In 1997 the Mir Core Module had been in orbit since 1986, for a total of 11 years, 6 years longer than its design life. The United States and the Russian Federation were in the middle of an agreement to create the International Space Station. The first part of this agreement let NASA astronauts to live and work on the Mir space station alongside Russian cosmonauts. Examining the effects of the space environment on Mir was essential in planning for the construction of the ISS. To this end, NASA and ROSCOSMOS cooperated to bring back a piece of one of Mir’s solar arrays to see how it had fared in the environment of low-Earth Orbit. The array selected was one brought to Mir by the Kvant-1 module in April 1987 and installed via spacewalk on June 12 and June 16 by Yuri Romanenko and Aleksandr Laveykin. The array was 10.6 m long and had a total area of 24 m². It generated 2 kW of power, with an efficiency of about 12%—that is, 12% of incident light was converted into electricity.
But after ten years in space the efficiency of the solar array had dropped to nearly 5%. Since this type of array was one the Russians were planning to use on their ISS modules, NASA and ROSCOSMOS agreed to bring the array back to Earth and examine it. The EVA to retrieve the array took place on November 3, 1997, and was conducted by cosmonauts Anatoliy Solovyov and Pavel Vinogradov. The array was stored in the Core Module until the Space Shuttle could return it to Earth.
Space Shuttle Endeavour arrived on January 24, 1998, and took possession of the array. It also traded other supplies and transferred astronaut Andrew Thomas and returned home David Wolf, who had been aboard Mir since September 1997. Endeavour undocked on January 29 and landed on January 31. The solar array, which had been placed in a protective bag during its time in storage, was quickly removed after Endeavour entered the Orbiter Processing Bay and then taken to the clean room in the Spacehab Laboratory at KSC for examination.
The visual examinations of the array came first. Engineers saw a translucent white film spread in splotches across the array on both sides. Organic residue was also present in blotches around the solar panels. The white-painted hand rails on the sides of the array were unevenly discolored into shades of tan and brown.
After the initial visual and microscopic investigation, Russian authorities left one of the eight panels with NASA and returned the rest to RSC Energia for testing by Russian engineers.
The Russian team also provided NASA with information on how the arrays were constructed, and several unflown solar cells of the same vintage. The solar panel given to NASA was transferred to NASA Glenn Research Center. At the time, Russian solar arrays were built in layers of glass cloth, solar cells, optical solar reflectors, and a sheet of cover glass, which are held together by silicone adhesive. A backside cloth covers and protects the optical solar reflectors and is coated with an organic adhesive designated BF-4. Organic threads penetrate all the layers, binding the array together and providing the structural rigidity necessary to withstand the stress of launch, and to operate in the vacuum of space. NASA engineers set out to determine the cause of the drop in efficiency of the solar array and what might be done to prevent arrays on the International Space Station from following the same path. Determining the cause of the discoloration on the formerly-white handrails was simple. Chemical analysis of the paint revealed it was probably a standard Russian paint designated AK-573, and used zinc oxide as a pigment with silicone and acrylic binders. These rails were painted white to keep them from retaining too much heat and possibly damaging the array or the rails themselves. During their exposure to space, the rails solar absorptance had gone from 0.294 to 0.528, increasing by 80%. NASA engineers quickly concluded that the damage to the handrails was due to exposure to ultraviolet radiation from the Sun and exposure to atomic oxygen from the very upper reaches of Earth’s atmosphere. They arrived at this conclusion based on their experience with the Long Duration Exposure Facility, a satellite with various materials exposed to space that was left in orbit by the Space Shuttle for a year, and was then returned. Low-Earth orbit is above most of the atmosphere, but there are still high-energy atoms and particles that occupy that region. Atomic oxygen (as opposed to the molecular oxygen normally encountered on Earth) is extremely corrosive, especially at high energies. But determining the cause of the solar cell degradation was not as simple. First, engineers tested the transmitivity of the cover glass and found it to be at least 90%. So that was not the problem. Then they examined the white contamination, which they found to be largely composed of silicate. The white material did not significantly obstruct the passage of light, nor did it reflect very much. That wasn’t it either.
In order to test the properties of the solar cells themselves, they were removed from the array and taken to the Spectrolab Large Area Pulsed Solar Simulator. There, they were compared with unflown copies of the same cells. The experiments showed there was little degradation in the efficiencies of the solar cells themselves. The contaminated cells showed only a loss of about 0.58% efficiency, compared to the pristine cells provided by the Russians. So contamination by the white film definitely wasn’t the cause. But strangely, when the entire array was exposed to the LAPSS, it confirmed the efficiency loss reported by the Russians, and now had an efficiency of only 4.8%, when the Russians estimated an original efficiency of 12%. The ultimate conclusion of the NASA engineers was that the film and other contaminates had formed when the organic and inorganic adhesives had been exposed to direct sunlight and atomic oxygen in orbit. This made the solar array appear “dirty,” but by itself did not impact performance. The solar array had lost 58% percent of its efficiency during its 10 years in orbit. NASA only believed 5% was due to contamination by the film. Rather, NASA thought the remaining loss came from “thermal hot spotting,” electrical arcing, and orbital debris/micrometeorites and, in particular, high energy particles from the Sun and outer space.
Considering that Zarya and Zvezda have been in space since 1998 and 2000 respectively, it’s an interesting question as to how much longer Zvezda’s solar arrays will hold out. Zarya’s have been retracted to make way for the radiator extending from the Integrated Truss Structure, leaving them inoperable.