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Why our solar-storm sats corrode – and probably not what you expected

My dear, I do believe I have the vapors ... in spaaace

Spacecraft monitoring the Sun for potentially deadly solar storms have been degrading – and now scientists think they've worked out why.

Boffins at America's National Institute of Standards and Technology (NIST) and the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics (LASP) have spent years trying to figure out why the UV filters in these probes degrade over time.

Filters allow spacecraft to analyze the sunlight's light spectrum across specific wavelengths. Equipment used to direct UV light from 100 nanometres to 10 nanometres to detectors tends to deteriorate. Within a few years of being in space, the filters on some satellites sent to study solar flares or coronal mass ejections filters degrade significantly. 

The only satellite we've studied is the Solar Dynamics Observatory," Charles Tarrio, a physicist at NIST directly involved with the research, told The Register. "We can't say whether this or isn't the only satellite that's experienced this mechanism of degradation, but I'd be surprised if it is the only one."

The performance of a filter that allowed 50 per cent of 30-nanometer UV light through to the detector, for example, could drop to 25 per cent in a year, and even further to ten per cent in five years. Now, a team of researchers studying the issue believe they have figured out what's causing the problem.

Water vapor released by thermal blankets, used to control the temperature of instruments onboard satellites, stick to the surface of the UV filter. The water vapor is broken down by sunlight, leaving hydroxide molecules on the surface of the instrument. Rays of UV light release electrons from the filter's aluminum components, which ionize the hydroxide molecules.

The negative ion pulls the positive aluminum ion from the filter and interacts to create aluminum oxide. Layers of aluminum oxide are deposited on the surface of the filter over time, making them cloudy and less effective at absorbing UV light, according to a study published in the Solar Physics journal. 

Robert Berg, co-author of the paper and a physicist at NIST, said in a statement discovering water was the culprit for why sun-facing satellites corroded "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 satellite," he said.

"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. Putting everything together, I'm convinced Water is responsible for the filter degradation."

The team believes future satellites should have filters containing tubes that block water vapor from their surfaces, and be built with layers of carbon to prevent aluminum ions from interacting with the hydroxide molecules. 

They discovered water was causing the issue by exposing filter samples to UV light by using NIST's Synchrotron Ultraviolet Radiation Facility. The radiation created by the synchrotron particle accelerator simulates solar activity and space weather.

They found aluminum oxide started forming on the UV filters after they subjected them to the light, and believe similar levels of oxidation experienced by real satellites would have been replicated in their samples if they had left them exposed to radiation for ten months.

Oxidation requires oxygen and water, the researchers said. "It had to be something that can emit water for five years continuously at reasonably constant rates," Tarrio said. They pinpointed the source of moisture as thermal blankets inside the satellite.

"We think the source of the water vapor is insulating blankets near the entrance apertures of the instruments. The blankets are made from multiple layers of aluminized Mylar and polyethylene terephthalate (PET). The PET is known to absorb about water vapor in the atmosphere," he explained to us.

"When sunlight strikes it and warms it up, it outgasses water vapor, and because the water is stored in the volume of the material rather than just on the surface, it can continue outgassing at a reasonably constant rate for many years."

The team is now working to test different materials for filters that will be less likely to oxidize in space. ®

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