After nearly 7 years of service providing power to the International Space Station (ISS), the P6 solar photovoltaic power module (P6) was moved from its original location on the Z1 truss to its permanent location on the port outboard truss of the ISS. When the crew of the STS-120 mission redeployed the solar array in its new location in 2007, the array started to tear in two places as shown in Figure 10.1.1, forcing the crew to stop deployment at 90 percent of completion.
If this had been a robotic spacecraft, the only clues available to ground controllers would have been those provided by telemetry to guide their analysis of what had caused the failure, with the result perhaps being a conclusion that the failure was only mechanical, or a design flaw.
However, the STS-120 astronauts were able to cut away the frayed guidewire that caused the tear in the panels and bring it back to Earth for analysis. NASA scientists examined the frayed end of the guidewire with a scanning electron microscope, discovering damage to it in the form of melting that was characteristic of fusing from the impact of a meteoroid or orbital debris. Further examination using a narrow-focus electron microprobe and energy dispersive x-ray spectrometer to analyze the chemical elements in the impact zone resulted in the conclusion that the damage was caused by orbital debris, not a meteoroid.
Currently, most robotic spacecraft do not have systems sophisticated enough to provide the kind of detailed information necessary to say conclusively that an MMOD impact caused a failure of some part or system. However, clues that MMOD may be a cause are found by correlating a failure with data on the environment at the time (e.g., the spacecraft passed through a meteor shower or an orbit known for large amounts of debris), or an unexplained change in momentum. In this case, NASA’s astronauts, and scientists, were fortunate to retrieve a piece of hardware directly involved in damage to the spacecraft and to be able to thoroughly examine that hardware in the lab. Without it, they would not have had all of the pieces of the puzzle to solve this mystery.1
FIGURE 10.1.1 Two tears approximately 30 cm and 90 cm long in the P6 solar array wing 4B after an attempt to redeploy the array during the STS-120 mission. SOURCE: Courtesy of NASA, from D. Ross, E. Christiansen, and D. Lear, MMOD damage to the ISS solar array guidewire, Orbital Debris Quarterly News 15(1):4, January 2011.
1 D. Ross, E. Christiansen, and D. Lear, MMOD damage to the ISS solar array guidewire, Orbital Debris Quarterly News 15(1):4, January 2011.