ESA declares the Sentinel-1B mission over after payload resuscitation ends

What would you do with an otherwise healthy spacecraft?

ESA has admitted defeat and declared the mission of its Sentinel-1B spacecraft is at an end after attempts to deal with a borked power bus ended in failure.

The issue was an unwelcome Christmas gift from orbit for controllers and resulted in the payload of the Sentinel-1B being inoperative since December 23, 2021.

The Sentinel-1 mission consists of two polar-orbiting satellites which provide continuous radar mapping of the Earth. The constellation was expected to transmit data for at least seven years and had enough fuel onboard for twelve years.

Sentinel-1A was launched in 2014 and has had an eventful life with debris encounters and the odd swift manoeuvre to dodge space junk. Sentinel-1B went up on a Soyuz in 2016. Both are part of the Copernicus program.

However, the power issue that befell Sentinel-1B left its C-band Synthetic Aperture Radar inoperative, thus somewhat defeating the point of the spacecraft. In an exhaustive anomaly summary report [PDF], engineers documented efforts to recover the payload (as well as ensuring the same thing would not happen to Sentinel-1A) before the towel was reluctantly thrown in.

As for what caused the issue with the C-SAR Antenna Power Supply (CPAS) 28V regulated bus, "The most probable common cause," wrote ESA, "is the soldering process used to repair a ceramic capacitor on both the main and the redundant CAPS 28V regulator boards."

That said, without a look at the spacecraft or more telemetry than is available, something else might have happened. Either way, modifications have been made to Sentinel-1C (due for launch on a Vega-C in 2023) and will be implemented on Sentinel-1D to improve robustness and reliability.

All of which leaves ESA with a spacecraft that is pretty much fully functional aside from its payload. Though it seems a shame, a safe disposal is the most likely option, once Sentinel-1C is safely in orbit. "Due to manpower operational constraints," explained ESA, "the execution of the Sentinel-1B disposal cannot take place in parallel to the Sentinel-1A operations and to the Sentinel-1C related activities."

One of the engineers responsible for flying the spacecraft told The Register there was a certain inevitability regarding the end of mission after the failure of all the resuscitation attempts. While he noted "it definitely will be disposed because we need to meet our space debris obligations," he added that "some people have already started concocting ideas about what we could do with it."

"The risk is low," he went on, "but there's always the threat of an Enivsat-like catastrophic failure that would limit our ability to deorbit. That would of course expose us to justifiable criticism about why we didn't deorbit the dead spacecraft earlier."

Since the deorbit process will be a lengthy one, there is also the opportunity for some tests "on the way down."

ESA has a long history of keeping spacecraft going way past their expiration date. However, with the payload of Sentinel-1B declared inoperative the agency is taking a responsible stance regarding disposal and debris. Certain other operators would do well to take note.

We'll leave the last word to engineer Thomas Ormston. ®

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