The interstellar comet 3I/Atlas, discovered in July 2025 by the ATLAS telescope in Chile, reached its perihelion on October 29, 1.4 astronomical units from the Sun. Astronomers from agencies such as NASA and ESA recorded changes in the object’s color and speed after this closest approach. The change occurs due to exposure to solar heat, which causes sublimation of volatile materials in the core.
These transformations draw attention because 3I/Atlas is the third confirmed visitor from outside the Solar System, following ‘Oumuamua and 2I/Borisov. Observations from the James Webb Space Telescope reveal a composition rich in carbon dioxide, eight times higher than water, which differs from local comets.
The comet’s hyperbolic trajectory, with an initial speed of 210 thousand km/h, confirms its external origin. After perihelion, the object heads towards Venus on November 3, without risk to Earth.
Comet color changes
3I/Atlas exhibited reddish and greenish tones before perihelion. After the pass, images from probes such as SOHO and GOES-19 show a predominant blue glow.
This color arises from the emission of ionized gases by the Sun, such as cyanogen and carbon dioxide. The reddish dust contrasts with the blue gas, creating the observed effect.
Spectral analyzes indicate that the coma, a cloud of gas and dust surrounding the nucleus, expanded from 13,000 to 18,000 kilometers. The brightness increased to magnitude 9, visible in amateur telescopes.
Factors that drive speed
Ice sublimation reduces the comet’s mass, increasing its acceleration. At perihelion, the speed reached 68 km/s, higher than expected by gravity alone.
Gas ejections create non-gravitational forces, with radial and transverse components measured at 1.66 x 10^{-6} au/day². This explains the gain of 60 m/s in the last 20 days.
Solar coronagraph observations confirm the absence of a striking dust tail, reinforcing the gaseous influence.
- Core diameter: between 440 meters and 5.6 km.
- Estimated mass: more than 33 billion tons.
- Approximate age: 7 billion years, before the Sun.
Chemical composition revealed
The James Webb Telescope detected a high concentration of CO2 in the coma, unprecedented in comets. Water appears at low levels, suggesting formation in cold regions far from stars.
Nickel vapor and gaseous cyanide appear in proportions similar to solar comets, despite their interstellar origin. This mixture indicates prolonged exposure to cosmic radiation over billions of years.
AccelerateContinuous ration, without variation outside the orbital plane, intrigues scientists. Computational models predict a loss of 10% of mass in months, due to evaporation of volatiles.
Researchers plan International Asteroid Warning Network exercises to refine orbital measurements by January 2026. Probes like ESA’s Juice will attempt observations in March, near Jupiter.
Space probe observations
Mars probes, such as Mars Express, captured data in October, 30 million km away. After perihelion, the comet reappeared on October 31, visible from ground-based telescopes in December.
The orbital geometry aligned 3I/Atlas with the ecliptic plane, facilitating images of heliospheres. Steady brightness at magnitude 14.7 suggests sustained activity without disintegration.
Future visitor trajectory
The comet will pass Venus at 97 million km on November 3. In December, it will approach Earth at 1.8 astronomical units, without threat.
In March 2026, it will cross close to Jupiter at 54 million km, allowing studies of gravitational interaction. After that, it will return to interstellar space at 221 thousand km/h.
This passage offers a unique window into analyzing primitive materials, possibly originating in the early Universe.
