A rare encounter: How Juice came to observe 3I/ATLAS

An unexpected visitor

On 1 July 2025, an asteroid alert telescope in Chile confirmed an interstellar object had entered our Solar System. The icy body - 3I/ATLAS - is the latest of only three interstellar objects known to have passed through our solar neighbourhood. 

Rare and hard to predict, scientists rushed to determine 3I/ATLAS’s orbital trajectory and see what observation options would be available to them. 

“Almost since the time of discovery, we realised that the geometry of the orbit would allow observations from the Juice spacecraft, which would observe the comet from a completely different angle than what we can do from Earth,” says Marco Fenucci, Mathematician and Near-Earth Objects Dynamicist at ESA’s Near-Earth Object Coordination Centre (NEOCC).

Calculations predicted that Juice would be the closest spacecraft to 3I/ATLAS right after the icy object reached perihelion, in November 2025.

“Preparations for things like payload pointing campaigns or flybys are usually in the order of nine months,” explains Juice Spacecraft Operations Manager (SOM) Angela Dietz. “When ATLAS came, we knew there was not a lot of time.”

With only four months to prepare a brand-new observation campaign for a completely uncharacterised interstellar object, there was no time to lose. 

“With observations in November, we had to complete the planning by the end of September and uplink the commands by mid-October,” Angela explains. “The mission manager and I agreed to streamline the workflow by temporarily skipping the Science Operations Centre (SOC) step. Normally, SOC supports calibration and data processing, but working directly with the instrument teams allowed us to move faster,” she says.

In the months leading up to the encounter, NEOCC worked closely with the flight dynamics team at the European Space Operations Centre (ESOC) towards producing increasingly refined trajectory solutions. “This was essential to point the Juice instruments to image 3I/ATLAS,” says Marco. 

Challenges in deep space

As 3I/ATLAS observation plans continued, Juice faced its own challenges. 

Leading up to Juice’s Venus flyby on 31 August 2025, a surprise communication failure days before the manoeuvre sparked round-the-clock troubleshooting. The anomaly was quickly resolved before the flyby, setting the spacecraft on track to its 3I/ATLAS encounter.

“Because of the proximity to the Sun, after the flyby, Juice remained in its hot-cruise phase, with its high-gain antenna still pointed at the Sun to act as a heat shield,” Angela explains. This hot cruise phase would extend until after the observation campaign. 

By early October, NEOCC delivered final information on 3I/ATLAS's position, which gave the Juice flight control and flight dynamics teams the confidence to finalise their observation plans within the spacecraft’s hot constraints. 

The encounter

Juice officially began its 3I/ATLAS observations on 2 November 2025, continuing through to 25 November. The closest approach was on 4 November at about 0.4 AU (roughly 60 million km).  

This ideal distance paired well with Juice’s payload. As a mission designed and equipped to study Jupiter’s icy moons, Juice’s scientific instruments were a match made in heaven for the icy interstellar body. Specifically, the spacecraft used five of its instruments – JANUS, MAJIS, UVS, SWI, and PEP – to take measurements of 3I/ATLAS.  

Thermal constraints limited these observations to six 45-minute slots and one final 4-hour slot. Together, these generated 126 science files with a total of 11.18 Gbits of data. 

But the team would have to wait to see the results.

With Juice still flying in its hot configuration after the observation campaign, the spacecraft could not yet turn back to Earth to transmit the data. Instead, all observations were stored onboard the mass memory (SSMM) unit. 

Only after the spacecraft entered cold-cruise phase in mid-January 2026 would the high bit rate downlink be possible.

Data at last

At last, the long-awaited data downlink took place in two 11-hour passes on 17 and 20 February 2026 via ESTRACK’s New Norcia and Malargüe deep space antennas respectively. 

The first pass ran smoothly through the night and into the early hours of the next day.  

The second and final pass took place early on 20 February. Upon acquisition of signal and nominal telemetry readings, the transfer began. As the 16‑minute signal delay ended, the navigation camera images finally reached the control room, triggering instant smiles. 

The rest of the 11-hour downlink ran as expected. By 19:21 CET the 3I/ATLAS observation campaign officially ended. But it left a lasting impact on the mission team.

We came. We saw. Now, we venture on.

“That’s the nice thing in our job – it is always a team effort of many parties involved. I think, the fact that we could optimise this campaign in little time and maximise the output is something to be proud of!” says a beaming Angela.

“At Jupiter, we will perform flybys of the icy moons at a high cadence, sometimes only a couple of weeks apart. The 3I/ATLAS campaign has made me even more confident that Juice can quickly achieve scientific objectives with short warning times, and that complex operations can be planned and executed within very limited timeframes,” says a happy Federico Giannetto, Juice Spacecraft Operations Engineer.

The science teams are now pouring over the data to see what it will reveal about the interstellar object. They will meet this week to discuss the findings.  

With preparations already underway for Juice’s upcoming Earth flyby in September, Angela reflects on the significance of this interstellar achievement. “Someone from mission analysis said to me: in the future, we will remember Juice for the double lunar-Earth flyby and 3I/ATLAS. Nobody’s ever done a lunar-Earth flyby or been so close to an interstellar object’s perihelion. These are very rare and remarkable things!”