Comet 3I/ATLAS – frequently asked questions

What is comet 3I/ATLAS?

Comet 3I/ATLAS is an interstellar object, meaning that it comes from outside our Solar System. It is only the third of its kind ever observed, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Its shape and behaviour indicate that it is a comet: an icy object releasing dust and gas, with one or more 'tails' of charged particles and dust. 

Why is it important?

These comets are absolutely foreign. Every planet, moon, asteroid, comet and lifeform in our Solar System share a common origin. But interstellar comets are true outsiders, carrying clues about the formation of worlds far beyond our own.

When and how was it detected?

It was first spotted on 1 July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile. Its unusual trajectory immediately raised suspicions that it originated from interstellar space. This was later confirmed by astronomers around the world, and the object was given its name: 3I/ATLAS. 

Why the name ‘3I/ATLAS’? 

Traditionally, comets were named after the person who discovered them. Nowadays, we follow a coded naming system developed by the International Astronomical Union. The first letter indicates the category that the comet falls under – in this case ‘I’ for ‘Interstellar’. The number ‘3’ before the ‘I’ indicates that the comet is the third of its kind. The word ‘ATLAS’ refers to the programme that operates the telescope that first spotted the comet. 

An alternative name for the comet is C/2025 N1 (ATLAS). In this naming style, 'C' indicates the object is a non-periodic comet, '2025' indicates the year of discovery, and 'N1' is code for the comet being the first to be discovered in July. 

Find out more about how comets are named here. 

Can you give some quick facts about the comet? 

Size: a few hundred metres to a few kilometres across 

Speed: roughly 250 000 km/h at closest approach to the Sun – the highest ever recorded for a Solar System visitor 

Age: billions of years old – indicated by its breathtaking speed, evidence that it has been drifting through space for a very long time 

Closest approach to Earth: about 270 million km (19 December 2025)

Closest approach to Mars: 29 million km (3 October 2025)

Closest approach to the Sun: 203 million km (29 October 2025)

Does it pose a danger to Earth?

The closest the comet will come to Earth is about 270 million km – that’s about 1.8 times the distance between Earth and the Sun. During its closest approach to the Earth, it will be on the other side of the Sun. It poses no danger to our planet or any other planets in the Solar System.

Is ESA’s Planetary Defence Office tracking the comet?

Yes. ESA's Planetary Defence Office responded promptly to the discovery, with ESA astronomers contributing to global efforts to track the comet’s path using telescopes in Hawaii, Chile and Australia. They are also looking for evidence of its existence in older data – a process known as ‘precovery’.

The planetary defence team then used data from ESA's ExoMars Trace Gas Orbiter collected in early October to improve the comet's predicted location by a factor of ten.

These efforts are part of ESA’s broader mission to detect, track, and characterise near-Earth objects (NEOs) – though 3I/ATLAS is not considered one, due to its distance from our planet. 

Click here for interactive map of 3I/ATLAS's trajectory in the Orbit Visualisation Tool from ESA's NEO Toolkit. 

During the last couple of months, comet 3I/ATLAS was hidden behind the Sun. As of early November it reappeared in our skies, offering astronomers another window for study.

What is happening to 3I/ATLAS as it travels through the Solar System?

3I/ATLAS is an active comet. Observations by the NASA/ESA Hubble Space Telescope in early August already captured a dust plume ejected from the Sun-warmed side of the comet, and the hint of a dust tail streaming away from its nucleus. Comets originating within our own Solar System display similar behaviour. 

As the comet continued to approach the Sun, it lost more and more mass as frozen gases transform into vapour, carrying dust and ice into space. Observations in late August by the NASA/ESA/CSA James Webb Space Telescope of the coma, or ‘halo’, surrounding the comet already revealed carbon dioxide, water, carbon monoxide, carbonyl sulphide and water ice being released as the comet heated up.

A more recent image from ESA's Juice mission, taken when the comet was near its closest approach to the Sun, shows the coma and a hint of two comet tails. 

Find out more about the structure of a comet here.

Is ESA planning to visit 3I/ATLAS?

The comet made its closest approach to the Sun on 29 October 2025, passing just inside the orbit of Mars. Since that moment, it has been speeding away from the centre of the Solar System. This was far too soon, and the comet is moving far too quickly, to contemplate sending a spacecraft to visit or land on it. Instead, we are observing 3I/ATLAS from a distance with our space telescopes and planetary missions.

What ESA missions are observing 3I/ATLAS?

In the first months after its discovery, the comet was observed by space telescopes close to Earth, including the NASA/ESA Hubble Space Telescope and the NASA/ESA/CSA James Webb Space Telescope. Hubble observed the comet again in December 2025. 

In autumn 2025, ESA turned interplanetary voyagers such as Mars Express, ExoMars Trace Gas Orbiter and the Jupiter Icy Moons Explorer (Juice) in the direction of the comet to make further observations with eyes from all over the Solar System.

In October, 3I/ATLAS passed through the field of view of the ESA/NASA Solar and Heliospheric Observatory (SOHO). It was expected to be too faint for SOHO to see, but detailed image processing and overlaying (or 'stacking') subsequent telescope images ultimately generated this image. 

In late November and early December, X-ray telescopes XRISM and XMM-Newton observed the comet, revealing a diffuse X-ray glow around the comet nucleus. This makes 3I/ATLAS the first interstellar comet to have been observed in X-ray light. 

When and how are ESA’s Mars’s missions observing 3I/ATLAS?

ESA’s Mars Express and ExoMars Trace Gas Orbiter (TGO) observed the comet with several instruments around its closest approach to Mars on 3 October 2025. On that date, 3I/ATLAS was around 29 million km from Mars.

Read more about their observations here.

Read about how ExoMars TGO's observations helped refine the trajectory of the comet here.

When and how is ESA’s Juice mission observing 3I/ATLAS?

Of all ESA interplanetary spacecraft, the Jupiter Icy Moons Explorer (Juice) is likely to have the best view of 3I/ATLAS in a very active state. Juice made observations in November 2025 using several instruments, including cameras, spectrometers and a particle sensor.

As Juice is currently close to the Sun, it is using its main high-gain antenna as a heat shield. It is using its smaller medium-gain antenna to send data back to Earth at a much lower rate. It is also far from Earth, on the other side of the Sun. Therefore, we don’t expect to receive scientific data from Juice’s observations of 3I/ATLAS until February 2026. The team did download a quarter of a Navigation Camera (NavCam) image, which revealed an active comet.

Is ESA planning any missions to visit a comet in the future?

Following in the footsteps of ESA’s Rosetta mission, which landed on Comet 67P in 2014, ESA is currently developing the next-generation comet mission, Comet Interceptor. Comet Interceptor will be the first mission to visit a comet coming directly from the outer reaches of the Sun's realm, carrying material untouched since the dawn of the Solar System. It is also possible – though very unlikely given their rarity – that Comet Interceptor could visit an interstellar comet.

Find out more about Comet Interceptor here.