Comet 3I/ATLAS, detected in July 2025 by the ATLAS system in Chile, reached perihelion on October 30 and exhibited mass loss greater than 13% due to ice sublimation. Astronomers from institutions such as NASA and ESA recorded non-gravitational acceleration during this period, caused by the ejection of gases. The object, originating from another star system, follows a hyperbolic trajectory towards interstellar space.
Preliminary observations indicate that the degassing process released compounds such as carbon dioxide and water, altering its visual appearance. Ground- and space-based telescopes have captured data that confirm cometary behavior, but with anomalies regarding local objects.
- Ejection of gases drove radial acceleration of 1.1×10⁻⁶ au/day².
- Orbital deviation of four arc seconds was measured by ALMA.
- Brightness increased fivefold between September and November 2025.
Trajectory and initial detection
3I/ATLAS entered the Solar System at high speed, exceeding 50 km/s, and was identified as interstellar due to its orbit not linked to the Sun. Telescopes in Hawaii and Australia confirmed its nature in July, with an estimated diameter between 400 meters and 5.6 km.
This early detection allowed continued monitoring, including close passes to Mars on October 3 at 30 million km. Probes like Mars Express have collected initial spectra, revealing micrometer-sized dust in the coma.
Mass loss due to degassing
Exposure to solar heat caused intense sublimation of ice during perihelion. This phenomenon released gases and dust, resulting in a loss of up to 10% of the total mass in months. Calculations indicate an evaporation half-life of six months for the nucleus.
Engineers at NASA’s JPL measured the impact as equivalent to a natural rocket, with gas plumes visible in November and December. ESA’s JUICE spacecraft plans additional observations to quantify the ejection.
- Carbon dioxide dominates material release.
- Water and carbon monoxide were detected in high concentrations.
- Loss of mass slightly alters heliocentric velocity.
Acceleration beyond gravity
Data from the ALMA observatory recorded transverse acceleration of 3.7×10⁻⁷ au/day², incompatible with pure gravitational forces. The asymmetric ejection of gases explains the deviation, similar to Oort comets, but with greater intensity.
Astronomers adjust orbital models weekly to track the movement. This acceleration does not change the general route, but highlights differences�as in interstellar visitors. The proximity of 268 million km to Earth occurs on December 19, without risk of collision.
The phenomenon reinforces studies on interactions with solar radiation on objects from distant origins. Observations from probes such as STEREO-A confirm the impulse as a result of accelerated sublimation.
Color and brightness variations
Initially reddened by dust, 3I/ATLAS adopted greenish and bluish tones as it approached the Sun, indicating chemical reactions in compounds such as cyanide and ammonia. This mutation occurred for the second time, with brightness reaching magnitude 9 in October.
SOHO and GOES-19 probes captured the increase in luminosity, linked to the release of ionized gases. The blue color suggests dominance of C2 and NH2 emissions, contrasting with red tones typical of solar dust.
Chemical composition revealed
Analysis by James Webb and Keck detected enrichment in iron and nickel, as well as CO2, CO and H2O at high levels. These elements indicate exposure to cosmic radiation for billions of years, shaping the core.
Spectra show the absence of common organics in solar comets, suggesting formation in an environment of different pressure and temperature. Nickel vapor registered peaks in August, close to the Sun.
- Cyanogen contributes to bluish tones.
- Micrometric dust grains cause initial red coma.
- Rota�The irregularity was listed as one of the eight observed anomalies.
Global monitoring ongoing
Telescopes in Chile, Hawaii and Australia monitor the output of 3I/ATLAS, focusing on brightness decay and potential fragmentation. ESA and NASA integrate data from multiple missions to refine analyses.
This continuous surveillance fills in gaps during solar conjunctions and reveals dominant gas emissions. The object, estimated to be 10 billion years old, offers insights into early stellar evolution.
