02/04/2026
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‘Excessive however not unique,’ – a glimpse at Comet 3I/ATLAS via the eyes of the European Area Company’s Jupiter Icy Moons Explorer (Juice).
In November 2025, Juice was in the best place, on the proper time, with the best tools to watch interstellar comet 3I/ATLAS simply after its closest strategy to the Solar. Our mission operations groups switched on 5 of Juice’s science devices to gather details about how the lively comet was behaving on the time.
Following a three-month wait to obtain the info on Earth, scientists engaged on every of those devices have spent the previous few weeks delving into the photographs, spectra and numbers. Outcomes are nonetheless preliminary, work continues to be ongoing, however listed below are 5 issues we’ve already realized.
1. The comet was releasing 70 Olympic swimming swimming pools of water vapour each day
On 2 November 2025, simply 4 days after 3I/ATLAS had made its closest strategy to the Solar, Juice’s Moons And Jupiter Imaging Spectrometer (MAJIS) detected that the comet was spewing out 2000 kg of water vapour each second – equal to 70 Olympic swimming swimming pools per day.
Comets – true to their ‘soiled snowballs’ nickname – are principally manufactured from ice. As they strategy the Solar, this ice turns to gasoline and escapes the comet. The quantity of water vapour leaving 3I/ATLAS isn’t distinctive, however it’s on the excessive facet of what we might anticipate from a comet near the Solar, primarily based on what we’ve seen earlier than in comets like 67P (300 kg per second) and Halley (20 000 kg per second).
These numbers rely so much on the scale of a comet and its distance from the Solar. MAJIS detected 3I/ATLAS once more on 12 and 19 November, because it was shifting away from the Solar. By 12 November, the quantity of water vapour being launched by the comet didn’t appear to have lowered considerably. The instrument crew is planning to analyse the info from 19 November within the coming weeks.
2. Most of this water vapour was being released in the direction of the Sun
Juice’s Submillimeter Wave Instrument (SWI) also detected water vapour from 3I/ATLAS, revealing that most of it was being released from the Sun-facing side of the comet. It also appears that a lot of this water vapour is not actually coming directly from the solid part of the comet (its nucleus), but from icy dust grains that have escaped into a surrounding halo of dust and gas (its coma).
The SWI team are continuing to look into the data to determine how much ‘light’ water (normal H2O) 3I/ATLAS is releasing. It is interesting to compare this to the amount of ‘semiheavy’ water (HDO) from the comet, which has been measured by the ALMA and Webb telescopes. This ratio is a very necessary quantity in our research of the Universe, giving a type of ‘fingerprint’ that describes how and the place an object fashioned.
ALMA and Webb discovered this ratio to be unexpectedly and very excessive for 3I/ATLAS – presumably as a result of the comet was born in a really chilly and really historic setting, the place it was uncovered to a number of ultraviolet radiation from younger stars. The SWI crew is investigating whether or not the Juice knowledge again up these findings.
3. Gasoline and dirt stretch a minimum of 5 million km from the comet’s nucleus
Juice’s Ultraviolet Imaging Spectrograph (UVS) captured mild coming from oxygen, hydrogen and carbon atoms within the gasoline and dirt surrounding and trailing behind the comet. Oxygen, hydrogen, carbon and dirt emit photons of sunshine at particular wavelengths, which UVS recorded as counts per second.
UVS noticed these gasoline parts and dirt stretching as much as greater than 5 million km from 3I/ATLAS’s nucleus. Gasoline and dirt are widespread round lively comets, with tails generally reaching as much as 10 million km lengthy.
4. This interstellar comet looks… just like a normal comet!
Juice’s high-resolution science camera, JANUS (short for ‘Jovis Amorum ac Natorum Undique Scrutator’ – or ‘Scrutiniser of Jupiter, and all his loves and descendants’) also saw 3I/ATLAS spewing gas and dust.
Despite being over 60 million km from 3I/ATLAS, JANUS clearly reveals the coma in which the nucleus is hiding, as well as two tails. One tail stretches away from the Sun, and the other follows the path taken by the comet through the Solar System. We can also see fainter shapes within the coma and tails that indicate various processes and interactions with the radiation, particles and magnetic field 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 from the Solar System during a close approach to the Sun.
5. 3I/ATLAS is supporting our planetary defence efforts – perhaps not in the way you might think
Juice’s Navigation Camera (NavCam) is specially designed to help Juice navigate around Jupiter’s icy moons following arrival in 2031. The encounter with 3I/ATLAS allowed us to do something totally 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 seems to come from the direction of the Milky Way’s disc, and was therefore likely created more than 10 billion years ago.
NavCam had a much closer view of 3I/ATLAS, from a different angle to Earth-based telescopes, and when the comet was not visible from Earth. This meant that ESA’s Planetary Defence 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 hazardous asteroids – showed how powerful observations from deep-space missions can be to precisely calculate the orbits of comets or asteroids that cannot immediately be seen from Earth.
What’s more, because a comet’s trajectory is affected slightly by the release of dust and gas, the team is starting to use the trajectory measurements based on NavCam images to understand what materials – and how much of them – the comet is leaving in its wake.
What’s next for Juice?
Instrument teams will continue to study the data, with many teams planning to publish papers on their results in the coming months.
“3I/ATLAS is a rare and unexpected visitor, its arrival came as a complete surprise,” says Olivier Witasse, ESA Juice Project Scientist. “But when we realised that Juice would be close to the comet around its closest approach to the Sun, we realised what a unique opportunity this was to collect a once-in-a-lifetime dataset.”
He continues: “Observing the comet was challenging, with no guarantee of success, but in the end, it turned into a great bonus for Juice during its journey to Jupiter.”
The closest Juice came to 3I/ATLAS was about 60 million km, whereas it will see Jupiter’s moons from just a few hundred kilometres away. Even so, being designed and equipped to study icy moons, Juice’s instruments were a great match for the icy interstellar comet.
We still have five years to wait before Juice arrives at Jupiter in 2031, but all its instruments will be switched on once again in September 2026 when Juice returns to Earth for another gravity assist.
“The data we are already seeing from Juice’s instruments is really promising,” says co-Project Scientist Claire Vallat. “We are getting more excited about how well they work and how much we will reveal about Jupiter and its icy moons in the 2030s.”
