Cometa – Foto: Nazarii Neshcherenskyi/istock
NASA, in collaboration with the European Space Agency (ESA), is closely tracking the interstellar comet 3I/ATLAS, the third object of extrasolar origin detected in the Solar System. Identified on July 1, 2025, by the ATLAS telescope in Chile, the comet follows a hyperbolic orbit with an eccentricity above 5, confirming its origin outside our stellar system. It will reach perihelion, its closest point to the Sun, on October 29, 2025, at 1.36 astronomical units, about 203 million kilometers, between the orbits of Earth and Mars. This proximity exposes the nucleus to intense solar radiation, which may evaporate volatile compounds and lead to potential fragmentation, as seen with comet 2I/Borisov in 2020.
Recent images from the Nordic Optical Telescope, captured between July and September 2025, show jets of dust and ice directed toward the Sun. The composition of 3I/ATLAS, rich in carbon dioxide and metals, suggests formation in a cold environment, similar to the Kuiper Belt of another star.
- Speed of 245,000 km/h confirms hyperbolic trajectory.
- Extreme negative polarization is unprecedented in known comets.
- Ecliptic alignment aids tracking by ground-based telescopes.
Discovery and initial trajectory
The ATLAS telescope in Río Hurtado detected 3I/ATLAS at 670 million kilometers from the Sun, with pre-discovery data from NASA’s TESS satellite. Observations confirmed cometary activity at 6.4 astronomical units, with hydroxyl release derived from water.
The active surface, exceeding 8%, is unusual for solar comets, and the nucleus has an estimated diameter between 440 meters and 5.6 kilometers, enveloped by a reddish coma.
Unique chemical composition
Spectral analyses reveal carbon dioxide eight times more abundant than water, suggesting chemical processes in distant protoplanetary disks. The presence of metals like nickel, detected by the James Webb Space Telescope, distinguishes 3I/ATLAS from local comets.
Preserved water ice causes unusual light scattering, and the absence of heavy metals aligns the comet with ancient stars in the Milky Way.
Hubble images from July 2025 showed a faint tail driven by solar radiation. Stable activity indicates significant volatile reserves, with no brightness surges until September.
Current observation challenges
Since October 2025, 3I/ATLAS has been transiting behind the Sun, limiting ground-based observations. The GOES-19 satellite’s CCOR-1 coronagraph captured images on October 21, revealing a gas and dust coma.
Amateur astronomer Worachate Boonplod confirmed an anti-tail in processed data. Ground telescopes will resume observations in November, when the comet appears at dawn in Virgo and Leo, with a magnitude of 11.5.
Proximity to Mars on October 3, at 28 million kilometers, was recorded by ESA’s Trace Gas Orbiter, revealing plasma interactions.
Contributions from space missions
NASA’s Europa Clipper will align with 3I/ATLAS’s ion tail in late October, collecting particles at 300 million kilometers from the Sun. On November 4, ESA’s Juice mission will pass 64 million kilometers away, obtaining multi-spectral data.
The Perseverance rover captured the comet as a bright spot in Mars’ sky on October 4. The James Webb Telescope detected water ice and metals, while NASA’s SPHEREx analyzes ice properties.
Lessons from Borisov’s collapse
Comet 2I/Borisov fragmented in March 2020, after its December 2019 perihelion, with the nucleus splitting into two fragments 180 kilometers apart. Volatile evaporation and accelerated rotation caused the rupture, releasing dust and carbon monoxide.
This event revealed the dispersion of organic molecules between stars, suggesting an origin near red dwarfs. For 3I/ATLAS, telescopes will monitor changes in November, with a passage 0.65 astronomical units from Venus on November 3.
Post-perihelion monitoring
In March 2026, 3I/ATLAS will cross Jupiter’s orbit, 54 million kilometers away, allowing NASA’s Juno probe to record gravitational interactions. Telescopes like Canada-France-Hawaii and Vera C. Rubin will begin astrometry on November 27, 2025.
If fragmentation occurs, dust trails will be detectable by celestial surveys, expanding data on interstellar objects. Global telescope networks ensure continuous tracking despite NASA’s budget constraints.
