Attack of Atomic Oxygen: Molecule Slithers into the Spotlight
Decades ago, NASA engineers figured out how to prevent a highly pervasive molecule in low-Earth orbit from destroying materials commonly used on the external surfaces of spacecraft and components. Imagine their surprise when scientists discovered signs of degradation deep within the Hubble Space Telescope.
“We thought we understood the problem,” said Nancy Carosso, chief engineer of Goddard’s Contamination and Coatings Engineering Branch. She and her colleagues now have found evidence that the internal surfaces of Hubble’s Corrective Optics Space Telescope Axial Replacement (COSTAR) instrument had degraded during its many years in space. COSTAR was returned home after astronauts replaced it in 2009 with the Cosmic Origins Spectrograph (COS).
Engineers Nancy Carosso and David Hughes are pictured here with spacecraft components that had been damaged due to exposure to atomic oxygen.
The culprit? Based on ground tests and other sleuthing, the degradation likely was caused by low-velocity atomic oxygen molecules finding their way inside the instrument and ricocheting off the surfaces, Carosso said.
COS Problems Triggered Inquiry
Scientists discovered the COSTAR problem by accident during an investigation into why COS was showing signs of possible degradation. Although COS’s science capabilities are in no immediate danger, the sensitivity of the instrument’s windowless cesium-iodide detectors appears to be degrading five percent or more per year.
As part of the investigation, which is still continuing, engineers with the Contamination and Coatings Engineering Branch asked the Smithsonian Institution — now COSTAR’s official guardian — for permission to obtain a sample of COSTAR’s ribbon cable. Though engineers couldn’t see any visible damage to the cable, a look under the microscope and chemical analysis told a different story. The sample was pitted — a telltale signature of exposure to atomic oxygen.
Even if atomic oxygen isn’t causing COS’s degradation, the presence of atomic oxygen inside Hubble is a warning to other instrument designers, said David Hughes, a senior engineer who examined COSTAR. Developers of the Air Force’s Special Sensor Ultraviolet Limb Imager, for example, also have noticed degradation of their instrument’s cesium-iodide detector. “We might be the first people to have thought that atomic oxygen is to blame, but we’re not the first to have experienced the molecule’s ill effects,” Hughes said.
Been There, Done That
This isn’t the first time engineers have waged war against the corrosive effects of atomic oxygen.
Engineers first became aware of its harmful effects in the 1980s. Materials exposed to the molecule while the Space Shuttle orbited the Earth at high velocities — particularly those enshrouding payloads stashed in the opened cargo bay — showed obvious signs of serious wear. The high-velocity impacts etched or “frosted” Kapton, a commonly used film used as a thermal insulator on instruments. Components made of silver Teflon returned home ripped. Some coatings and materials even oxidized to the point where they had disappeared.
To understand what had caused the damage, NASA engineers carried out flight experiments on various Space Shuttle missions and the International Space Station. They developed predictive models. “We learned from the Shuttle experience. We learned what materials to use and what materials not to use on the outer surfaces of spacecraft and instruments operating in low-Earth orbit,” Carosso said. “We thought we had the problem licked.”
Based on her team’s analysis, however, that doesn’t appear to be the case.
More Analysis Proposed
To avoid future degradation of instrument components, Carosso and Hughes have proposed a multi-faceted research effort to find out just how the molecule slithers its way into instruments. They also plan to establish a laboratory where they would test different detector materials to determine which are immune to degradation.
The information is critical to instrument designers, Hughes added. Atomic oxygen, already plentiful in low-Earth orbit, increases in relation to solar activity. Unfortunately for potentially vulnerable instruments, the Sun is entering its solar maximum, the cyclical period of greatest solar activity.
“We never suspected that atomic oxygen could get inside a spacecraft,” Carosso explained. “We thought we understood the problem for high-energy atomic oxygen ruining external surfaces; now we see evidence of internal surfaces being degraded. So now, here we go again,” Carosso said. “We’re the ones building hardware. We need to thoroughly understand what’s going on here.”
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