An exceptionally heavy interplanetary probe is on an eight-year journey to Jupiter, using the gravity of Earth and Venus to propel it on its path toward the gas giant. Just weeks before its scheduled flyby of Venus, the European Space Agency’s JUICE mission went silent, threatening its ability to perform the planetary encounter.
Unable to communicate with the spacecraft, teams of engineers got to work on figuring out the problem under a tight schedule, hoping their efforts would reach JUICE as it cruises millions of miles away.
JUICE, or JUpiter ICy moons Explorer, is currently on its way to Venus to perform a gravity assist maneuver on August 31, following the resolution of a pesky software glitch that had weakened the spacecraft’s signal. Mission control managed to reestablish communication with the spacecraft just in time to prepare it for its upcoming flyby, pulling off an impressive recovery of the mission as it heads toward its target.
Waiting not an option
The team detected the anomaly on July 16 as JUICE was flying above a ground station in Cebreros, Spain. ESA’s deep space antenna was unable to establish contact with the spacecraft, raising concerns that JUICE was in a dreaded survival mode triggered by multiple onboard system failures.
“Losing contact with a spacecraft is one of the most serious scenarios we can face,” Angela Dietz, JUICE spacecraft operations manager, said in a statement. “With no telemetry, it is much more difficult to diagnose and resolve the root cause of an issue.”
The spacecraft would automatically reset in 14 days, but the team could not wait that long and risk missing JUICE’s scheduled encounter with Venus. “Waiting was not an option. We had to act fast.” Dietz added. “Waiting two weeks for the reset would have meant delaying important preparations for the Venus flyby.”
Instead, the team of engineers behind the mission began to blindly send commands toward JUICE’s presumed location in space. That proved to be challenging, as the spacecraft is currently located 124 million miles (200 million kilometers) away on the other side of the Sun. Each rescue signal would take 11 minutes to reach the spacecraft, and the team would then have to wait another 11 minutes to hear back from JUICE.
The dreaded software timing bug
Nearly 20 hours later, a command signal finally reached the spacecraft, triggering a response. Thankfully, the team found JUICE in good condition, and no system failures were detected.
As it turns out, a software timing bug caused JUICE’s signal to become too weak to detect from Earth. JUICE has built-in software that switches its signal amplifier on and off using an internal timer. The timer restarts from zero once every 16 months, but if the software happens to be using the timer at the same moment it restarts, then the signal amplifier remains switched off, silencing JUICE’s calls to Earth.
The team was able to resolve the issue and is now devising ways to ensure JUICE’s signal is always heard across deep space. “We have identified a number of possible ways to ensure that this does not happen again, and we are now deciding which solution would be the best to implement,” Dietz said.
JUICE launched on April 14, 2023, carrying a suite of remote sensing, geophysical, and in situ instruments to explore Jupiter and its three ocean-bearing moons—Ganymede, Callisto, and Europa. The 13,227-pound (6,000-kilogram) spacecraft is expected to arrive at the gas giant system in 2031, using a series of gravity assists to pick up speed. This week’s Venus flyby is the second of four planned gravity assist maneuvers.
JUICE will also use Earth to reach its required transfer velocity through an upcoming flyby in September 2026 and another one planned for January 2029.