Mark McCaughreanJul 3

Welcome to the neighbourhood, 3I/ATLAS, if only briefly 😬✌️

A new interstellar object has been found skimming through our solar system, only the third after 1I/'Oumumua & 2I/Borisov ☄️

This new object, detected by the ATLAS telescope in Chile, will make its closest approach to the Sun in late October, inside the orbit of Mars, & then barrel out again at more than 200,000 kmh 👀

More at: https://www.esa.int/ESA_Multimedia/Images/2025/07/ESA_observes_interstellar_comet_3I_ATLAS

#SpaceScience

Image credit: NASA/JPL-Caltech

Show threadMark McCaughrean

3I/ATLAS is thought be around 20km in size, so considerably larger than either of the previous interstellar objects.

It's currently out near the orbit of Jupiter, but there have been reports of some faint comet-like activity – that should become more apparent in the weeks months to come.

Jul 3 at 4:31pmWeb
Show threadMark McCaughreanJul 3

This is potentially excellent news for ESA's Comet Interceptor mission, due to launch later this decade.

It adds further weight to the idea that these interstellar interlopers may be quite common, raising the chances that Comet Interceptor will be able to target a flyby past one from its Sun-Earth L2 loiter orbit.

And with the Rubin Observatory now online, surveying the whole southern sky every three nights, many more such objects may soon be discovered.

Show threadMark McCaughreanJul 3

Some further information here, including the possibility that 3I/ATLAS is in fact smaller than 20km in size, if it's already an active comet – the outgassing material would make it look brighter for a given size of nucleus.

https://www.theguardian.com/science/2025/jul/03/nasa-interstellar-comet-solar-system-a11pl3z-3i-atlas

Scientists spot mystery object believed to come from beyond solar system

Astronomer says object could be further evidence that ‘interstellar wanderers’ are common in galaxy

The Guardian