Astronomers from international observatories detected comet 3I/Atlas, the third interstellar object confirmed to enter the solar system, in July 2025. The visitor, originating from a distant star system, advances at a speed of 57 km/s, higher than the Sun’s gravitational escape, which guarantees its exit after a brief passage. The comet’s hyperbolic trajectory, observed in November 2025, allows detailed studies of external chemical compositions without risk of orbital capture.
The discovery occurred by the ATLAS system in Chile, and initial data indicate that 3I/Atlas reached perihelion on October 29, 2025, 1.36 astronomical units from the Sun, between the orbits of Mars and Earth.
- Recorded speed: 57 km/s, above Oumuamua’s 26 km/s and Borisov’s 33 km/s.
- Minimum distance to Earth: 1.8 astronomical units on December 19, 2025.
- Estimated age: Up to 14 billion years, older than the solar system.
Initial detection and orbital confirmation
Comet 3I/Atlas has appeared in pre-discovery observations since May 2025, but was only made official on July 1 by the ATLAS telescope in Río Hurtado, Chile. Analysis from the Minor Planet Center confirmed its hyperbolic orbit with an eccentricity of 6.139, indicating an interstellar origin. Telescopes such as Hubble and Gemini captured initial images showing a diffuse coma and faint tail, with no signs of fragmentation.
Nordic Optical Telescope observations in July revealed clear comet activityara, with reddish dust similar to that of Borisov. The speed relative to the Sun, of around 221 thousand km/h at discovery, reinforces that the object does not belong to the local system.
Distinctive chemical composition
Spectra obtained by the James Webb Space Telescope detected traces of carbon dioxide, carbon monoxide and nickel vapor in the coma of 3I/Atlas, rare proportions in solar comets. The low water fraction, about 4%, suggests prolonged exposure to interstellar radiation over millions of years of travel.
Color changes in the coma, from reddish to bluish near perihelion, indicate the release of volatile gases under solar heating. Data from the ExoMars probe, in orbit around Mars, recorded jets of up to 35 km/s in hexagonal formation, without anomalous non-gravitational accelerations beyond the expected sublimation.
- Elements detected: CO2, CO, H2O and carbonyl sulfide.
- Anomalies: Thermal excess of 5 thousand K and hydroxyl emission in1,665 MHz.
- Mass loss: Greater than 13% at perihelion, compatible with natural outgassing.
Planetary trajectory and interactions
The 3I/Atlas pass included close approaches to planets, such as 0.19 AU of Mars on October 3, 2025, enabling Mars Express imaging. On November 3, the comet was 0.65 AU from Venus, and in March 2026, it will approach Jupiter at 54 million km.
Computer models predict angular deviation by the solar gravitational slingshot, but the kinetic energy maintains the course towards deep space. The 5-degree orbital inclination aligns with the ecliptic plane, facilitating visibility in both Earth’s hemispheres. Parker Solar Probe observations confirmed no risk to Earth or satellites.
The almost retrograde orbit, of 175 degrees, differentiates 3I/Atlas from local comets, which orbit prograde. After perihelion, the comet gains altitude in the constellations of Virgo and Leo, with an apparent magnitude of 11 to 12, accessible to amateur telescopes.
Real-time observations
Ground-based and space-based telescopes have been adjusting daily tracking of 3I/Atlas since November 2025. Images from the Virtual Telescope Project in Austria on November 8 revealed a complex ion tail with five jets, measuring 5 coma arc minutes.
The International Asteroid Warning Network’s global campaign, from November 27, 2025 to January 27, 2026, brings together data from multiple countries to refine the trajectory. Satellites like TESS detected early cometary activity in May, at 6.4 AU from the Sun.
Comparison with previous visitors
3I/Atlas differs from Oumuamua, which showed unusual gaseous acceleration in 2017, and Borisov, with an aqueous composition in 2019. While Oumuamua lacked a visible coma, 3I/Atlas displays well-defined dust and ion tails, with a stable brightness of magnitude 14.5 to 14.7.
Spectroscopy indicates unusual metals, such as nickel, absent in typical solar comets. The irregular rotation and thermal emission patterns suggest evolution under galactic radiation, contrasting with the observed minimal non-gravitational acceleration.
The estimated age of 7.6 to 14 billion years makes it potentially the oldest studied comet belonging to the thick disk of the Milky Way.
Future visibility and monitoring
Amateur astronomers report early morning sightings of 3I/Atlas starting November 3, 2025, in the constellation Virgo, 9 degrees above the eastern horizon. Until November 17, the object gains 3 degrees in height per day, migrating to Leo.
ESA’s Juice mission plans observations in November 2025, using cameras and spectrometers to map the coma. Juno, in March 2026, will capture data during its passage by Jupiter. The comet will weaken below magnitude 12 in December, but will provide insights into distant planetary formation.
Insights into stellar origins
The 3I/Atlas analysis points outfor ejection from its original system by gravitational interactions or stellar explosions, millions of years ago. Its composition, with traces of nitrogen and cyanogen, reflects varied formation processes in ancient stars.
SPHEREx data from August 2025 confirms water ice and organic compounds, offering clues about outer protoplanetary disks. The detection of hydroxyl absorption reinforces that outgassing explains observed accelerations, without the need for exotic hypotheses.