Scientists find the answer to the mystery of cloudy filters on satellites

There’s a mystery going on in some satellites facing the Sun, and scientists from the National Institute of Standards and Technology (NIST) and the Laboratory for Atmospheric and Space Physics (LASP) are on the case. The team has been trying to find out what shadows and compromises the performance of small, thin metal membranes that filter sunlight as it enters detectors that monitor the sun’s ultraviolet (UV) rays.

These detectors can warn us of impending solar storms – bursts of radiation from the sun’s surface – that could reach Earth and temporarily disrupt communications or interfere with GPS readings.

Last year, the team disproved the prevailing theory: that this haze was a build-up of carbon on the surface of the filters from organic sources left on the satellite.

Now, in a series of three new papers, the same team from NIST and LASP have made a strong case for what they believe is the true culprit: oxidation caused by water, which, along with UV light from the sun, produces a thick layer of aluminum oxide—a lot thicker than previously thought possible – which blocks incoming rays.

As a bonus, the researchers believe they have identified the source of the water: thermal blankets, which are used to control the temperature of instruments on a spacecraft. This information could help researchers improve the performance of future satellites that rely on this type of filter, perhaps by adding hardware that limits the filters’ exposure to the area around the thermal blankets, or by using different materials as part of the filters themselves.

The first of the three newspapers was published today in Solar physics.

“As far as I know, we are the only ones looking at filter oxidation due to exposure to ultraviolet light,” said Charles Tarrio of NIST.

Proving that water is responsible for the problem “was kind of a one-two punch,” said NIST physicist Robert Berg. “Punch one physically showed that this chemical process involving water can cause something comparable to what we actually see happening in the satellites. And number two says that when you create a theoretical model that takes everything into account, the numbers match up. quantitatively with what we see in the satellites.

“Putting it all together, I’m convinced,” Berg said. “Water is responsible for the breakdown of the filter.”

#No filter

Most of the light produced by the Sun is visible and ranges from red light, with a wavelength of about 750 nanometers (nm, billionths of a meter), to violet light, with a wavelength of about 400 nm. Among other wavelengths, the sun also emits relatively small amounts of light in the extreme ultraviolet (EUV) range, which extends from 100 nm down to just 10 nm – wavelengths too short for human eyes to see.

Although small, that EUV signal is useful because it increases with the solar flares that can interfere with communications on Earth or cause problems with GPS. EUV signals also give scientists a heads-up on hours or even days before more destructive phenomena such as coronal mass ejections reach Earth. These explosions of charged particles can overload power lines or increase radiation exposure to airline crew and passengers.

A critical piece of equipment on the sun-facing space detectors are the aluminum filters, each smaller than a postage stamp, which block all but the EUV light between 17 nm and 80 nm wavelength.

Even if they begin life in space and send plenty of EUV light into their range, within just a few years they can lose a significant amount of transmission capability. For example, a filter can start by letting 50% of 30 nm EUV light through to the detector. That figure can drop to 25% within a year, and 10% within five years.

Scientists believed that some unknown substance must grow or become deposited on the filters, causing them to darken in just months and limiting the amount of light entering the detectors. The leading theory was that carbon was outgassed from the instrument itself and was deposited on the filters.

When NIST and LASP staff disproved that last year, they turned their attention to what they believed to be a much more likely explanation: the process of oxidation, in which oxygen atoms from water molecules (H₂O) combines with aluminum atoms from the filter itself (Al) to form a hazy layer of aluminum oxide (Al)₂O₃). (By the way, a thin layer of aluminum oxide naturally covers every aluminum object on Earth, from soda cans to frying pans.)

Scientists already knew that exposing an aluminum surface to UV light in the presence of water can grow extra layers of oxide beyond those that naturally form. But there was no existing theory that could explain how the aluminum oxide could grow thick enough to cause this obscure problem.

Scientists decided to thoroughly investigate how the presence of water can affect the filters, to find out what was really going on.

SURF is up

NIST scientists wanted to test their water theory in a controlled setting: a machine that would effectively allow them to create space weather. Called NIST’s Synchrotron Ultraviolet Radiation Facility (SURF), the device is a space-sized particle accelerator that uses powerful magnets to move electrons in a ring. The movement generates EUV light, which can be deflected through specialized mirrors to hit targets like the satellite filters being tested.

Despite exposing their sample filters to lab-made UV light for as long as 20 days, they were unable to grow oxide layers thick enough to explain the cloudiness of real space filters. But the oxide layers were still much thicker than predicted by the accepted theory.

The researchers believe that with further exposure they would have reached the required thickness. They also suggested that the sample filters would need to be exposed to the SURF beam for approximately 10 months to achieve the same oxide thickness as the filters in the actual room.

The team took a different tack and also conducted modeling studies. The finished models match almost exactly what astronomers see in real aluminum filters in space.

An important part of the new model’s success is that it accounts for the fact that electrons are scattered as they move within the aluminum filters. This dispersion slows their progress, which affects the dynamics of oxide growth.

“This is the first model that takes scattering electrons into account, and it uses parameters that are consistent with what is expected in the literature for each of the steps in the chemical reaction,” Berg said.

Just add water

However, for the models to work, one important piece of information was missing: a significant source of water that could fuel this reaction.

“It had to be something that could emit water for five years continuously at reasonably constant rates,” Tarrio said. “That set Bobby [Berg] out on this quest to find, what the hell could this be? What might be a suitable source? And he found it.”

The most likely source, concludes Berg, is thermal blankets. These are made from a type of plastic called polyethylene terephthalate (PET), known to trap water on Earth. This water is usually not a problem for most equipment.

“It was hard to think of anything else that would hold that kind of water,” Berg said.

Future work, the researchers hope, might include testing different materials for the filters that would still be transparent at the relevant wavelengths, but would not be subject to oxidation.