Scientists detect X-ray glow from interstellar comet 3I/ATLAS extending 250,000 miles into space

For the first time ever, astronomers have captured two distinct X-ray views of an interstellar comet, providing an unprecedented glimpse into the interplay between a visitor from beyond our solar system and solar wind.

Interstellar comet 3I/ATLAS, the third confirmed object known to have originated outside our solar system, has now been imaged in X-ray light by both the European Space Agency's (ESA) XMM-Newton observatory and the X-Ray Imaging and Spectroscopy Mission (XRISM) led by the Japanese space agency JAXA in partnership with NASA and ESA. These X-ray observations allow scientists to detect and study gases that other instruments can't easily spot, according to a statement from ESA.

Comets shine in visible light when sunlight reflects off dust and gas escaping their icy core, while X-ray light tells a very different story. In space, the interaction between fast-moving charged particles from the sun — also known as solar wind — and a comet's surrounding gas cloud produces X-ray emissions. Detecting that glow lets scientists trace where and how these interactions occur and what kinds of gases are present at levels that optical telescopes might miss.

While NASA's James Webb Telescope and other instruments have already spotted abundant water vapor, carbon monoxide and carbon dioxide in 3I/ATLAS's coma, X-ray observations are uniquely sensitive to lighter gases such as hydrogen and nitrogen that are otherwise hard to detect.

The first X-ray observation of 3I/ATLAS was made by the XRISM space telescope, which observed the comet for 17 hours between Nov. 26 and 28. The resulting image was captured using XRISM's soft X-ray telescope, Xtend, whose field of view spans roughly 1.2 million square miles (3 million square kilometers), revealing X-ray emission extending about 250,000 miles (400,000 kilometers) from the comet's nucleus — evidence that the comet's gas is being energized by the solar wind, according to a statement from ESA releasing the image.

The XRISM data also carry spectral signatures of elements such as carbon, nitrogen and oxygen, which helps scientists begin to disentangle the mix of particles released from the comet's nucleus and how they interact with the high-energy environment near the sun, ESA officials said in the statement.

Shortly after, ESA's XMM-Newton X-ray observatory studied 3I/ATLAS for about 20 hours on Dec. 3, when the comet was roughly 175–177 million miles (282–285 million km) from the spacecraft. The image was captured using the telescope's most sensitive X-ray instrument, the European Photon Imaging Camera (EPIC)-pn, revealing a distinct X-ray glow (shown in red) surrounded by fainter gradients. These features mark regions where the solar wind is interacting with gas streaming off the comet, according to a statement from ESA releasing the image.

"3I/ATLAS presents a new opportunity to study an interstellar object, and observations in X-ray light will complement other observations to help scientists figure out what it is made of," ESA officials said in the statement.

Together, X-ray, optical, infrared and radio observations are offering fresh insights into 3I/ATLAS as it makes its rare journey through the inner solar system, with its upcoming closest approach to Earth expected around Dec. 19.