Interstellar comet 3I/ATLAS rapidly brightened as it zoomed behind Sun, 'it's bluer than our star,' reveals new study

Comet 3I/ATLAS is a newly identified interstellar object, meaning that it comes from outside our Solar System. It is only the third of its kind ever observed, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. It was discovered on July 1, 2025.

The comet has been flying around the sun to reach perihelion (its closest point to sun) on Thursday (October 30).

It couldn't be seen from Earth. However, the European Space Agency (ESA) said its astronomers began using ground-based telescopes in Hawaii, Chile, and Australia to monitor 3I/ATLAS's progress.

So far, the comet has been observed by space telescopes close to Earth, including the NASA/ESA Hubble Space Telescope and the NASA/ESA/CSA James Webb Space Telescope.

Additionally, some of the satellites tracking it include the National Oceanic and Atmospheric Administration's GOES-19 weather satellite, NASA's Polarimeter to Unify the Corona Heliosphere (PUNCH) mission, as well as NASA and ESA's Solar and Heliospheric Observatory (SOHO), according to Universe Today.

Despite some rather frenzied speculation that comet 3I/ATLAS could be an alien spacecraft, most astronomers are confident that this interstellar visitor is a space rock from an unknown star system far away.

The team estimates that, at perihelion, the comet will have brightened to roughly magnitude 9 — still too faint to be seen with the unaided eye, but bright enough to be seen by good backyard telescopes, if it were visible from Earth.

The study relied on space-based solar instruments such as GOES-19 and SOHO, and found that the comet was “distinctly bluer than the sun”, which was consistent with gas emissions contributing substantially to the comet’s increased brightness near perihelion, according to the study's authors.

This is expected of comets, which heat up as they approach the sun, causing surface ices to sublimate into gases that wrap around the comet’s body and contribute to its tail.

Solar radiation ionises the gas, causing further brightening.

Meanwhile, NASA had earlier said that 3I/ATLAS is traveling through our solar system at "the highest velocity ever recorded for a solar system visitor."

"This breathtaking sprint is evidence that the comet has been drifting through interstellar space for many billions of years... The longer 3I/ATLAS was out in space, the higher its speed grew," NASA said.

In fact, 3I/ATLAS could be the oldest comet ever seen, experts have said. One study even suggested it's around 3 billion years older than our 4.6 billion-year-old solar system.

The comet is also likely the largest interstellar object ever seen, though researchers are still pinning down its exact size. Hubble Space Telescope data suggest that 3I/ATLAS has a maximum width of about 3.5 miles (5.6 km).