Comet 3I/ATLAS
The third visitor from another star, found in July 2025 and watched by a fleet of spacecraft across the Solar System.
Key facts
- Designation
- 3I/ATLAS (C/2025 N1)
- Type
- Interstellar comet
- Orbital period
- none: it is on a one-way, hyperbolic path
- Closest to the Sun
- 1.36 AU (about 203 million km), on 29 October 2025
- Farthest from the Sun
- none; it is leaving for good
- Tilt of orbit
- about 175° (almost exactly backwards in the plane of the planets)
- Nucleus
- probably a few kilometres or less; Hubble images suggest about 2 to 3 km
- Discovered
- by the ATLAS survey telescope at Río Hurtado, Chile, on 1 July 2025
- Returns
- never: it is heading out of the Solar System
Orbit
3I/ATLAS is moving faster than either earlier interstellar visitor, at about 58 km/s relative to the Sun far from it. Its orbit has an eccentricity of about 6, very far from a closed ellipse. It travels almost exactly in the plane of the planets, but backwards. It passed Mars at about 29 million km on 3 October 2025, the Sun on 29 October 2025, and Earth at about 270 million km in December 2025, then went on past Jupiter in March 2026. It was never any danger to Earth.
Size and nature
When it was found it already had a coma of dust, so it was clearly a comet. Telescopes found carbon dioxide in large amounts, water, and atoms of nickel in its gas. Its high speed suggests that it comes from an old part of the Milky Way, and it may be older than the Sun, though that cannot yet be tested.
History
The ATLAS survey, which scans the sky for asteroids that might approach Earth, picked it up on 1 July 2025 when it was inside Jupiter's orbit. Earlier survey pictures from June showed it too. It was given the letter I as the third interstellar object, after 'Oumuamua in 2017 and Borisov in 2019.
Missions
No spacecraft could be sent to meet it, but many already in space turned to look. Orbiters at Mars, including Mars Express, observed it as it passed that planet. The asteroid probes Psyche and Lucy photographed it on their way to their targets, and spacecraft that watch the Sun, including SOHO, saw it while it was hidden from Earth behind the Sun's glare.
Space missions
- Mars Express Europe's long-lived Mars orbiter, which has made the closest flybys of the moon Phobos.
- Psyche The first visit to a world rich in metal, which may be the exposed core of a shattered planet.
- Lucy A twelve-year tour of the Trojan asteroids, possible fossils of planet formation.
- SOHO A solar observatory that has become the greatest comet discoverer in history.
- Comet Interceptor A mission that will wait in space for a pristine comet, then split into three to meet it.
Key dates
| Date | What happened |
|---|---|
| 1 July 2025 | The ATLAS survey discovered 3I/ATLAS, the third known interstellar object. |
| 3 October 2025 | The interstellar comet 3I/ATLAS passed Mars at about 29 million km. |
| 29 October 2025 | The interstellar comet 3I/ATLAS passed closest to the Sun. |
Did you know?
It was the first interstellar object watched from spacecraft at another planet.
Its path crosses the planets' plane almost flat, so it passed several worlds in turn.
The I in its name stands for interstellar; the 3 means it was the third.