NASA confirmed the detection of comet 3I/ATLAS on July 1, 2025, by the ATLAS telescope in Chile, as the third interstellar object to cross the Solar System. This celestial body, originating from another star system, travels at 209 thousand kilometers per hour and poses no collision risk to Earth. Astronomers monitor its approach to the Sun, scheduled for October 29, at 210 million kilometers away.
The comet displays intense activity, with jets of dust and gas expelled toward the Sun, forming a tail about 10 thousand kilometers long. Its composition, rich in carbon dioxide, challenges traditional cometary formation models and suggests origins in distant cold regions.
Preliminary studies indicate that 3I/ATLAS may be over seven billion years old, predating the Sun’s formation. The US space agency activated observation protocols to analyze its evolution during the passage.
Designation and origins of 3I/ATLAS
The prefix “3I” marks the comet as the third confirmed interstellar object, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. The ATLAS acronym refers to the NASA-funded terrestrial impact alert system responsible for the initial discovery.
Its hyperbolic orbit confirms the trajectory external to the Solar System, with entry recorded in May 2025, at 6.4 astronomical units from the Sun. Observations from NASA’s TESS satellite detected early activity in this phase.
Initial detection process
Telescopes in Chile and Spain captured images of the comet in July, revealing a diffuse coma with reddish color, indicative of organic dust. Pre-discoveries from the Zwicky Transient Facility extend records to June 2025.
Confirmation of cometary activity occurred on July 2, with subtle elongations suggesting a tail. These initial data allowed precise calculations of speed and direction.
Spectroscopic analyses identified elevated CO2 levels in the coma, eight times higher than water, altering orbital predictions. NASA integrated these measurements to refine future detection models.
Chemical composition revealed
The coma of 3I/ATLAS is dominated by carbon dioxide, with low water presence, possibly due to thermal barriers in the nucleus. Observations from the Swift telescope captured ultraviolet brightness from hydroxyl in July and August, confirming limited ice decomposition.
Studies from August 2025, published by European astronomers, noted progressive reddening of the coma throughout July, a sign of surface evolution. This feature aligns with comets like 2I/Borisov, but with greater intensity.
The nucleus diameter ranges from 300 meters to 11 kilometers, with mass estimated in billions of tons, according to Hubble data. The emitted dust, from micrometric grains, contributes to the observed reddish coloration.
Infrared instruments from the James Webb Telescope analyze organic materials, such as irradiated tholins, to map the original formation environment. These preliminary results indicate processes in cold protoplanetary disks.
Observations by space missions
The comet passed near Mars on October 3, 2025, at 28 million kilometers, recorded by probes like Trace Gas Orbiter and Perseverance. Images show the object as a bright point in the Martian sky.
In November, ESA’s Juice probe will attempt captures with cameras and spectrometers, coordinating with NASA’s Europa Clipper. Particles from the ion tail may interact with Clipper instruments in late October.
- Hubble Telescope: July images reveal solid ice nucleus with drop-shaped dust envelope.
- James Webb: Infrared spectroscopy detects volatile compounds at 277 million kilometers.
- Vera C. Rubin: June to July observations measure coma growth from 13 thousand to 18 thousand kilometers.
The passage by Jupiter occurs in March 2026, allowing additional analyses by Juice before interstellar exit.
Trajectory and continuous monitoring
The speed of 209 thousand km/h accelerates the comet toward perihelion, with no outbursts detected until August 2025. NASA coordinates with the International Asteroid Warning Network for training exercises from November 2025 to January 2026.
After October, the object crosses Jupiter’s orbit and leaves the Solar System, becoming unobservable from ground telescopes in December. Apps like Sky Tonight guide amateur visualizations, with magnitude reaching 12.
The global campaign integrates data into the interstellar objects catalog, predicting annual detections with the future LSST. These efforts improve planetary defense protocols for similar visitors.
Activity and expelled jets
Gas and dust jets, with 10 thousand km extension, point to the Sun due to sublimation of heated ices. Dust production rate is estimated at 30 kg per second in July, assuming 100-micrometer grains.
Negative polarization of reflected light suggests irregular surface, aligned with solar comets. Observations from the Nordic Optical Telescope confirm active diffusion since July.
Constant brightness avoids fragmentations, unlike 2I/Borisov. Computational models project stability until perihelion, focusing on dust measurements to understand ejectors.
Speculations and scientific confirmations
Initial hypotheses of artificial origin, raised by a Harvard study, were refuted by NASA with evidence of natural cometary behavior. Tom Statler, chief scientist, highlighted compliance with observed standards.
The atypical composition reinforces studies on galactic evolution, without indications of anomalies beyond chemistry. The astronomical community prioritizes empirical data to discard unproven theories.
Continuous observations validate 3I/ATLAS as a relic from ancient star systems, contributing to global catalogs. NASA emphasizes the absence of risks, focusing on scientific advances.
