulation, then unscrew some two dozen screws and cut several wires, all while wearing bulky spacesuit gloves. He was able to remove the damaged box much quicker than expected, however. Nelson then installed and reconnected the new electronics box using clips instead of screws for the electrical connectors. The entire operation took less than an hour. The crew finished their Solar Max repairs by placing a baffle cover over the X-ray Polychromator to vent its exhaust gas away from the other Solar Max instruments.

With the servicing chores completed ahead of schedule, van Hoften was given the go-ahead to fly engineering tests of the Manned Maneuvering Unit inside the cargo bay before he and Nelson returned inside Challenger.

A New Life for Solar Max

The rest of that day and night were used for checkouts of the satellite. Terry Hart picked up Solar Max with the arm and held it up and away from the orbiter while engineers verified that the new attitude control system and electronics box worked perfectly. When these checkouts were complete, Solar Max was released by the arm to begin its "second life" in orbit.

Following a thirty day operational checkout period, the satellite will resume its study of the Sun for another two years or more. Among its most important tasks will be to monitor the solar constant, or total energy output of the Sun. Solar Max discovered that this energy output was declining during the time it observed the Sun in 1980, and if it continues, this downtrend could have an effect on the Earth's climate.

California Landing

Their mission complete, Challenger's crew returned to Earth on Friday, April 13, one day later than originally planned. The shuttle was scheduled to land at the Kennedy Space Center in Florida, but bad weather at the prime landing site forced a change to Edwards Air Force Base in California.

On Challenger's 107th revolution around the Earth, the Orbital Maneuvering System engines were fired to drop it out of orbit. Just over an hour later, at 5:38 a.m. PST, Challenger landed on Runway 17 at Edwards, bringing to a close history's first on-orbit satellite repair mission .