Astronomers gather more clues about interstellar comet 3I/ATLAS

The latest observations of 3I/ATLAS suggest it resembles comets from the outer reaches of our solar system, but may be smaller than initially estimated

By Alex Wilkins

8 August 2025

A Hubble Space Telescope image of the interstellar comet 3I/ATLAS. Background stars appear as streaks because the telescope was tracking the comet

NASA, ESA, David Jewitt (UCLA)/Joseph DePasquale (STScI)

Telescopes trained on the interstellar comet 3I/ATLAS have revealed that it looks much like comets found in the outer reaches of our own solar system. However, puzzling features, such as a surprisingly large amount of water coming off it even while far from the sun, might give us clues about the ancient star system it originally came from.

Objects from other systems passing through ours are extremely rare: 3I/ATLAS, discovered in July, is only the third known interstellar visitor after the comets ‘Oumuamua in 2017 and Borisov in 2019. There is only a window of a few months to observe the newcomer, which is barrelling towards the sun at around 60 kilometres per second, before it loops around it and is flung out of the solar system forever.

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So far, scientists have used its speed to infer that it might come from a star system billions of years older than our own. Early observations also suggested it was around 20 kilometres wide, but there has been little information on the vast plume of water or gases, or both, that it leaves in its wake, called its tail, which can tell us about the composition of the comet itself.

Toni Santana-Ros at the University of Barcelona in Spain and his colleagues have now used several ground-based telescopes to observe the comet and its tail, and found that it contains a relatively low to moderate amount of dust. The amount of dust in its tail also appears to be growing as it gets closer to the sun and warms up, which is similar to the behaviour of comets from the outer solar system. “It’s a regular object. There is nothing especially weird on it,” says Santana-Ros.

Astronomers have also observed the comet from space. Researchers working with the Hubble Space Telescope found it may be much smaller than it first seemed, possibly between 320 metres and 5.6 kilometres wide.