ESA: Euclid vision cleared after being fogged by ground ice, after launch
The European Space Agency (ESA) today announced that engineers have successfully “de-iced” the optics of its new Euclid space telescope that developed after it was launched in July 2023.
It was always expected that water could gradually build up and contaminate Euclid’s vision, as it is very difficult to build and launch a spacecraft from Earth without some of the water in our planet’s atmosphere creeping into it. For this reason, there was an ‘outgassing campaign’ shortly after launch where the telescope was warmed up by onboard heaters and also partially exposed to the Sun, sublimating most of the water molecules present at launch on or very near Euclid’s surfaces. A considerable fraction, however, has survived, by being absorbed in the multi-layer insulation, and is now being slowly released in the vacuum of space.
After a huge amount of research – including lab studies into how minuscule layers of ice on mirror surfaces scatter and reflect light – and months of calibrations in space, the team determined that several layers of water molecules are likely frozen onto mirrors in Euclid’s optics. Likely just a few to few tens of nanometres thick – equivalent to the width of a strand of DNA – it’s a remarkable testament to the mission’s sensitivity that it is detecting such tiny amounts of ice.
While Euclid’s observations and science continue, teams have come up with a plan to understand where the ice is in the optical system and mitigate its impact now and in the future, if it continues to accumulate.
It appears this new process has worked, according to a short update at the link.
Normally spacecraft are vented both on the ground during thermal testing, as well as when they reach orbit. It appears some of these normal procedures were either insufficient for Euclid’s needs, or threatened its optics if done as usual. This press release suggests that Euclid required very targeted venting processes that would not harm its sensitive optics, and that the procedures have worked.
I must admit I am suspicious of these claims. During development and after launch Euclid has had a number of problems. First, back in 2017 the NASA instrument on the telescope had to be completely rebuilt when it was found to be defective. Second, after launch engineers discovered unexpected light leaks on the mirror that now limit where it can look. Third, the telescope required a software patch to fix its pointing system, which was confusing cosmic rays for guide stars, causing it to shift positions randomly.
I can’t help wondering if this icing on the mirrors was also due, not to actual planning as suggested by ESA’s press release, but to poor ground testing and engineering that missed what is a common problem on spacecraft and thus required a post-launch improvised fix. I admit I might be wrong, but I still wonder.
On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.
The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.
The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.
"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News
The European Space Agency (ESA) today announced that engineers have successfully “de-iced” the optics of its new Euclid space telescope that developed after it was launched in July 2023.
It was always expected that water could gradually build up and contaminate Euclid’s vision, as it is very difficult to build and launch a spacecraft from Earth without some of the water in our planet’s atmosphere creeping into it. For this reason, there was an ‘outgassing campaign’ shortly after launch where the telescope was warmed up by onboard heaters and also partially exposed to the Sun, sublimating most of the water molecules present at launch on or very near Euclid’s surfaces. A considerable fraction, however, has survived, by being absorbed in the multi-layer insulation, and is now being slowly released in the vacuum of space.
After a huge amount of research – including lab studies into how minuscule layers of ice on mirror surfaces scatter and reflect light – and months of calibrations in space, the team determined that several layers of water molecules are likely frozen onto mirrors in Euclid’s optics. Likely just a few to few tens of nanometres thick – equivalent to the width of a strand of DNA – it’s a remarkable testament to the mission’s sensitivity that it is detecting such tiny amounts of ice.
While Euclid’s observations and science continue, teams have come up with a plan to understand where the ice is in the optical system and mitigate its impact now and in the future, if it continues to accumulate.
It appears this new process has worked, according to a short update at the link.
Normally spacecraft are vented both on the ground during thermal testing, as well as when they reach orbit. It appears some of these normal procedures were either insufficient for Euclid’s needs, or threatened its optics if done as usual. This press release suggests that Euclid required very targeted venting processes that would not harm its sensitive optics, and that the procedures have worked.
I must admit I am suspicious of these claims. During development and after launch Euclid has had a number of problems. First, back in 2017 the NASA instrument on the telescope had to be completely rebuilt when it was found to be defective. Second, after launch engineers discovered unexpected light leaks on the mirror that now limit where it can look. Third, the telescope required a software patch to fix its pointing system, which was confusing cosmic rays for guide stars, causing it to shift positions randomly.
I can’t help wondering if this icing on the mirrors was also due, not to actual planning as suggested by ESA’s press release, but to poor ground testing and engineering that missed what is a common problem on spacecraft and thus required a post-launch improvised fix. I admit I might be wrong, but I still wonder.
On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.
The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.
The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.
"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News
I think if we compare Euclid’s vision contamination problem with other telescopes and spacecraft that we have watched launch, we would discover that the others did not mention this problem. In fact, that Euclid’s vision was a problem tells us that something in the design, assembly, or launch preparation process went wrong. JWST is run at a very cold temperature, and if they had water on the mirrors, they would never get it off.
Robert wrote: “Normally spacecraft are vented both on the ground during thermal testing, as well as when they reach orbit.”
Yes, this is an important process. This post bring up three related topics: venting, offgassing, and outgassing.
Venting a spacecraft is the process of letting the air out during launch (or thermal vac. pump down). Generally, there are some form of screens that leave openings to allow for the rapid venting during launch. The next time you watch a launch, notice that thirty seconds after lift off the vehicle is still fairly deep in the atmosphere, but a minute later it is high enough to be in a hard vacuum. Not only does the spacecraft have to vent, but so does the rocket’s fairing, otherwise the difference in pressure can cause things to pop open in an uncontrolled or an unscheduled, rapidly disassembling manner.
Offgassing is when volatile molecules or atoms cover the surface of the materials and spend hours, days, or years coming off the surface into the vacuum. This is one of the reasons for wearing gloves when working on flight hardware; it prevents (reduces) the oils from fingerprints from becoming one of these offgassing materials.
Outgassing is a similar problem, except that the volatiles have been absorbed into the material and spend a long time (or forever) outgassing. Nylon is rarely used on spacecraft, because it can absorb 6% of its mass in water.
Thermal vacuum tests spend some time at their first high temperature waiting for the venting, the offgassing, and the outgassing to reach acceptable levels. The end of a thermal vac. test can be tricky, because backfilling with air can result in layers of water molecules sticking to the spacecraft and chamber surfaces, which will then take a while to offgas during the next time it goes into vacuum. The trick is to backfill the chamber half way with dry nitrogen, so that layers of nitrogen molecules coat all the surfaces rather than water. Nitrogen molecules offgas quicker and are less likely to stick to other surfaces after launch, but it takes a while for them to offgas and be replaced by sticky water molecules during the rest of the tests and the launch preparations.
There are two main problems with volatile materials (molecules). High voltage (over 150 volts for air) can cause corona breakdown, where the air molecules ionize and carry charges between higher voltage locations and lower voltage locations. It causes problems for electronics and especially for signals (data, commands, etc.). Once the pressure gets low enough, this phenomenon won’t happen, and it becomes safe to turn on the high voltage parts of the spacecraft. Volatile materials can come off of one surface and land on another surface, especially when the first surface is warm and the second is cold. Thus, optical surfaces can become contaminated.
Optical surfaces are more than just lenses and mirrors for collecting data, they are also the solar arrays that want all the energy getting to the photovoltaic substrate rather than be absorbed or scattered by a layer of water molecules. Optical surfaces are also the thermal radiators that want to look at the cold of deep space not the warmth of water molecules. They are the Thermal blankets that are just as finicky as the radiators.
So the question that this article does not answer is: what did Euclid’s designers or operators do that allowed this problem to occur?
Euclid calibration scientist here. Euclid was designed to be an extremely low-outgassing spacecraft. Everything was thoroughly baked out. But during assembly and storage in clean rooms, humidity will unavoidably be reabsorbed. For example, the multilayer thermal insulation (MLI) absorbs 1% of its own weight in form of water within 24h after baking out in a normal atmosphere. This is an unavoidable problem all satellites using MLI have (and that’s essentially all of them). It’s just that Euclid has to measure incredibly precise and needs great thermal stability, that’s why we can’t just do a thermal decontamination to get rid off the ice like other satellites can do. For example, Hubble’s WFPC2 camera needed decontamination every 28-49 days for 15 years. It’s routine stuff, but for Euclid a few nanometers of ice are already a problem.
Mischa Schirmer: Thank you for the clarification. It would have been to Euclid’s advantage to have explained this more clearly in the press release. When press releases are vague about such things, it immediately raises questions that the press office is trying to hide a problem.
NASASpaceFlight has a little more detail on this issue:
https://www.youtube.com/watch?v=QjhFrEPRtto#t=963 (1 ½ minutes)
Mischa Schirmer mentioned that the thermal blankets (multilayer thermal insulation (MLI)) can absorb water at 1% of its own weight. Venting these insulating blankets is difficult, as they are made of many layers of aluminized mylar. Venting is done through many small holes in each layer so that the air eventually works its way from the center layer to the outside.
https://en.wikipedia.org/wiki/Multi-layer_insulation
I don’t know whether mylar (PTE) absorbs water, similar to nylon, but water can stick to the inner layers of the MLI like it does to other surfaces. Since these blankets can be thin, perhaps five layers (my recollection is JWST uses five layers that are much more open than thermal blankets), or thick, dozens of layers, depending upon the thermal protection needed, and since they can cover a lot of surface area of a spacecraft, they could end up with quite a bit of water, and they could offgas/outgas for years, since the holes are small and the paths tortuous.
It is a reason to halfway backfill the thermal vacuum chamber with dry nitrogen before using air the rest of the way.