3I/ATLAS

At a glance

Key points

Its water is rich in deuterium and it may be older than the Sun.

• Most hyperbolic orbit ever measured (e = 6.14)
• Water about 30× richer in deuterium than our comets
• Possibly older than the Sun

Where it is now

As of October 2026, 3I/ATLAS lies about 11.8 AU from the Sun in the direction of Gemini, beyond Saturn's orbit and pulling away at nearly 59 km/s. It made its closest pass to Earth on 19 December 2025, at about 1.8 AU, and swept within about 0.36 AU of Jupiter on 16 March 2026. Its activity has faded with distance from the Sun. It is not bound to the Sun and will never return.

Seen from other worlds

From October to early December 2025 the comet sat too close to the Sun in Earth's sky to observe, so spacecraft elsewhere had the better view. ESA's ExoMars Trace Gas Orbiter imaged it from Mars orbit in early October 2025, about ten times closer than Earth-based telescopes; those were the first positions measured from another planet accepted by the Minor Planet Center, and they sharpened its predicted path tenfold. In November ESA's Juice, en route to Jupiter, watched it with five instruments from about 60 million km.

A typical comet, in some ways

Juice's spectrometer estimated a water output of about 2,000 kg per second soon after perihelion: high, but within the range of Solar System comets, between 67P (about 300 kg/s) and Halley (about 20,000 kg/s). Its camera showed a coma with two tails. The chemistry is what sets it apart: the carbon dioxide, the methane and the deuterium-rich water.

Chemistry from another star

JWST found a coma dominated by carbon dioxide rather than water, and later the first methane seen on an interstellar object. ALMA measurements reported in 2026 put its water's deuterium-to-hydrogen ratio about 30 times that of Solar System comets, pointing to a colder, darker birthplace.

Watched by a fleet

Hubble, JWST, SPHEREx, Swift and spacecraft at Mars, including MRO and Perseverance, all observed it. Europa Clipper and Psyche turned their instruments toward it on their way out. NASA also lists TESS, MAVEN, Lucy, Parker Solar Probe, PUNCH and the ESA/NASA SOHO observatory among the craft that recorded it.

What we still don't know

Its true size, and which part of the galaxy it came from. Kinematic estimates suggest an age over 7 billion years.

Missions

1. 1990–

   Hubble Space Telescope

   NASA / ESAActive

   Ultraviolet to near-infrared imaging from low Earth orbit.

   Mission page (opens in a new tab)

2. 2021–

   James Webb Space Telescope

   NASA / ESA / CSAActive

   Infrared observatory at Sun–Earth L2, about 1.5 million km from Earth.

   Mission page (opens in a new tab)

3. 2023–

   JUICE

   ESAEn route

   Earth flyby Sep 2026, another in Jan 2029; Jupiter arrival Jul 2031, then orbit Ganymede.

   Mission page (opens in a new tab)

Related entries

• 1I/ʻOumuamuaThe first known interstellar visitor
• 2I/BorisovThe first interstellar comet
• JUICEESA's Jupiter Icy Moons Explorer

Sources

1. Salazar Manzano et al. 2026, Nature Astronomy (opens in a new tab)
2. NASA Science: 3I/ATLAS (opens in a new tab)
3. JPL Small-Body Database: 3I/ATLAS (opens in a new tab)
4. arXiv 2603.00782 (opens in a new tab)
5. NASA Science: Webb detects methane on 3I/ATLAS (opens in a new tab)
6. NASA JPL: Horizons ephemeris system (opens in a new tab)
7. ESA: ESA pinpoints 3I/ATLAS's path with data from Mars (opens in a new tab)
8. ESA: Five things Juice has revealed about Comet 3I/ATLAS (opens in a new tab)

Last reviewed 10 Oct 2026 against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.