The detective of the dark universe of the European Space Agency (ESA), the EUCLID probe, has located its guide stars, which it had lost due to a misidentification. It can now begin investigating dark matter and dark energy. Dark matter represents 85% of the matter in the universe but is invisible. Dark energy causes the cosmos to expand at an ever-increasing rate.

Cosmic rays and charged particles from the Sun

EUCLID departed on July 1 and embarked on a four-week journey to the Lagrange Point L2. While the probe safely reached its destination, its operators noticed an issue after the spacecraft captured its first incredible images of the cosmos. EUCLID’s fine guidance sensor had difficulty finding its guide stars, which it uses for navigation. These guide stars are crucial for allowing it to point to specific areas in the sky.

The cause of this problem was cosmic rays, charged particles that the Sun emits during periods of high solar activity. Cosmic rays were hitting the guidance sensor, creating signals that EUCLID mistakenly identified as stars. Additionally, scattered sunlight and X-rays were interfering with the spacecraft. As a result, artifacts caused by this interference occasionally outnumbered the real stars identified.

A prominent example of the effect of this flaw on EUCLID’s operations is an image of a distant star field showing strange rings and loops. While suggestive, they are not useful in the search for patterns that might reveal clues about dark energy and dark matter. This type of issue often occurs during the commissioning phase of a spacecraft.

A Euclid image shows the rings and swirls that occurred when the spacecraft's fine guidance sensor intermittently lost its guide stars. Credit: ESA
A Euclid image shows the rings and swirls that occurred when the spacecraft’s fine guidance sensor intermittently lost its guide stars. Credit: ESA

Corrective software

The mission team created a software patch that was applied for the first time, according to ESA officials. After being updated and undergoing 10 days of in-orbit testing, the Fine Guidance Sensor now functions as expected, and EUCLID’s guide stars have been re-located.

“Our industrial partners, Thales Alenia Space and Leonardo, returned to the drawing board and revised how the Fine Guidance Sensor identifies stars” Micha Schmidt, head of EUCLID spacecraft operations, stated in the note. “After a tremendous effort and in record time, we were provided with new onboard software to install on the spacecraft. We carefully tested the software update step by step in actual flight conditions for target observation”.

EUCLID probe is now ready to resume the crucial performance verification phase, interrupted in August, during which final tests will be conducted. “The performance verification phase, interrupted in August, is now fully resumed, and all observations have been made correctly” explains Giuseppe Racca, the project head. “This phase will last until the end of November. We are confident that the mission’s results will prove exceptional, and regular scientific observations can begin afterward”. This is the final step before EUCLID can begin investigating the dark universe.

Mission objectives

EUCLID will survey about one-third of the sky above Earth, looking back over 10 billion years of cosmic history. It will map 3D models of galaxies to see how the universe has taken shape and what role dark matter has played. The probe will also examine large-scale galactic disturbances to see the influence of dark energy as it pushes galaxies farther apart.

“Now comes the exciting phase of Euclid testing under scientific-like conditions. We look forward to receiving the first images showing how this mission will revolutionize our understanding of the dark universe” said Carole Mundell, ESA’s scientific director.

Stefano Gallotta
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