ESA tracks rare interstellar comet

A visitor from beyond the void

The comet 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 formal designation: 3I/ATLAS, indicating its status as the third known interstellar object.

3I/ATLAS is approximately 670 million kilometres from the Sun and will make its closest approach in late October 2025, passing just inside the orbit of Mars. It is thought to be up to 20 kilometres wide and is travelling roughly 60 km/s relative to the Sun. It poses no danger to Earth, coming no closer than 240 million kilometres – over 1.5 times the distance between Earth and the Sun.

ESA tracks the interstellar interloper

ESA’s Planetary Defence Office responded promptly to the discovery. Automated detection systems alerted ESA’s astronomers, who are contributing to global efforts to track the comet’s path and to find evidence of its existence in older data – a process known as ‘precovery.’

ESA astronomers are using telescopes in Hawaii, Chile, and Australia to monitor the comet’s progress. Some of these telescopes are owned by ESA, others are provided as part of longstanding partnerships.

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

Scientists will now be interested in learning more about this interstellar visitor’s composition and behaviour. 

3I/ATLAS is an active comet. If it heats up sufficiently as it nears our star, it could begin to sublimate – a process in which frozen gases transform directly into vapor, carrying dust and ice particles into space to form a glowing coma and tail.

However, by the time the comet reaches its closest point to Earth, it will be hidden behind the Sun. It is expected to reappear by early December, offering astronomers another window for study.

A remnant from a distant world

What makes interstellar objects like 3I/ATLAS so extraordinary is their absolutely foreign nature. While every planet, moon, asteroid, comet and lifeform that formed in our Solar System shares a common origin, a common heritage, interstellar visitors are true outsiders. They are remnants of other planetary systems, carrying with them clues about the formation of worlds far beyond our own.

It may be thousands of years until humans visit a planet in another solar system and interstellar comets offer the tantalising opportunity for us to touch something truly otherworldly.

These icy wanderers offer a rare, tangible connection to the broader galaxy – to materials formed in environments entirely unlike our own. To visit one would be to connect humankind with the Universe on a far greater scale.

ESA mission will intercept the unknown

To this end, ESA is preparing the Comet Interceptor mission. The spacecraft will be launched in 2029 into a parking orbit at the Sun–Earth Lagrange Point 2 (L2), lying in wait for a suitable target – a pristine comet from the distant Oort Cloud that surrounds our Solar System, or, unlikely but highly appealing, an interstellar object.

While it is improbable that we will discover an interstellar object that is reachable for Comet Interceptor, as a first demonstration of a first rapid response mission that waits in space for its target, it will be a pathfinder for possible future missions to intercept these mysterious visitors.

Whether it’s 1I/‘Oumuamua, 2I/Borisov, 3I/ATLAS or the future target of an interceptor mission, each new interstellar visitor reminds us that we are part of a vast and dynamic galaxy – and that sometimes, the Universe comes to us.