After contacts with Jupiter Icy Moons Explorer (Juice) were restored, the European Space Agency (ESA) felt more at ease. For now, the spacecraft is heading toward Venus while it is getting ready for a gravity-assist touchdown on August 31.
Launched in April 2023, the probe started treating its controllers in quite when, on July 16, ESA’s deep space antenna at Cebreros, Spain, was unable to successfully establish communication with the spacecraft at the originally planned time of 0450 CEST. When New Norcia station of ESA was also unable to establish communication, controllers recognized the issue was with Juice, despite the fact that the ground station seemed to be operating OK.
It is possible that Juice could have changed to survival mode and lowered its speed, sweeping its antenna across the Earth once an hour, if there had been a serious issue. Still, not a sound came from the spaceship.
Angela Dietz, Juice Spacecraft Operations Manager said:
“Losing contact with a spacecraft is one of the most serious scenarios we can face. With no telemetry, it is much more difficult to diagnose and resolve the root cause of an issue.”
It’s possible that the signal transmitter or amplifier malfunctioned, or that the antenna had somehow become misaligned. With the nearing of Venus flyby, teams rushed to put together a solution immediately given that they were unable to wait additional 14 days for the next computerized spacecraft reset. Instead, in the hopes that a backup antenna may pick them up, the team tried to broadcast commands blindly into space, where Juice was supposed to be.
Each order takes 11 minutes to reach the spacecraft, and it takes a further 11 minutes for a response to arrive, therefore it took more than 20 hours. The medium-gain antenna was unsuccessfully pointed back toward Earth six times. However, contact was restored after a directive to activate the signal amplifier, which increases the power of the signal Juice transmits toward Earth.
What really transpired continued to be a mystery, however the spacecraft was in excellent condition. But it was discovered that, it was one of those scenarios which plenty of software engineers has witnessed: an unforeseen combination of variables unfolding at exactly the wrong minute.
A timer that the software of spacecraft utilized to turn the signal amplifier on and off was the problem. Once every six months period, the timer starts all over again from zero and continues to accumulate.
The signal of Juice is too weak to be transmitted from Earth if the function that turns on the amplifier also happens to be utilizing the timer at the exact moment the timer restarts.
Dietz gave a statement to The Register:
“The downlink amplifier is controlled by an onboard function (OBCP) that uses an internal timer for wait/sleep statements. If called near the wrap-around point for a duration extending beyond it, the timer fails to wake the function, leaving the amplifier permanently off.
This exact scenario had occurred. The OBCP itself is used to schedule the transmitter amplifier on operations. We need this additional logic on-board to avoid radiation of the downlink through forbidden zones – there are scientific sensors in certain parts of the field of view of the antenna.”
Dietz said:
“Possible mitigations of this anomaly would be for example, a regular controlled reset of this timer, or replacing this timer with a different one, which wraps around only after 100 years. We might require a spacecraft software change (which is a complex activity), though other options would be easier.”
Thankfully, the next time the timed wraparound happens is in 15 months, by which Juice will have finished its next flyby of Earth on its way to Jupiter.
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