100 Facts About 3I/ATLAS

3I/ATLAS is only the third confirmed interstellar object ever observed passing through our Solar System, discovered on July 1, 2025 by the survey telescope Asteroid Terrestrial‑impact Last Alert System (ATLAS) in Chile. It follows a hyperbolic, unbound trajectory, which means it didn’t originate in our Solar System — it’s a visitor from another star system. Observations from powerful telescopes show it is an active comet, with an icy nucleus surrounded by a glowing coma of gas and dust. Detailed spectral data reveal the coma is dominated by carbon-dioxide and contains water, carbon monoxide and other compounds — a chemical signature quite different from most Solar System comets. Its nucleus size is uncertain (estimates range from a few hundred meters to a few kilometers), and as it swings past the Sun and heads back into interstellar space, 3I/ATLAS offers scientists a rare chance to study material from beyond our own planetary neighborhood — shedding light on how comets form around other stars and what the building blocks of distant planetary systems might be like.

3I/ATLAS is only the third confirmed interstellar object ever seen passing through our Solar System — after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. It is an active comet following a hyperbolic, unbound trajectory, which means it will not stay in our Solar System and is destined to return to interstellar space.

Below are 100 definitive facts about this extraordinary visitor.

Discovery & Origin (1–20)

3I/ATLAS is the third known interstellar object.
Its official names are 3I/ATLAS and C/2025 N1 (ATLAS).
It was discovered on July 1, 2025.
The discovery was made by the ATLAS telescope in Río Hurtado, Chile.
ATLAS is NASA-funded and designed for planetary defense.
Its temporary internal designation was A11pl3Z.
“3I” denotes the third interstellar (I) object detected.
Its orbit was declared interstellar on July 2, 2025.
Pre-discovery images dating June 5–25, 2025, were later identified.
Those images were initially obscured by the Galactic Center’s star density.
Its approach direction was from Sagittarius, toward the Galactic Center.
Its velocity was far too high to originate in the Solar System.
It arrived on an extremely hyperbolic orbit.
Its hyperbolic excess speed is about 58 km/s.
This is faster than both ‘Oumuamua and Borisov.
It is likely a long-traveled body formed around another star.
Initial estimates place its age at 3–14 billion years, older than the Sun.
It may have originated in the Milky Way’s thick disk.
It is possibly the oldest comet ever observed.
Its incoming path suggests it wandered interstellar space for billions of years.

Physical Characteristics (21–40)

3I/ATLAS is an active comet.
It has a solid icy nucleus.
Its nucleus is surrounded by a coma of gas and dust.
Hubble data indicates its nucleus is <1 km, possibly a few hundred meters wide.
Broader estimates range from 320 m to 5.6 km due to coma interference.
It contains water ice and water vapor.
JWST detected carbon dioxide as a dominant volatile.
Its CO₂ abundance greatly exceeds its water abundance (~8:1 ratio).
CO is also present in the coma.
Carbonyl sulfide (OCS) was detected.
The Very Large Telescope found cyanide gas (CN).
Atomic nickel vapor was detected in unexpected amounts.
Its nickel-to-iron ratio is unusually high.
Its spectrum resembles some carbonaceous meteorites.
It shows heavy isotope enrichment, including possible carbon-13 enhancement.
It produces spiral jets, revealing a rotating nucleus.
These jets indicate cryovolcanic-style activity.
Water activity was observed far from the Sun, at >4 AU.
Early dust activity indicates very volatile-rich materials.
Its coma spans thousands of kilometers.

Trajectory & Speed (41–60)

Its orbit is strongly hyperbolic and unbound.
At discovery, it was traveling ~221,000 km/h.
At perihelion, its speed increased to ~246,000 km/h.
It is faster than ʻOumuamua and Borisov.
It passed the Sun on October 29, 2025, at 1.36 AU distance.
It passed behind the Sun (solar conjunction) on October 21, 2025.
It passed Mars at 0.19 AU on October 3, 2025.
It will pass Earth at 1.8 AU on December 19, 2025.
It will later pass Jupiter at 0.36 AU on March 16, 2026.
This Jupiter encounter may slightly adjust its outbound path.
It will exit the Solar System toward the Gemini constellation.
Its outbound radial velocity will be ~58 km/s.
It cannot be captured by the Sun or planets.
It poses no threat to Earth.
Solar System passage time is roughly 5,000 years inbound and outbound.
Gravity assists from passing stars over eons may have accelerated it.
Its high inclination and direction differ from solar apex expectations.
Its trajectory is one of the most precisely measured for an interstellar object.
Mars-based observations improved its orbit prediction 10-fold.
It will never return once it departs.

Observability & Imaging (61–80)

At discovery, it was around magnitude 17–18 (very faint).
It showed little early activity, causing initial classification uncertainty.
By July 2, 2025, several telescopes confirmed cometary features.
The Nordic Optical Telescope confirmed a diffuse coma.
The Teide Twin Telescope confirmed activity independently.
It brightened to magnitude 10–12 near perihelion.
It was never visible to the naked eye.
It required moderate to large telescopes for imaging.
It temporarily became unobservable during solar conjunction.
It reappeared in early December 2025.
By late December 2025, it faded below magnitude 12.
Amateur telescopes captured it via stacking techniques.
An amateur in Utrecht photographed it at magnitude 10.
Hubble imaged it on July 21, 2025.
JWST analyzed its chemistry in detail.
The Mars Reconnaissance Orbiter (HiRISE) photographed it from millions of miles away.
MAVEN detected hydrogen atom emissions from its water vapor breakdown.
ExoMars Trace Gas Orbiter captured it in a multi-frame GIF sequence.
NASA’s Psyche and Lucy spacecraft also imaged it during cruise phases.
STEREO and PUNCH solar missions observed it near the Sun.

Scientific Importance (81–95)

It offers a rare sample of matter from another star system.
Its chemistry implies some exoplanetary systems produce CO₂-rich comets.
The water detection proves interstellar objects can hold Solar-System-like ices.
Its activity far from the Sun challenges models of comet behavior.
Its composition differs strongly from Solar System comets, showing diversity.
It helps refine models of volatile retention in cold environments.
It hints that interstellar comets may be altered by cosmic rays.
It may carry isotopic signatures from ancient galactic environments.
It contributes to understanding ejected planetesimals from other stellar systems.
It provides new constraints on interstellar object population rates.
Its observational campaign tests planetary defense orbit-determination tools.
It bridges the gap between Solar System comets and extrasolar chemistry.
It supports models of galactic mixing of solid materials.
It informs theories on panspermia (though not evidence of life).
It may influence future mission planning, like ESA’s Comet Interceptor.

Mysteries & Open Questions (96–100)

Its birthplace in the galaxy remains unknown.
Its internal structure is still unmeasured.
The thickness of its cosmic-ray–altered “crust” is uncertain.
Its true nucleus size, shape, and rotation remain imprecisely known.
After it fades, we may never observe another object exactly like it.

100 Facts About 3I/ATLAS