Chance encounter in space: JANUS camera captures interstellar comet 3I/ATLAS
ESA's Jupiter probe, JUICE, was unexpectedly perfectly lined up for observations of a comet from deep interstellar space
Ideal observation conditions for JUICE: Die Sonde hatte von ihrer Position auf der anderen Seite der Sonne etwa auf Höhe der Bahn des Planeten Venus aus einen guten Blick auf 3I/ATLAS. © ESA/JUICE/JANUS
This image is a composite of red, green and blue filter images of 3I/ATLAS taken by the JANUS camera from a distance of more than 180 million kilometres. The comet appears to glow green because the gases surrounding the nucleus emit light in green wavelengths. Background stars appear in different colours depending on their temperature. © ESA/JUICE/JANUS
The Jupiter Icy Moons Explorer (JUICE) mission, en route to Jupiter on a journey lasting over seven years, was not originally scheduled to conduct scientific operations so early in its voyage. Then, in the summer of 2025, a comet was discovered: the interstellar visitor 3I/ATLAS, heading towards the inner Solar System. JUICE was perfectly positioned to observe the active comet as it approached the Sun. Among the instruments activated was the JANUS camera system, which captured 120 images of 3I/ATLAS and transmitted them back to Earth, revealing details of the comet's coma, as well as various outflows of gas and dust. The images also highlight the outstanding performance of the JANUS camera, to which the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) made significant hardware contributions. JUICE is a mission of the European Space Agency (ESA).
Right place, right time
From autumn 2025 onwards, 3I/ATLAS became unobservable from Earth as it continued its journey around the Sun – precisely during the comet's most active and scientifically interesting phase. Fortunately, the JUICE probe, positioned on the far side of the Sun, near Venus's orbit, had a clear view of 3I/ATLAS. Although the JUICE mission's operational plan did not actually include any scientific activities during the interplanetary journey to the Jovian system, the project team swiftly devised an observation campaign.
Among the instruments involved in this impromptu campaign was JANUS, the multispectral camera on board the JUICE spacecraft. Between 5 and 25 November 2025, the JANUS team designed and conducted a series of observations shortly after the comet's perihelion – the point at which a comet is closest to the Sun. JANUS captured more than 120 images using seven filters across a spectral range of 380 to 1015 nanometres (blue to near-infrared). The closest distance to the comet was 63 million kilometres.
“JANUS has delivered outstanding results much earlier than anticipated, revealing very faint structures in the comet's tail and providing precise radiometric data on the brighter coma near the nucleus,” said Ganna Portyankina from the DLR Institute for Planetary Research in Berlin. Ganna is Co-Principal Investigator (PI) of JANUS and leads the instrument’s operations together with PI Pasquale Palumbo from the Institute for Space Astrophysics and Planetology in Rome.
Typical behaviour despite interstellar origins
Due to the position of the JUICE spacecraft and the receiving antennas on Earth, the image dataset was not downloaded until 19 February 2026. The long wait was rewarded with outstanding images showing, for the first time, the comet's intense activity shortly after perihelion. 3I/ATLAS displayed an extensive coma, within which the comet's nucleus is ‘hidden’, as well as a tail and various morphological features such as jets, jet-like structures, streams, filaments and outgassing events. These were recorded in both panchromatic and composite colour images.
The observations indicate that, despite its interstellar origins, 3I/ATLAS behaves like a typical Solar System comet as it approaches the Sun. The dataset will enable detailed study of the morphological structures, photometric intensity and the evolution of the coma and tail on short and medium timescales. The coma – a cloud of dust and gas – surrounds the nucleus of 3I/ATLAS, which is thought to measure no more than five kilometres, and the two identified tails.
In addition to JANUS, four other JUICE instruments contributed to this campaign. Together, they gathered information that will shed light on the comet's behaviour and composition. The science teams are currently analysing spectrometry data to determine the comet's chemical composition, as well as measurements to characterise the physical properties of the emitted particles.
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German Aerospace Center (DLR)
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DLR press release, 7 April 2026