Five things Juice has revealed about Comet 3I/ATLAS

In November 2025, Juice was in the right place, at the right time, with the right equipment to observe interstellar comet 3I/ATLAS after its closest approach to the Sun. Our mission operations teams turned on Juice’s five science instruments to gather information about how the active comet was behaving at that time.

After waiting three months to get the data back to Earth, scientists working on each of these instruments have spent the last few weeks poring over photos, spectra and numbers. The results are still preliminary, the work is still ongoing, but here are five things we’ve already learned.

1. The comet was releasing water vapor from 70 Olympic swimming pools every day

On November 2, 2025, just four days after 3I/ATLAS made its closest approach to the Sun, Jus’s Moon and Jupiter Imaging Spectrometer (MAJIS) discovered that the comet was spewing 2000 kilograms of water vapor every second – the equivalent of 70 Olympic swimming pools per day.

Comets – living up to their ‘dirty snowball’ nickname – are made mostly of ice. As they get closer to the Sun, this ice turns into gas and escapes from the comet. The amount of water vapor leaving 3I/ATLAS is not extraordinary, but it is on the high side of what we would expect from a comet close to the Sun, given what we have previously seen in comets like 67P (300 kilograms per second) and Halley (20 000 kilograms per second).

These numbers depend largely on the comet’s size and its distance from the Sun. MAJIS detected 3I/ATLAS again on 12 and 19 November, as it was moving away from the Sun. By November 12, the amount of water vapor being released by the comet did not decrease significantly. The instrument team plans to analyze data from November 19 in the coming weeks.

2. Most of this water vapor was being released in the direction of the Sun

Jus’s submillimeter wave instrument (SWI) also detected water vapor from 3I/ATLAS, which showed that most of it was coming from the Sun-facing side of the comet. It also appears that much of this water vapor is not actually coming directly from the solid part of the comet (its nucleus), but from icy dust particles that have escaped into the surrounding halo of dust and gas (its coma).

The SWI team is looking at data to determine how much ‘light’ water (normal H₂o) 3i/Atlas is being released. It is interesting to compare this with the amount of ‘semiheavy’ water (HDO) from the comet, measured by the ALMA and Webb telescopes. This ratio is a really important number in our study of the universe, providing a kind of ‘fingerprint’ of how and where an object formed.

ALMA and Webb found this ratio unexpectedly and much higher for 3I/ATLAS – possibly because the comet was born in a very cold and very ancient environment, where it was exposed to much more ultraviolet radiation from young stars. The SWI team is investigating whether the juice data supports these findings.

3. Gas and dust extend at least 5 million km from the comet’s nucleus

Jus’s ultraviolet imaging spectrograph (UVS) captured light coming from oxygen, hydrogen and carbon atoms in the gas and dust moving around and behind the comet. Oxygen, hydrogen, carbon and dust emit photons of light at specific wavelengths, which are recorded by the UVS as counts per second.

UVS observed these gas elements and dust extending more than 5 million km from the nucleus of 3I/ATLAS. Gas and dust are common around active comets, whose tails sometimes reach up to 10 million km long.

4. This interstellar comet looks just like a normal comet!

Jupiter’s high-resolution science camera, Janus (short for ‘Jovis Amorum AC Natorum Undic Scrutator’ – or ‘Investigator of Jupiter, and all its loved ones and descendants’), also spotted 3I/ATLAS spewing gas and dust.

Despite being more than 60 million km away from 3I/ATLAS, JANUS clearly reveals the coma in which the nucleus is hidden, as well as two tails. One tail extends away from the Sun, and the other follows the path taken by the comet through the Solar System. We can also see faint shapes within the coma and tail that indicate various processes and interactions with radiation, particles, and magnetic fields from the Sun. The JANUS team is currently investigating these shapes in more detail.

Overall, JANUS shows that, despite its interstellar origin, Comet 3I/ATLAS was behaving like a typical comet of the Solar System during its approach to the Sun.

5. 3I/ATLAS is supporting our planetary defense efforts – maybe not in the way you might think

Juice’s navigation camera (NavCam) is specifically designed to help Juice navigate around Jupiter’s icy moons following its arrival in 2031. The encounter with 3I/ATLAS allowed us to do something completely unexpected with it.

We have already used telescopes on and around Earth to estimate the location and path of Comet 3I/ATLAS through the Solar System. It appears to have come from the direction of the galaxy’s disk, and so was probably formed more than 10 billion years ago.

NavCam observed 3I/ATLAS at a much closer angle than Earth-based telescopes, from a different angle, and when the comet was not visible from Earth. This meant that ESA’s Planetary Defense team could line up NavCam images from throughout November to get a better idea of ​​the comet’s changing position and trajectory.

In this way, the team – which usually tracks potentially dangerous asteroids – showed how powerful observations made from deep space missions can be in accurately calculating the orbits of comets or asteroids that cannot be immediately seen from Earth.

What’s more, because the comet’s trajectory is slightly affected by the ejection of dust and gas, the team is beginning to use trajectory measurements based on NavCam images to understand what material – and how much of it – the comet is leaving behind.

What’s next for Juice?

The instrument teams will continue to study the data, with several teams planning to publish papers on their results in the coming months.

“3I/ATLAS is a rare and unexpected visitor, its arrival was a complete surprise,” says ESA JUS project scientist Olivier Vitesse. “But when we realized that Juice would be at the closest comet ever to the Sun, we realized what a unique opportunity this was to collect a once-in-a-lifetime dataset.”

He adds: “Observing the comet was challenging, with no guarantees of success, but in the end, it turned out to be a great bonus for Juice during the trip to Jupiter.”

The closest Jus to 3I/ATLAS was about 60 million km away, while it will view Jupiter’s moons from only a few hundred kilometers away. Nevertheless, being designed and equipped to study icy moons, Jus’s instruments were a great match for icy interstellar comets.

We still have a five-year wait before Juice reaches Jupiter in 2031, but all of its instruments will be operational once again in September 2026 when Juice returns to Earth for another gravity assist.

“The data we’re already seeing from the Juice devices is really promising,” says co-project scientist Claire Vallat. “We’re more excited about how well they work and how much we’ll uncover about Jupiter and its icy moons in the 2030s.”