The European Space Agency (ESA) spacecraft, known as the Jupiter Icy Moons Explorer (Juice), is fully prepared for its upcoming and crucial maneuver – a gravity-assist flyby of Venus, scheduled for August 31. This important phase of the mission is proceeding as planned after experts successfully resolved a serious communication anomaly that temporarily cut off all contact between the spacecraft and Earth, leaving the control center in a complete blackout.

The problem, which unexpectedly appeared during a routine communication window with a ground station on July 16, prevented Juice from sending telemetry data, which is crucial information about its status and system health. Thanks to the exceptionally fast and flawlessly coordinated action of the teams at ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany, and in collaboration with the spacecraft's manufacturer, Airbus, communication was re-established. The speed of the response was critical to prepare everything for the upcoming encounter with the planet Venus, a maneuver that is vital for continuing the journey to Jupiter.

Drama in deep space: Contact lost with Juice

The anomaly began when ESA's deep space antenna in Cebreros, Spain, failed to establish contact with the Juice spacecraft at the scheduled time, 04:50 Central European Summer Time on July 16. Initial checks immediately ruled out a problem with the ground station, which alerted the Juice mission control team at ESOC. Additional attempts to establish contact via ESA's New Norcia station in Australia were also unsuccessful, confirming the engineers' worst fears – the problem was with the spacecraft itself, hundreds of millions of kilometers away from Earth.

With no signal and no telemetry, the engineers faced the worst-case scenario. There was a serious concern that Juice had automatically entered so-called "survival mode." This is a last resort, an automatic protocol activated in the event of multiple failures in key systems. In this state, the spacecraft begins to rotate slowly, with its antenna "sweeping" the area of the sky where Earth should be once an hour, in the hope that a signal will be picked up. However, no such intermittent signal was detected, which further deepened the mystery and concern.

"Losing contact with a spacecraft is one of the most serious scenarios we can face," said Angela Dietz, Juice Spacecraft Operations Manager. "Without telemetry data, diagnosing and resolving the underlying cause of the problem becomes immensely more difficult."

The team's entire focus turned to the communication subsystem. Engineers suspected two main possibilities: either the medium-gain antenna was misaligned, or there was a failure in the transmitter or signal amplifier. It should be noted that Juice is designed for the extremely cold environment of the Jovian system. The spacecraft is currently much closer to the Sun, so it must constantly keep its large high-gain antenna pointed towards the Sun to use it as a massive heat shield. For this reason, a smaller, steerable medium-gain antenna is used for communication with Earth during this phase of the journey.

Race against time: 20 hours of uncertainty

Two recovery strategies were considered. The first was to wait for the spacecraft's next automatic system reset, which occurs every 14 days. The second, much riskier, was to send commands "in the blind" into space, in the direction where Juice was supposed to be, hoping that one of the auxiliary low-gain antennas would receive them.

"Waiting was not an option," explained Angela. "We had to act quickly. Waiting two weeks for an automatic reset would have meant delaying crucial preparations for the Venus flyby, which could have jeopardized the entire mission."

Sending commands "in the blind" was a huge challenge. At that moment, Juice was about 200 million kilometers from Earth, on the other side of the Sun. It took 11 minutes for each signal sent to reach the spacecraft. After that, the team had to wait another 11 minutes to see if any response would arrive. Six initial attempts to command the medium-gain antenna to point back towards Earth were unsuccessful. Recovery efforts continued through the night and lasted for nearly 20 exhausting hours, with a focus on manually switching on different components of the spacecraft's communication system.

Finally, one of the commands managed to reach Juice and trigger a response. The command activated the signal amplifier that boosts the strength of the signal Juice sends back to Earth. Contact was re-established, and to the great relief of the entire team, the data showed that the spacecraft was in excellent condition. No systems had failed, and all telemetry was nominal.

The cause discovered: A subtle software bug

A detailed analysis revealed that the root cause of the problem was a subtle software bug related to the system's internal time. The software function that switches the signal amplifier on and off relies on an internal counter. This counter continuously increases and resets to zero once every 16 months. An incredible coincidence occurred: the function tried to access the counter at the exact millisecond it was resetting to zero. Due to this error, the amplifier remained off, and Juice's signal was too weak to be detected from Earth.

"It was a very subtle bug, but one that we were prepared to investigate and solve," said Angela Dietz. "We have identified several possible ways to ensure this never happens again and are now deciding which solution would be best to implement."

Despite the high stakes and technical complexity, the recovery performed by the ESA mission operations team was achieved with minimal disruption to the overall mission schedule. It was an example of outstanding teamwork under immense pressure, where a calm and methodical approach enabled the recovery of the spacecraft without any permanent consequences.

Venus as a cosmic slingshot: Preparations for the key maneuver

Once the anomaly was resolved, the Juice mission team returned to its primary task: preparing for the Venus flyby. Juice will make its closest approach to Venus on Sunday, August 31, at 07:28 Central European Summer Time, completing the second of four planned gravity assists. These assists, also known as "gravity slingshots," use a planet's gravitational field to increase the spacecraft's speed and change its trajectory without expending precious fuel.

Since it is designed for the cold and dark environment of the outer Solar System, Juice must adapt to the intense solar heat near Venus. To protect its sensitive components, the spacecraft uses its main high-gain antenna as a heat shield. Due to these thermal constraints, its remote sensing instruments cannot be active during the flyby, which means no new photos of Venus will be taken.

A complex path to Jupiter

To reach Jupiter directly, Juice would need to leave Earth at a speed of 11 km/s. However, Juice is one of the heaviest interplanetary spacecraft ever launched, with a mass of almost 6000 kg. With such a massive payload, its Ariane 5 launch vehicle provided it with an exit velocity of "only" 2.5 km/s.

The spacecraft gains the rest of the required velocity precisely through gravity assist maneuvers. This week, it will use Venus's gravity to bend its trajectory around the Sun and gain speed relative to Earth. This flyby will give Juice a significant speed boost. When it next encounters Earth in September 2026, the spacecraft will have already reached the required transfer velocity to Jupiter of 11 km/s. However, Jupiter will not yet be in the right place in its orbit at that time.

Therefore, Juice will use the 2026 Earth flyby to further fine-tune its trajectory. After another orbit around the Sun, the spacecraft will return to Earth for a final flyby in January 2029. This last maneuver will send Juice on its final trajectory to intercept Jupiter in July 2031.

About the Juice mission

The European Space Agency's Jupiter Icy Moons Explorer (Juice) mission represents humanity's next bold step in exploring the outer Solar System. Its goal is to conduct detailed observations of the gas giant Jupiter and its three large moons believed to harbor oceans of liquid water beneath their icy crusts – Ganymede, Callisto, and Europa. This ambitious mission will characterize these intriguing worlds as potential habitats for past or present life, using a powerful suite of remote sensing, geophysical, and in-situ measurement instruments.

Juice will closely monitor Jupiter's complex magnetic, radiation, and plasma environment and its interaction with the moons, studying the Jovian system as an archetype for gas giant systems throughout the universe. It was launched on an Ariane 5 rocket from Europe's Spaceport in Kourou in April 2023. It has an eight-year journey ahead of it, which includes the aforementioned flybys of Earth and Venus. Upon arrival, it will perform 35 flybys of the three large moons while orbiting Jupiter, before changing orbit to enter orbit around Ganymede, thus becoming the first spacecraft in history to orbit a moon of another planet.