Astronomers from NASA and international institutions identified comet 3I/ATLAS as the third confirmed interstellar object to cross the Solar System. The comet, discovered on July 1, 2025 by the ATLAS telescope in Chile, follows a hyperbolic orbit that confirms its origin outside the local solar system. This passage occurs without any threat to Earth, with the closest approach to the planet estimated at 1.8 astronomical units, or about 270 million kilometers.
The object reached its perihelion, the closest point to the Sun, around October 30, 2025, at a distance of 1.4 AU, equivalent to 210 million kilometers, within the orbit of Mars. Preliminary observations indicate that the comet exhibits typical cometary activity, with the release of gases and dust, which allows studies on its chemical composition. The discovery reinforces global monitoring of celestial bodies to understand interstellar dynamics.
Origin and naming of the cosmic visitor
The ATLAS telescope, funded by NASA and located in Rio Hurtado, Chile, reported the first 3I/ATLAS images to the Minor Planet Center. Previous observations, retrieved from telescope archives on different continents, date back to June 14, 2025.
The designation “3I” reflects its position as the third known interstellar object, afterós ‘Oumuamua in 2017 and 2I/Borisov in 2019. The suffix “ATLAS” honors the detection system that identified it, designed for warning of terrestrial impacts.
Trajectory and calculated distances
The comet’s hyperbolic orbit excludes any possibility of return to the Solar System, confirming its nature as a single passenger. It travels at speeds of up to 221,000 kilometers per hour, accelerating as it approaches the Sun.
- Minimum distance to the Sun: 210 million kilometers, in October 2025.
- Approach to Earth: 270 million kilometers, without risk of collision.
- Passage by Mars: Recorded on October 3, 2025, 28 million kilometers from the planet.
Astronomers predict the comet will cross Jupiter’s orbit in March 2026, before moving away permanently.
Initial discovery and confirmation of activity
The first report arrived on July 1, 2025, with pre-discovery data extending to June. Telescopes such as the one at the Palomar Observatory in California contributed to mapping its initial position.
Observations from the Nordic Optical Telescope on July 2 confirmed the diffuse coma, indicating release of dust and gases. The reddish color of the coma, observed in July, suggests evolution in the surface composition as the comet heats up.
Initial studies estimate the core between440 meters and 5.6 kilometers in diameter, based on Hubble images captured on July 21, 2025, 446 million kilometers from Earth.
NASA Instruments in Joint Action
Several NASA space telescopes have aimed sensors at 3I/ATLAS since July. Hubble revealed a teardrop-shaped cocoon of dust breaking away from the icy core, with background stars appearing as streaks due to the fast trajectory.
The James Webb Space Telescope on August 6, 2025, detected infrared spectra of water ice, carbon dioxide, and carbon monoxide in the coma. Probes on Mars, such as the Perseverance rover and the Mars Reconnaissance Orbiter, recorded the comet as a bright spot in October.
Chemical composition revealed by observations
Swift Ultraviolet/Optical Telescope analyzes identified water vapor through traces of hydroxyl in July and August 2025. The dominant presence of carbon dioxide suggests formation in cold regions far from its original star.
SPHEREx observed between August 7 and 15, confirming release of carbonyl sulfide and water ice. These detections indicate an estimated age of between 3 and 11 billion years, offering clues about planetary formation processes in other systems.
The coma evolved to redder tones in July, reflecting changes in dust and gases as the comet approaches the Sun. No bright bursts were recorded until November 2025, maintaining stable activity.
Orbital probes on Mars captured images from 3I/ATLAS in October 2025. ESA’s Trace Gas Orbiter recorded the diffuse coma 28 million kilometers from the planet, confirming emission of hydroxyl derived from water.
The Chinese probe Tianwen-1, from Martian orbit, obtained the first close views between October 1st and 3rd, showing the comet as a point object against the sky. This data complements ground-based observations, improving orbital models.
Rovers such as Curiosity and Perseverance contributed shallow photos, integrating multiple perspectives for global analysis.
Future visibility and monitoring
The comet remained accessible to ground-based telescopes until September 2025, becoming invisible close to the Sun. It reappears in December 2025, with an estimated magnitude of 12, viable for amateur observers with equipment with at least an eight-inch aperture.
The International Asteroid Warning Network plans an observation campaign from November 27, 2025 to January 27, 2026, focused on precise astrometry. Telescopes like the Vera C. Rubin will continue tracking until the exit of the Solar System.
These final observations will allow us to refine size and composition estimates, contributing to catalogs of interstellar objects.
