NASA’s SPHEREx space telescope carried out observations of the interstellar comet 3I/ATLAS between August 7 and 15, 2025. The instrument captured data in near infrared, allowing analysis of its chemical composition and physical structure. This is the third time that an object from outside the Solar System has been detected, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. The mission takes place in Washington, NASA headquarters, with teams processing the results to understand distant cosmic origins.
Discovered on July 1, 2025 by the ATLAS telescope in Chile, the comet travels at more than 210 thousand km/h. It poses no threat to Earth and follows a hyperbolic trajectory outside the Solar System. Complementary observations from other NASA telescopes provide context on its size and activity.
- Diameter of the coma: approximately 26,400 km, equivalent to twice that of Earth.
- CO₂ detection: gaseous coma extends 348 thousand km.
- Initial activity: signs of coma observed since May 2025, 6.4 astronomical units from the Sun.
Chemical composition revealed by SPHEREx
SPHEREx data identified strong water ice absorption and bright carbon dioxide emission from the comet’s coma.The observations occurred when the object was 470 million km from the Sun. This multispectral analysis helps map volatile molecules released by solar heating.
The detection of CO₂ suggests formation in a cold region, beyond the frost line in its original star system. NASA teams integrate these findings with studies of other instruments to refine models of interstellar comets.
Trajectory and chronology of the passage
Comet 3I/ATLAS entered the Solar System from the constellation Sagittarius. Ground-based telescopes confirmed comet activity on July 2, 2025, with diffuse coma and tail elongation of 3 arc seconds.
On July 21, Hubble captured an image 445 million km from Earth, estimating a drop-shaped dust nucleus. Webb observed on August 6, detecting water vapor and hydroxide ions. SPHEREx continued from August 7th to 15th, focusing on infrared emissions.
Passage by Mars took place on October 3, 30 million km away, with images from orbiters such as Mars Express. On October 21, solar conjunction temporarily occulted the object. Reappearance in November showed rapid brightening and possible mass loss.
Contributions from multiple NASA telescopes
Hubble provided an estimate of the nucleus’s size, which was difficult to resolve due to its speed. Images from July 21 revealed an envelope of dust streaked with background stars.
Webb, with a NIRSpec spectrograph, mapped flows of CO₂ at 4.3 micrometers and H₂O at 2.7 micrometersros. These data confirm the release of carbon monoxide and carbonyl sulfide.
Swift detected the presence of water vapor on July 30th and August 1st. TESS indicated early activity in May.
Other missions, such as Perseverance and Curiosity on Mars, monitor indirect effects during approach.
Estimated age and origin
Preliminary studies indicate an age of 3 to 11 billion years for 3I/ATLAS, older than the Solar System. Its CO₂-rich composition implies the formation of a protoplanetary disk far from the parent star.
If it comes from the thick disk of the Milky Way, it could have low metallicity, with an abundance of heavy elements 40% of that of the Sun. VLT observations detected emission of atomic nickel, without equivalent iron, in July and August.
The reddish color of the coma, similar to 2I/Borisov, reflects processed dust. Color changes observed three times since discovery suggest variations in material ejection.
Post-perihelion observations and perspectives
After perihelion in October, telescopes like Lowell Discovery captured images on October 31, showing faint bluish hue. The comet moves north, visible in amateur telescopes in the Northern Hemisphere.
In December, Earth’s approach to 270 million km allows for more data. NASA coordinates missions like Psyche and Lucy for continued tracking until March 2026, when it passes by Jupiter.