The European Space Agency (ESA) has shared the story of how engineers brought a mission back from the brink after a micrometeoroid strike, an equipment failure, and an impressive solar storm.
It is more than ten years since ESA’s Gaia observatory was launched from Kourou aboard a Russian Soyuz to an orbit around the second Sun-Earth Lagrange point (L2,) approximately 1.5 million kilometers from Earth. The mission is to monitor the positions and motions of stars and the exoplanets that orbit them.
According to ESA, Gaia has identified 214 exoplanet candidates and is capable of observing approximately 3,400 stars per second.
It was only designed for a six-year mission but, as is so often the case, has outlived those initial expectations. In 2023, engineers estimated that the probe will exhaust its cold gas propellant — essential for precision pointing — sometime in the second quarter of 2025. The mission would then transition to a post-operations phase, which is due to be completed on December 31, 2030.
However, it almost ended sooner than planned. Earlier this year, the probe was struck by a micrometeoroid, which damaged the spacecraft’s protective cover. Although Gaia is designed to withstand such impacts, this one hit at just the right angle and speed to do damage.
The damage allowed a tiny bit of sunlight to disrupt the probe’s instruments occasionally.
While engineers were grappling with the problem, Gaia was hit by a charge coupled device (CCD) failure. Its “billion-pixel” camera uses 106 CCDs to convert light into electrical signals. While not conclusively proven to be the cause, the failure happened at around the same time as a burst of energetic particles erupted from the Sun and caused social media to be filled with shots of green and purple skies.
Gaia was designed to handle radiation, but, according to ESA, “It is possible that the storm was the final straw for this piece of the spacecraft’s aging hardware.”
The failed CCD was vital for Gaia’s ability to confirm the detection of stars. Without that functionality, the probe began to register thousands of false detections.
Edmund Serpell, Gaia spacecraft operations engineer at ESOC, said, “Gaia typically sends over 25 gigabytes of data to Earth every day, but this amount would be much, much higher if the spacecraft’s onboard software didn’t eliminate false star detections first.”
Sending a crew to fix the spacecraft isn’t possible. At 1.5 million kilometers away, it cannot be physically repaired. Instead, engineers tweaked the software, effectively changing the threshold at which the observatory decides that a faint point of light must be a star. The change has dramatically reduced false detections attributed to the light leakage and the CCD problem.
Gaia probably has less than a year of precision pointing left before its propulsion finally runs out. However, that year could yield all manner of discoveries. We, therefore, raise a glass to the creativity and determination of the team of engineers responsible for pulling the spacecraft back from the brink. ®
