Astronomers at the ATLAS system detected the interstellar comet 3I/ATLAS on July 1, 2025, at the Río Hurtado observatory in Chile. The object, the third confirmed to originate outside the Solar System, follows a hyperbolic orbit with an eccentricity exceeding 5, confirming its interstellar origin.
Its velocity relative to the Sun reaches 58 km/s, surpassing predecessors 1I/ʻOumuamua and 2I/Borisov. The comet will reach perihelion on October 29, 2025, at 1.4 astronomical units from the Sun, near Mars’ orbit.
This proximity exposes the nucleus to intense radiation, sublimating volatile ices and potentially causing fragmentation. Initial observations show early activity, with gas emissions detected beyond 6 astronomical units.
Estimated nucleus diameter: between 440 meters and 5.6 kilometers.
Dominant composition: carbon dioxide, eight times more abundant than water.
Approximate age: over 7 billion years, older than the Solar System.
36 hours for 3I/ATLAS reappearance from behind the Sun and fact is, no one knows what we will see emerging.Stay tuned 🛸 eyes to the skies.#3IATLAS pic.twitter.com/Uv6zYnIcTZ— Astronomy Vibes (@AstronomyVibes) October 27, 2025
Early activity and unexpected emissions
The comet displays a reddish, elongated coma since July 2025. Telescopes like the Nordic Optical captured dust and ice jets directed toward the Sun between July and September.
This solar-facing configuration differs from typical tails and suggests the ejection of heavy particles, above hundreds of micrometers. Solar radiation drives sublimation, forming a coma spanning thousands of kilometers.
Chemical composition reveals distant origins
Spectral analyses from the James Webb Space Telescope identified carbon dioxide, water, carbon monoxide, carbonyl sulfide, and water ice in the coma. The low water vapor abundance suggests possible thermal inhibition in the nucleus.
This high CO2 proportion indicates formation near the carbon dioxide ice line in its parent protoplanetary disk. Metals like nickel, detected in emissions, distinguish the object from local comets.
The extreme negative polarization, unprecedented in known comets, points to fine dust and elevated radiation processes during its interstellar journey.
Observations from multiple space missions
The European Space Agency and NASA coordinate tracking with probes like ExoMars Trace Gas Orbiter and Mars Express. In October 2025, these spacecraft captured images of the coma from thousands of kilometers, without resolving the nucleus due to distance.
NASA’s Swift Observatory measured hydroxyl production at 40 kg/s, comparable to a fire hydrant at full flow, even at three times the Earth-Sun distance. This early rate suggests a complex structure, possibly with detaching ice fragments.
SPHEREx data confirm CO2 signatures, reinforcing classical cometary activity.
The Juice probe, near the Sun, used a medium-gain antenna for observations, with data analysis expected by February 2026 due to solar conjunction.
Fragmentation risks at perihelion
Exposure to 33 gigawatts of solar radiation may accelerate uneven sublimation. Ground-based telescopes, like the 2.5-meter in the Canary Islands, recorded a transition from anti-tail to tail in September 2025.
Resulting fragments would be trackable by celestial surveys, expanding data on interstellar objects. The estimated mass exceeds 33 billion tons, with undetectable non-gravitational recoil.
Global monitoring and recent alignments
Telescope networks ensure continuous monitoring, despite a solar conjunction on October 21, 2025. The GOES-19 satellite observed the object during the event.
On October 27, 3I/ATLAS aligned with the Sun, comets Lemmon and R2 SWAN, plus Venus and K1/ATLAS. This configuration aids studies of dynamic interactions.
The object poses no threat to Earth, with a closest approach of 1.8 astronomical units in December 2025.
Post-perihelion trajectory and scientific legacy
After perihelion, the comet will accelerate out of the Solar System. Post-event observations in December 2025 will provide final clues about its integrity.
Low water activity may indicate internal thermal barriers, suppressing sublimation relative to CO2. These findings reshape models of formation in exoplanetary systems.
Hubble and Gemini studies reveal fine dust ejection, consistent with prolonged interstellar radiation exposure.
