Interstellar visitor 3I/ATLAS reappears: Astronomer captures first post-Sun encounter image of comet

The Gemini South telescope in Chile captured interstellar comet 3I/ATLAS racing through the solar system on 27 August. (Image: International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the ScientistImage Processing: J. Miller & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab))

After slipping behind the sun, the interstellar comet 3I/ATLAS has returned to view, delighting astronomers with fresh images captured from Arizona’s Lowell Observatory.

What makes this comet’s return significant?
Astronomer Qicheng Zhang used the observatory’s Discovery Telescope to snap what is believed to be the first optical post-perihelion image of the comet on Friday, 31 October. The photo, taken on Halloween, shows the rare visitor emerging after its close approach to the sun. Zhang, a postdoctoral fellow at Lowell Observatory, later confirmed that 3I/ATLAS is now visible through small telescopes. He shared an example on his Cometary blog on Sunday, 2 November.

According to Zhang, amateur astronomers across much of the Northern Hemisphere should soon be able to spot it. “All you need is a clear sky and a low eastern horizon,” he told Live Science. The comet may appear faint, but it is expected to grow brighter in the coming days.

Comet 3I/ATLAS appears as the bright white spot at the centre of the image, while the distorted dot above it is a star blurred by the comet’s movement. (Image: Qicheng Zhang/Lowell Observatory)

How fast is comet 3I/ATLAS travelling?
Scientists say 3I/ATLAS is moving at speeds above 130,000 mph (210,000 km/h). It is only the third interstellar object ever discovered and follows a remarkably straight, flat path through our solar system. After its discovery in July, it disappeared briefly while circling the sun, reaching its closest point—known as perihelion—on 29 October. At that moment, it was around 130 million miles (210 million kilometres) from the sun.

Even when hidden behind the sun, researchers continued tracking its movement using space telescopes. On 28 October, Zhang and a colleague published a study on the preprint server arXiv, suggesting that the comet brightened rapidly before perihelion and appeared distinctly bluer than the sun. This colour, they wrote, may indicate gas emissions contributing to its visible brightness.

What are astronomers learning now?
Zhang said more data is needed to confirm if the comet is still brightening. The Lowell Discovery Telescope is among the few large telescopes able to observe the comet so close to the horizon. From Earth’s view, 3I/ATLAS is moving northward, away from the northeastern horizon. Observers can look for it during morning twilight when it rises just above the horizon before sunrise.

Zhang used a smaller six-inch telescope to test viewing conditions ahead of scheduled telescope time. He captured the new image when the comet was about 16 degrees from the sun and five degrees above the horizon. He noted that although radio observations continued during perihelion, no other confirmed optical post-perihelion sightings have yet been reported.

Could 3I/ATLAS reveal more cosmic secrets?
As the comet moves away from the sun, its surface heats up, causing ice to sublimate into gas. This process helps scientists study its composition. Some early studies suggest the comet could be around three billion years older than our solar system, possibly making it the oldest ever observed.

There has been speculation that 3I/ATLAS might be an alien probe, but experts largely dismiss that theory. Most astronomers agree it is a natural comet from another star system within the Milky Way. Prolonged exposure to cosmic radiation may have formed a thick crust on its surface, obscuring clues about its origin.

Zhang expects more research and observations soon as 3I/ATLAS climbs higher in the morning sky. “The comet is rising quickly from the sun,” he said. “In about a week, it should be 25 or 30 degrees away, allowing many large telescopes worldwide to begin tracking it.”