ESA's Juice spacecraft in November 2025 used a rare geometry to observe the interstellar comet 3I/ATLAS (C/2025 N1) in a phase of enhanced activity after passing through perihelion. Although the mission is primarily dedicated to Jupiter and its icy moons, the scientific team opened targeted observation windows with five instruments — JANUS, MAJIS, UVS, SWI, and PEP — to catch the chemical, dynamic, and particle traces that the comet reveals during the period of its liveliest "coma and tail" behavior. This extraordinary opportunity came shortly after 3I/ATLAS made its closest approach to the Sun at the end of October, which is precisely the period when jets of gas and dust become most pronounced.

What is 3I/ATLAS and why is it important?

3I/ATLAS is only the third confirmed interstellar object to enter our Solar System, after 1I/ʻOumuamua (2017) and 2I/Borisov (2019). Unlike ʻOumuamua, which was without a classical tail and had a very unusual photometric signature, 3I/ATLAS behaves like a "proper" active comet with a clearly visible coma and tails. It is a natural body on a hyperbolic trajectory, whose composition and behavior carry information about the chemistry and thermophysics of distant planetary systems. That is precisely why the international community — from ground-based observatories to space orbiters — is coordinating to monitor this passage through the inner Solar System, aware that such opportunities are extremely rare.

November 2025: The first window for Juice

In the first half of November 2025, Juice began observations of 3I/ATLAS with multiple sensors. Initial observations were conducted as early as November 2, 2025, and on November 4, the most favorable approach occurred from the spacecraft's perspective — about 65 million kilometers from the comet (estimates from various ephemerides gathered in media and agency reports range around 64–66 million km). That moment came immediately after perihelion, when the comet was in its most active state. In imagery from the Navigation Camera (NavCam) — an engineering camera intended for guidance, not scientific photography — the comet is, according to preliminary downloads from the spacecraft, clearly visible as a condensed source surrounded by a diffuse halo, with indications of a plasma tail (ionized gases "pulled" by the solar wind) and a possible fainter dust tail formed by microscopic particles exiting the nucleus.

NavCam is not a calibrated high-resolution scientific instrument; its role is primarily autonomous navigation during future flybys of Jupiter's moons. Nevertheless, such "test shots" are valuable for checking the geometry and lighting of the scene and for a quick assessment of how pronounced the tails and coma are at the time of observation. Furthermore, it is accompanied by detailed measurements from JANUS (high-resolution optics), MAJIS and UVS (spectroscopy from ultraviolet to infrared), SWI (submillimeter "sniffing" of molecules and gas temperatures), and PEP (particles and plasma). This combination enables the correlation of appearance (photometry and morphology) with composition (lines of H₂O, CO₂, CO, fragments of OH, etc.) and environment (solar wind, ions, electrons), which is crucial for understanding the mechanisms that "drive" the comet after perihelion.

Why is data arriving only in February 2026?

Juice is configured in this cruise phase so that its main high-gain antenna (HGA) serves as a shield against solar heat. Because of this, communication is mostly switched to the medium-gain antenna, which drastically reduces data throughput. Consequently, the majority of scientific packages from the 3I/ATLAS observations are planned to be delivered to Earth only in February 2026, when the spacecraft changes its thermal orientation and returns to a standard connection via HGA. This is not a "delay on a whim," but compromises dictated by thermal protection and spacecraft safety in the part of the journey near the Sun. Scientists therefore currently have only limited telemetry snippets (e.g., a partially downloaded NavCam frame), while complete JANUS/MAJIS/UVS/SWI/PEP series will arrive according to a complex schedule in the coming weeks of 2026.

Mars as an "external tripod": Autumn measurements for a more precise trajectory

This November campaign relies on observations from Mars orbit conducted from October 1 to 7, 2025, when Mars Express and the ExoMars Trace Gas Orbiter (TGO) "hunted" 3I/ATLAS under extremely challenging conditions of low brightness and high speed. Although images from Mars Express had shorter exposure times and lower sensitivity for such dark targets, TGO managed to register the comet and, more importantly, deliver data that tenfold narrowed uncertainties in position and future trajectory. These ephemeris corrections were crucial to "lock" the observation windows from Juice onto the correct comet position in November, just when 3I/ATLAS was in its most active state after perihelion.

What exactly will Juice's instruments look for?

• JANUS (high-resolution camera): Images are expected to track changes in coma morphology and tail orientation over days. Although distance imposes resolution limits, the evolution of brightness and coma "asymmetry" will reveal the dynamics of material ejection.


• MAJIS and UVS (spectroscopy): They target diagnostic lines of water, carbon dioxide, and carbon monoxide, as well as traces of atomic hydrogen and other dissociation products. Comparison of UV and IR spectra will show thermal conditions and jet sources.


• SWI (submillimeter): Provides temperature and kinematics of gases; can separate "slow" and "fast" evaporation components, which is important for CO₂ and H₂O sublimation models.


• PEP (particles and plasma): Measures ionized products of the comet and interaction with the solar wind; signatures of the plasma tail, changes in particle density and energy, and potential discontinuities associated with changes in the solar wind are expected here.

In synergy, these instruments enable "from behavior to composition" analysis: we see how the coma expands, what is in it, how the tails bend and change, and what plasma "stamp" the environment leaves. When ground-based and heliospheric observations (e.g., UV and white light from other missions) are added to this data, a chronicle of the comet's activity post-perihelion is obtained — day by day, instead of a single static "snapshot".

"Anti-tail", plasma tail, and dust: What geometry does to our eye

A comet's tail is not one and the same thing at every moment. The plasma tail of ionized gas almost always extends opposite to the Sun because it is shaped by the solar wind. The dust tail — composed of particles of various sizes — follows the trajectory and can structurally look like an "anti-tail" when the plane of observation aligns with the orbital plane, making it appear that the tail points "toward" the Sun. For 3I/ATLAS after perihelion, precisely such perspective effects are expected: a stronger coma due to thermal inertia and geometrically conditioned changes in dust distribution. Juice's multi-date sets in November are based on the idea that a series of shorter measurements captures this dynamics better than a single long exposure.

What we know about 3I/ATLAS to date

Coordinated measurements from Mars and heliospheric missions (as well as ground-based observatories) confirm that 3I/ATLAS behaves like a natural, active comet. Ultraviolet and infrared signatures indicate the presence of water and carbon dioxide, and the morphology of the coma and tail matches expectations for a body that has recently passed its closest point to the Sun. Although speculations about "anomalies" appear in the public sphere, the agency consensus is based on multiple independent measurements and photometric analyses that correspond to a natural origin. Compared to 1I/ʻOumuamua and 2I/Borisov, 3I/ATLAS provides perhaps the richest multi-mission set to date — and Juice's campaign in November fits into that international mosaic.

Why the distant Juice is still "in the right place"

Although Juice's distance was greater than that of the Martian spacecraft in October, the timing of the November windows makes the difference: immediately after perihelion, the comet's activity is most pronounced. This increases the signal-to-noise for spectroscopic instruments and the chance to capture changes in tails conditioned by the solar wind. In other words, "farther, but louder" — and long enough to compare day-by-day, which is crucial for gas and dust dynamics.

Technical aspect: NavCam and "quick look", then science "in full resolution"

The Navigation Camera on Juice primarily serves for navigation, but the team used it to quickly validate that the comet is within the frame and that activity is visible. Due to link limitations (HGA as a solar shield, transmission via medium-gain antenna), only a partial crop of one NavCam frame was downloaded from the spacecraft — enough to confirm "what awaits us" in the full scientific sets. In the coming weeks of 2026, when geometry and thermal conditions improve, the systematic transmission of complete data series from JANUS, MAJIS, UVS, SWI, and PEP is planned, along with standard calibrations and quality checks.

Timeline: Where we are today and what follows

Viewed from the perspective of December 5, 2025, the sequence is clear: the perihelion of 3I/ATLAS occurred in late October; Martian observations lasted from October 1 to 7; Juice began observations on November 2, and the most favorable pass was on November 4 at approximately 65 million kilometers. The transmission of scientific packages from Juice is planned for February 2026, followed by processing, cross-comparison with data from other missions, and the publication of initial results. Additionally, continued ground and space observations of the comet are expected as its activity declines and it continues its journey toward the outer parts of the system.

The bigger picture of the mission: Juice on the way to Jupiter

The Juice mission was launched in 2023, and entry into the Jupiter system is scheduled for 2031. After a series of gravity assists and a long cruise, the spacecraft will end up in orbit around Ganymede — the first ever around a moon other than Earth's — through a series of close flybys of Europa and Callisto. Scientific goals include ice geology, subsurface oceans, the plasmasphere, and the magnetosphere of the Jupiter system, as well as an assessment of conditions that may indicate potential habitability. November 2025 with 3I/ATLAS is therefore a kind of "bonus window" — a unique opportunity to test the same instruments calibrated for icy moons on an active comet and enrich the database of volatile substance phenomenology in different environments.

What to expect from the results

When all sets arrive and undergo processing, the community will specifically seek answers to four groups of questions:

1. Volatile inventory and thermodynamics: H₂O/CO₂/CO ratios, temperatures and coma expansion velocities, dependence on time since perihelion and heliocentric distance.


2. Morphology and tail dynamics: Does the orientation of the plasma tail change with changes in the solar wind; how is dust distributed and under what conditions is the "anti-tail" visible.


3. Particle-plasma interaction: Energy and spectra of ions/electrons, possibility of detecting discontinuities, relationship with active jets from the nucleus.


4. Non-gravitational accelerations: How much gas and dust ejection "pushes" the nucleus; are ephemeris corrections needed and can parameters stably fit into standard cometary models.

3I/ATLAS is, by all accounts, a natural interstellar comet whose observations we now finally have from multiple points in the Solar System — from Mars, near the Sun, and from the perspective of a spacecraft on its way to Jupiter. Juice's observations from November 2025 should therefore fill the gaps: linking spectra and images with real evolutions of tails and the coma in the weeks following perihelion. This is the kind of "time series" that we often lacked with previous interstellar visitors.

Editor's Note: Due to the limited telemetry window in November, part of the content was validated by a short NavCam clip which served merely as confirmation of the comet's presence and visible activity. Scientific conclusions will be based on complete packages from the instruments JANUS/MAJIS/UVS/SWI/PEP, which arrive when the spacecraft's thermal geometry and communication regime allow.