An object streaking through the solar system has sparked fresh debate among astronomers after displaying unexplained acceleration and an unusual blue hue that one leading scientist says could hint at alien technology.

The National Aeronautics and Space Administration (Nasa), the American space agency, confirmed this week that the interstellar visitor, officially named 3I/ATLAS, and which is about three-quarters the size of Benoni, showed non-gravitational acceleration as it passed near the Sun. Such behaviour is typically caused by gas and dust erupting from a comet’s surface, yet a Harvard astrophysicist, Avi Loeb believes another explanation cannot be ruled out.

“Alternatively, the non-gravitational acceleration might be the technological signature of an internal engine,” Loeb wrote in a new paper, noting that 3I/ATLAS had turned: “bluer than the sun” as it brightened – an effect he said could stem from: “a hot engine or source of artificial light.”

The United States Naval Research Lab reported that the object shifted in colour from red to green to an unexpectedly bright blue while rapidly increasing in luminosity. Loeb said that if no significant gas cloud is observed when the body makes its closest pass to Earth on 19 December, when it would be around 267 million kilometres away, it could point to a propulsion system rather than natural outgassing.

Nasa, however, dismissed speculation that the object was of alien origin. Acting Nasa Administrator Sean Duffy insisted on X that 3I/ATLAS is: “the third interstellar comet to pass through our solar system. No aliens. No threat to life here on Earth.”

Loeb criticised Nasa for not yet releasing high-resolution images taken by the Mars Reconnaissance Orbiter when the 3I/ATLAS passed relatively close to the Red Planet in October. The release of these images have reportedly been delayed by the ongoing US government shutdown.