NASA’s James Webb Space Telescope (JWST) has delivered unprecedented new insights into the interstellar comet known as 3I/ATLAS, revealing that it is far more unusual—both in size and chemistry—than scientists initially believed.

A Surprising Revision of Its Size
Early observations suggested that the nucleus of 3I/ATLAS could be as large as 3.5 miles (5.6 kilometers) in diameter. However, more recent data from JWST, combined with observations from other telescopes, indicate that the solid nucleus is actually much smaller, likely less than 1 mile (1.6 kilometers) across. The earlier overestimate occurred because the comet’s coma—a dense cloud of gas and dust surrounding the nucleus—was mistakenly interpreted as part of the solid body.

Unusual and Extreme Chemistry
JWST’s Near-Infrared Spectrograph (NIRSpec) has revealed that the coma of 3I/ATLAS is extraordinarily rich in carbon dioxide. The ratio of carbon dioxide (CO₂) to water vapor (H₂O) is estimated to be around 8:1—roughly 16 times higher than what is typically seen in comets originating from our own solar system. This striking chemical signature suggests that 3I/ATLAS formed in a vastly different environment, possibly near a carbon-dioxide ice line or in a region exposed to intense radiation.

A Rare Glimpse Into Cosmic History
Discovered on July 1, 2025, by the ATLAS sky survey, 3I/ATLAS is only the third confirmed interstellar object ever observed passing through our solar system. Traveling at speeds exceeding 130,000 miles per hour (210,000 kilometers per hour), it is expected to reach its closest approach to the Sun on October 29, 2025. Because it originated beyond our solar system, the comet provides scientists with a rare opportunity to study pristine material that predates the formation of the Sun and planets.

As 3I/ATLAS continues its journey through the solar system, astronomers around the world are monitoring it closely, hoping this ancient interstellar visitor will shed new light on the processes that shaped planetary systems across the universe.

Recent discoveries on the interstellar comet 3I/ATLAS