Gemini North observations show changes in Interstellar Comet 3I/ATLAS after perihelion

On November 26, 2025, astronomers at the Gemini North telescope on Maunakea in Hawai’i observed the interstellar comet 3I/ATLAS using the Gemini Multi-Object Spectrograph.

The new observations showed that the comet’s appearance and activity had changed following its closest approach to the Sun on October 30, 2025.

After emerging from behind the Sun, the comet became visible near Zaniah, a triple-star system in the constellation Virgo.

According to Bryce Bolin from Eureka Scientific and his team, the coma of 3I/ATLAS displayed a faint greenish glow, displayed a faint greenish glow, and gases such as diatomic carbon were present in the coma​​​​​​.

Observational details from Gemini North

The Gemini North telescope team captured multiple exposures of 3I/ATLAS through four different filters: blue, green, orange, and red.

Science News noted that the comet was kept centered in the telescope’s field of view, which caused background stars to appear as colorful streaks due to their motion relative to the comet.

Earlier observations conducted at Gemini South in Chile showed that 3I/ATLAS had a red hue, while the new images revealed a faint green coloration.

NSF NOIRLab explained that this change in color was caused by gases in the comet’s coma, including diatomic carbon, which emit light at green wavelengths when exposed to solar radiation.

Bolin and his colleagues also indicated that the observations were part of a public outreach initiative coordinated by NSF NOIRLab in collaboration with Shadow the Scientists, allowing the public to access and observe the scientific process behind the data collection.

Activity and composition of 3I/ATLAS

Live Science reported that following perihelion, the comet’s activity increased as ice and dust were released into space.

This created a visible coma around the comet and a tail extending behind it.

The green coloration observed in the new images is consistent with the emission of diatomic carbon molecules as the comet’s surface heated.

According to NSF NOIRLab, many comets, including those within the solar system, produce a green hue when activated by sunlight.

3I/ATLAS was first detected in late June 2025 while traveling through the solar system at roughly 130,000 miles per hour on a hyperbolic trajectory that will not return to the solar system.

Observations from multiple observatories suggest that 3I/ATLAS is likely the largest and oldest interstellar object recorded, displaying sun-facing jets and evidence of exposure to cosmic radiation during its long interstellar journey, according to Science News.

Future monitoring and predictions

Scientists​‍​‌‍​‍‌​‍​‌‍​‍‌ from NSF NOIRLab pointed out that the comet’s disengagement from the Sun with its accompanying change in behavior is still a question.

In general, comets do not instantly respond to the solar heating of their outer layers but rather the heat has to penetrate their cold interiors.

Such a late response may lead to the emission of more gases or the triggering of cometary outbursts, as explained by Live Science.

Any changes in the brightness, activity, and the path of the comet are what astronomers are looking for while 3I/ATLAS is getting closest to Earth on December 19, 2025, at around 170 million miles. 

Besides the Gemini North observations, the data gathered from other ground-based observatories and spacecraft are being utilized to figure out the chemical composition, size, and the movement of the interstellar objects.

According to Science News, these investigations are intended to supply the knowledge of the formation of the first star systems in the Milky Way and the behavior of the objects coming from beyond the solar ​‍​‌‍​‍‌​‍​‌‍​‍‌system.

Stay tuned for more updates.