Scientists solve the mystery of cloudy filters on Sun-facing satellites

Scientists have finally figured out why solar filters on some Sun-facing satellites become mysteriously obstructed over time. A team from the National Institute of Standards and Technology (NIST) and the Laboratory for Atmospheric and Space Physics (LASP) have used experiments and modelling to explain why tiny, thin metal membranes that filter sunlight as it enters detectors that monitor the Sun's UV rays quickly lose their transmission abilities. These detectors give advance warning of impending solar storms that could reach Earth and temporarily disrupt communications or interfere with navigation systems.

Last year, the NIST and LASP team disproved a leading theory that carbonization is the cause of filter degradation. Now the same team has proved that water is responsible for the filter degradation.

They have also identified the source of water - thermal blankets, which are used to control the temperature of instruments on a spacecraft. The researchers believe that oxidation caused by water, which together with UV light from the Sun is producing a thick layer of aluminum oxide, much thicker than previously thought possible, that blocks incoming rays.

The researchers tested their water theory in NIST’s Synchrotron Ultraviolet Radiation Facility (SURF), a machine that effectively creates space weather. Despite being subjected to lab-made UV light for 20 days, their sample filters did not develop oxide layers of sufficient thickness to explain the cloudiness observed in real space filters. However, the oxide layers were significantly thicker than what the accepted theory predicted. The researchers speculate that additional exposure to UV light would have resulted in the required thickness.

The researchers estimate that it would take approximately 10 months of exposure to the SURF beam for the sample filters to attain the same oxide thickness as those present in actual space filters.

The team also conducted modelling studies. The new NIST models predicted an oxide growth that matches exactly the real-life oxide growth seen on the aluminum filters on two Sun-facing detectors on NASA's Solar Dynamics Observatory (SDO).

"Proving that water is responsible for the problem "was sort of a one-two punch." Punch one was physically showing that this chemical process involving water could cause something comparable to what we actually see happening in the satellites. And the number two punch is saying once you create a theoretical model that takes everything into account, then the numbers line up quantitatively with what we see in the satellites," said NIST physicist Robert Berg.

More details can be found here.