Comet 3I/ATLAS, the third interstellar object detected in the Solar System, will reach its closest proximity to Earth on December 19, 2025. On that date, the celestial body will pass approximately 270 million kilometers from the planet, a distance equivalent to almost twice the Earth’s orbital radius around the Sun. Astronomers from NASA and the European Space Agency (ESA) confirm that the comet’s hyperbolic trajectory presents no risk of collision, allowing continued scientific observations.
Discovered on July 1, 2025 by the ATLAS telescope in Chile, 3I/ATLAS travels at more than 210 thousand kilometers per hour, coming from the direction of the constellation Sagittarius. After reaching perihelion on October 29, 210 million kilometers from the Sun, the object reappeared in the Earth’s sky in early November, exhibiting changes in its coma and tail.
Experts use missions such as Hubble, James Webb and SPHEREx to analyze its composition, which includes carbon dioxide and water ice, revealing clues about ancient star systems. The close pass of Mars on October 3 allowed detailed images from probes such as Mars Express and ExoMars Trace Gas Orbiter.
Initial discovery and identification
Astronomers identified 3I/ATLAS as an interstellar comet shortly after its detection, thanks to preliminary observations that showed cometary activity. The object, initially cataloged as A11pl3Z, received the official designation C/2025 N1 (ATLAS) and 3I/ATLAS, highlighting its origin outside the Solar System.
The Hubble Space Telescope captured images on July 21, when the comet was 445 million kilometers from Earth.a, revealing a teardrop-shaped coma of dust. Initial studies estimate the core to be between 440 meters and 5.6 kilometers in diameter.
Forecasts indicate that the comet will remain visible to amateur telescopes until January 2026, before moving away permanently.
Chemical composition revealed by observatories
The James Webb Space Telescope observed 3I/ATLAS on August 6, 2025, detecting carbon dioxide dominant in the coma, which stretches nearly 350,000 kilometers and glows green. This cloud of gas and dust differs from local comets, suggesting formation under distant stellar conditions.
SPHEREx analyses, between August 7th and 15th, confirmed the presence of water ice and hydroxyl (OH) emissions, indicative of solar sublimation. The composition includes traces of atomic nickel, which intrigues scientists, but does not change its natural classification.
Terrestrial observations, such as those from the Teide Observatory in Spain, have recorded an anti-collar tail facing the Sun, with a lengthening of up to 30 arc seconds.
Changes observed post-perihelion
After perihelion, the comet exhibited increased brightness, reaching magnitude 7-8, and a bluer color than the Sun, according to data from the Swift Observatory. This anomalous acceleration, of 1.1×10⁻⁶ astronomical units per day squared, results from gas ejections, increasing the rate of mass loss from 150 kg/s to 2 million kg/s.
Images from the Chinese probe Tianwen-1, 30 million kilometers away, captured jets of material extending 1 million kilometers towards the Sun. Partial fragmentation, suggested by astronomers, explains the loss of 16% of mass, with no impact on orbit.
The MeerKAT radio telescope detected radio signals at 1665-1667 MHz, linked to OH molecules, reinforcing their organic nature.
Planetary trajectory and passages
- Passage by Mars: October 3, 2025, 28 million kilometers, observed by Perseverance, Curiosity and orbiters.
- Close to Venus: November 3, 2025, 97 million kilometers away.
- Approach to Earth: December 19, 2025, 269 million kilometers, on the opposite side of the Sun.
- Encounter with Jupiter: March 16, 2026, 54 million kilometers away, monitored by Juno and JUICE.
These distances guarantee safety, with the comet leaving the Solar System on a hyperbolic trajectory. The International Asteroid Warning Network (IAWN) coordinates tracking to refine orbit datamore.
Space mission observations
Missions like NASA’s Europa Clipper and Lucy redirected instruments to 3I/ATLAS in October, capturing infrared spectra. ESA’s JUICE will attempt observations in November, despite the distance of 64 million kilometers, with data expected in February 2026 due to transmission limitations.
The Gemini South Telescope recorded the tail in August, measuring 56,000 kilometers in length. These collections provide the first detailed glimpse of a post-perihelion interstellar visitor.
Astronomers predict that the object, estimated to be 7 billion years old, offers insights into galactic formation, exceeding the 4.6 billion years of the Solar System.
Rumors debunked by experts
Speculations about technological origin, driven by accelerations and composition, were refuted by NASA, which classifies 3I/ATLAS as a natural comet. Theories of extreme fragmentation or artificial signals lack evidence, according to bulletins from the Minor Planet Center.
The extreme negative polarization, similar to that of 2I/Borisov, results from interstellar dust, not intentional design. Observations confirm the absence of outbursts or non-gravitational behaviors beyond the expected sublimation.
Scientists emphasize that the focus remains on science, with global campaigns improving models of hyperbolic orbits.
Visibility for observers
The comet reappears in the November morning sky, visible in constellations such as Virgo with 30-centimeter aperture telescopes. In the Southern Hemisphere, detection requires dark skies before dawn.
- Peak magnitude: 12 in November, falling to 14 in December.
- Duration: Until January 2026, before permanent departure.
- Equipment: Insufficient 70 mm binoculars; prefer 8-inch refractors.
Amateurs in Austria and Spain have already recorded post-perihelion images, showing an expanding coma.
