The interstellar comet 3I/ATLAS, detected on July 1, 2025, by the ATLAS telescope in Chile, reaches perihelion on October 29, 2025, at 1.36 astronomical units from the Sun. This closest point occurs between the orbits of Earth and Mars, with no risk of collision with planets. The minimum distance from Earth reaches 270 million kilometers in December.
NASA monitors the object, the third confirmed of external origin to the Solar System, with a relative speed of 58 km/s to the Sun. The hyperbolic orbit, with eccentricity above 5, ensures a single passage through the system. Probes such as Mars Express and ExoMars Trace Gas Orbiter collect data during the approach.
- Composition includes CO2 and fine dust, identified by spectroscopy.
- Cohesive nucleus resists without initial outbursts, despite estimated age in billions of years.
- Alignment with Venus facilitates studies in October.
Initial discovery at Chilean observatory
The object appeared in observations from the Río Hurtado observatory in Chile in July 2025. Initially designated A11pl3Z, it received the 3I classification by the Minor Planet Center after interstellar confirmation.
Ground-based equipment recorded diffuse coma and subtle elongation, signs of cometary activity. The James Webb Space Telescope analyzed spectra in August, detecting water ice and metals such as nickel.
Trajectory and superior speed
Entry into the inner solar system occurred near Jupiter’s orbit, allowing early detections. The comet crossed the ecliptic plane in September, with brightness at magnitude 12-13.
Observers in equatorial regions captured the movement before solar conjunction. The speed of 58 km/s exceeds predecessors 1I/ʻOumuamua and 2I/Borisov.
Detailed chemical composition
Analyses reveal CO2 dominance in dust ejection, with extreme negative polarization. This feature indicates prolonged exposure to interstellar radiation, altering the surface.
The nucleus, with mass over 33 billion tons, releases about 40 liters of water per second at unusual distances from the Sun. Spectra from the Keck Observatory identified cyanide and absence of organics common in solar comets.
The reddish color of the dust evolves with solar proximity, pointing to varied thermal processes. Mass release totals two million tons since July, a minimal fraction of the estimated total.
Observations by space probes
European Space Agency probes recorded the comet on October 3, 2025, at 28 million kilometers from Mars. The Perseverance rover captured images as a bright point in the Martian sky.
Coronagraphs from the GOES-19 satellite detected the object on October 18, despite intense solar brightness. The Europa Clipper probe aligns with the ion tail at the end of October, collecting charged particles.
Monitoring at perihelion
Solar heating exposes the surface to 33 gigawatts of radiation, with potential for thermal collapse. Absence of initial outbursts suggests nucleus stability, unlike 2I/Borisov which fragmented in 2020.
Astronomers predict fine dust ejection post-perihelion, visible at magnitude 11.5. Sky survey networks track residual trails.
NASA protocols activated
Activation occurred via MPEC 2025-U142 bulletin on October 21, focusing on unpredictable trajectories. Collaborations with ESA integrate data from multiple agencies.
The object poses no threat to Earth, with minimal uncertainty ellipsoid. Software updates incorporate non-gravitational forces.
Close passages by Venus on November 3 and Jupiter in March 2026 allow additional observations. The comet becomes visible in November in the constellations of Virgo and Leo, with reappearance on November 11 before dawn. Telescopes of 20 cm detect the object at magnitude 12, with expanded coma and prominent tail in dark skies.
