In July 2025, astronomers detected a faint object moving quickly through the inner Solar System. Within days, orbital calculations confirmed that the object did not belong to the Solar System. It arrived from interstellar space. Astronomers named it 3I/ATLAS. The designation marked it as the third confirmed interstellar visitor ever discovered.

During its passage near the Sun, the European Space Agency’s JUICE spacecraft captured valuable observations of this rare visitor. The spacecraft was not designed to study comets like this. Still, its position in space created an unexpected opportunity. ESA released the findings from these observations in April 2026.

A visitor from beyond the Solar System

The ATLAS survey telescope discovered 3I/ATLAS on 1 July 2025. Soon after discovery, astronomers began measuring its orbit with increasing precision. Its trajectory showed a clear hyperbolic shape. That meant the Sun’s gravity could not hold it permanently. Objects on such paths come from outside the Solar System. They pass through once and continue their journey into interstellar space.

This discovery placed 3I/ATLAS among the rarest known small bodies ever observed. Interstellar objects offer a unique scientific opportunity. They carry material that formed around other stars. Their chemistry preserves clues about distant planetary systems.

Astronomers quickly organised an international observing campaign. Ground-based observatories tracked the comet across many wavelengths. Major space telescopes also joined the effort. ESA coordinated additional observations using several spacecraft already operating across the Solar System. These included Mars Express, the ExoMars Trace Gas Orbiter, and JUICE.

JUICE observed the comet from a unique position in space

The Jupiter Icy Moons Explorer launched in April 2023. Its primary destination is the Jupiter system. The spacecraft will eventually study Europa, Callisto, and Ganymede. During its long cruise toward Jupiter, it travelled through the inner Solar System along a carefully planned trajectory.

In late 2025, that trajectory placed JUICE in an ideal position to observe 3I/ATLAS. Scientists recognised the opportunity quickly. They prepared a targeted observation campaign within a short time.

JUICE observed the comet between 2 November and 25 November 2025. During this period, the spacecraft stayed about 60 million kilometres from the object. That distance still allowed detailed measurements of the coma and surrounding gas environment.

Five instruments took part in the campaign. These included the JANUS optical camera, the MAJIS spectrometer, the Submillimetre Wave Instrument, the Ultraviolet Imaging Spectrograph, and the spacecraft navigation camera system.

JUICE measured strong water vapour emission

One of the most important measurements came from the MAJIS spectrometer. The instrument detected strong water vapour emission from the comet shortly after perihelion.

Scientists measured a release rate near 2,000 kilograms per second. This level of activity places the comet within the range expected for active comets near the Sun. Solar heating triggered the sublimation of surface ice. Gas escaped into space, carrying dust particles outward.

These measurements confirm that the nucleus contains a large amount of volatile material. The activity level also matches behaviour seen in many Solar System comets moving through similar regions near the Sun.

Water emission continued for several days after perihelion. This observation suggests that active surface regions remained exposed to sunlight while the comet moved away from the Sun.

JANUS images revealed a classical comet appearance

The JANUS camera captured detailed images of the comet’s coma and tail structures. These images showed a bright central coma surrounding the nucleus. They also revealed two distinct tails extending away from the object.

One tail formed from ionised gas interacting with the solar wind. This tail pointed away from the Sun. The second tail formed from dust particles released from the nucleus. These particles followed paths shaped by the comet’s motion through space.

The images also revealed faint filaments and streamers inside the coma. These features formed when gas jets lifted dust from active surface regions. Similar structures appear in several well-studied Solar System comets.

The overall appearance surprised many scientists. Despite its interstellar origin, the comet behaved like a familiar member of the Solar System comet population. This similarity supports the idea that comet formation processes may follow comparable patterns in many planetary systems across the galaxy.

JUICE helped improve the comet’s orbital measurements

The spacecraft’s navigation camera system also played a crucial role during the observation campaign. It helped scientists refine measurements of the comet’s trajectory through the Solar System.

At several points during its passage near the Sun, the comet became difficult to observe from Earth. Its position placed it close to the Sun in the sky. This geometry limited ground-based observations. JUICE observed the comet from a different location in space during those periods. These observations improved the accuracy of orbital calculations.

Precise trajectory measurements help scientists understand how gas release affects comet motion. Escaping gas produces small forces that slightly change the orbit over time. Measuring these changes improves predictions of future positions.

Further reading

Right from the discovery of the interstellar comet 3I/ATLAS, DIYP has published several articles on its photographs and features. Here is the list of articles:

Clear skies!

---