The interstellar comet 3I/ATLAS, detected in July 2025 by the ATLAS telescope in Chile, triggered Nasa’s planetary defense protocols due to its unusual orbital behavior.
The International Asteroid Warning Network (IAWN) coordinates a training exercise from November 27, 2025, to January 27, 2026, to refine measurements of its hyperbolic trajectory.
The object, the third confirmed interstellar body, travels at over 210,000 km/h and reaches perihelion on October 29, 2025, at 210 million km from the Sun.
Its peculiar chemical composition, observed by the James Webb Space Telescope (JWST), justifies intensive monitoring without posing a threat to Earth.
Discovery and initial detection
The ATLAS telescope, funded by Nasa and located in Río Hurtado, Chile, identified 3I/ATLAS on July 1, 2025, near the constellation of Sagittarius.
Preliminary observations confirmed its extrasolar origin by its speed of 61 km/s, which prevents gravitational capture by the Sun.
The name “3I” indicates the third interstellar object, following ‘Oumuamua in 2017 and Borisov in 2019.
Data from the Harvard Minor Planet Center, via bulletin MPEC 2025-U142 on October 21, reinforced the need for international alerts.
Chemical composition revealed by JWST
The JWST captured data on August 6, 2025, with its Near-Infrared Spectrograph, showing a coma dominated by carbon dioxide (CO2).
This concentration, eight times higher than water, exceeds expected variations in solar comets by six times.
The presence of carbon monoxide, carbonyl sulfide, and water ice in minimal amounts suggests formation in cold conditions of another star system.
- CO2 level in the coma: over 80% of total detected gases.
- CO2/water ratio: 8:1, unprecedented in prior observations.
- OH (hydroxyl) emissions: active at over 450 million km from the Sun.
- Nickel vapor detected: indicates early sublimation of metals.
These findings, analyzed in an available preprint, highlight chemical differences between interstellar and local comets.
Trajectory and monitoring challenges
The hyperbolic orbit of 3I/ATLAS brings it to a minimum distance of 270 million km from Earth, with no collision risk.
During solar conjunction on October 21, 2025, the object is obscured by sunlight, limiting ground-based observations until December.
IAWN emphasizes “unique challenges” to predict variations, such as dust ejections altering the tail toward the anti-solar direction.
Probes like Europa Clipper and Hera may cross its tail between October 30 and November 6, 2025, enabling particle detection.
Researchers from the University of California and Oslo noted material release proportional to solar radiation, with dust and gas jets pointing toward the Sun in images from August 2, 2025, by the Two-Meter Twin Telescope in Spain.
This activity, unusual at such distances, accelerates the race for data before exiting the Solar System in March 2026, after passing Jupiter.
The nucleus, estimated between 320 meters and 5.6 km in diameter by Hubble on July 21, 2025, remains stable without detected outbursts until August.
Age and estimated origin
Computational models indicate 3I/ATLAS is over seven billion years old, predating the Solar System’s formation 4.6 billion years ago.
Possibly formed in the Milky Way’s thick disk, the comet was ejected by gravitational interactions and wandered for millions of years.
TESS observations from May 7 to June 3, 2025, reveal early cometary activity at 6.4 astronomical units from the Sun, linked to sublimation of non-aqueous volatiles.
The ESA’s Juice probe will observe it in February 2026 at peak activity, providing infrared and submillimeter spectra to map gaseous emissions.
Observations by multiple telescopes
Hubble recorded the drop-shaped nucleus with dust on July 21, 2025, at 445 million km from Earth.
SPHEREx observed from August 7 to 15, 2025, measuring thermal properties and dust composition.
Mars rovers, such as Perseverance and Curiosity, capture indirect data via reconnaissance orbits.
- Telescopes involved: Hubble, JWST, TESS, Swift, SOHO.
- Initial detection distance: 670 million km from the Sun.
- Average speed: 221,000 km/h.
- Visible reappearance: early December 2025.
These collective contributions enhance models of planetary formation in distant systems.
IAWN training for astrometry
IAWN’s astrometry campaign, from November 2025 to January 2026, tests methods for precise observations of active comets.
Focus on extracting orbital positions despite diffuse comas and variable ejections.
Global participants, including ESA and Asian agencies, share data to validate trajectories.
The exercise uses 3I/ATLAS as a practical case due to its prolonged Earth visibility.
The initiative improves protocols for future interstellar objects, integrating ground-based and space telescopes in real time.
