Monday, October 9, 2023

Europe's Euclid Telescope Was "Disoriented"

... but it's feeling much better now.  Shortly after sending back its first images over the summer (middle story) Euclid lost the guide stars it uses both to find its targets and to track them.  The probe is now said to be ready to get to its full science mission attempting to identify dark matter and dark energy.  

Euclid launched to investigate these cosmological mysteries, sometimes collectively known as the dark universe, on July 1 and took a four-week journey to Lagrange point 2, a gravitationally stable point in the Earth-sun system. Although Euclid reached its destination safely, its operators noticed a problem after the spacecraft took its first incredible images of the cosmos: Euclid's Fine Guidance Sensor was having trouble finding its guiding stars, which Euclid uses for navigation and thus are crucial to enabling it to point at precise areas of the sky.

Here's where the source article goes off the rails in my understanding of these things.  It opens by saying, "The cause of this issue was cosmic rays  —  charged particles that the sun emits during periods of high solar activity" but that's not what cosmic rays are.  Cosmic rays come from deep space, well beyond the solar system.  Charged particles coming from sun are called the solar wind or can be part of a Coronal Mass Ejection or CME, a part of more energetic solar flares.  You've heard of X-Ray flares?  Those are particularly energetic flares and bathe the solar system in X-rays (usually over a small angle)  Flares and CMEs can be encountered almost anytime, but are more common when the sun is in the more active portion of its solar cycle, which it is now.  

Coincidence of coincidences, as it turns out, the month of July was the second most active month of this solar cycle (25) so far in two different but vitally important measures, sunspot count and the 10.7 cm solar flux.  

The article goes on to say:

The cosmic rays were impacting the Fine Guidance Sensor, creating signals that Euclid was incorrectly identifying as stars. In addition, stray light from the sun and solar X-rays were interfering with the spacecraft. As a result, artifacts caused by this interference occasionally outnumbered the real stars being spotted by Euclid, meaning the spacecraft couldn't resolve the star patterns it needed to navigate.   

A quick visit to the NOAA Space Weather Prediction Center and a little editing results in this:

I can't get the two graphs to show the same little popup as the top (Sunspot Number) plot shows, but the bottom one says for July (red arrow), Monthly values: 177.53, Predicted values: 111.7, predicted range F10.7 94.6-130.3. includes this image from the Fine Guidance Sensor which shows "strange loops and lassos" that fall into the category of "pretty, but not pretty useful."  

An image that resulted from Euclid's Fine Guidance Sensor intermittently losing its guide stars. Image credit: ESA.

As you're probably aware, SW problems like this aren't completely unexpected, but thankfully aren't as serious as the one that took out the Russian Luna 25 probe.  Engineers started working on fixes as soon as the problem became apparent and developed a software patch.  The patch was tested out on a model of Euclid on the ground and once verified was uploaded to the operational Euclid out at L2.  Euclid is now ready to restart its all-important performance verification phase, interrupted in August, leading to final testing. It's expected to take until late November to fully check out the telescope and certify it for use.


  1. Interesting. They didn't take solar activity into account? Overlooked it? Discounted the effect it would have?

    Well, as we say in pilot training, "Live and learn, or crash and burn!" At least there's a software fix, as a hardware one would be slightly impossible...

    1. This is where I was puzzled. Solar cycle 25 is more active than 24 was, but that was the weakest cycle in a hundred years, so saying that July was the most active month in this cycle isn't exactly saying it's historically high. Back in cycle 23, in November of '03, there was a solar flare that was legitimately capable of being a Carrington event, retroactively classed as an X28 and the strongest flare seen in the satellite era, but cycle 23 overall was nowhere near as active a cycle as we've seen. Thankfully that flare was pointed 90 degrees away from Earth.

      In my mind, designing for something that's going to be away from our protective atmosphere and geomagnetic field has to consider that as a possibility. Maybe you don't design for the "once in 200 years" flare, but you have to know you're going to get flares.

      I've never heard of referring to solar activity as being cosmic rays, but I don't know if that was from the author on or directly from the ESA.