On 1 July 2025 the ATLAS survey telescope at Río Hurtado in Chile picked up a faint object 4.5 AU from the Sun, moving at 61 kilometres per second. That is far too fast for anything bound to the Sun. The next day the Minor Planet Center announced the third known interstellar object, after ʻOumuamua in 2017 and 2I/Borisov in 2019. Older images turned out to show it too: NASA’s TESS had caught it as early as 7 May.
The fastest visitor so far
Before the Sun’s gravity started pulling on it, 3I/ATLAS was crossing interstellar space at about 58 kilometres per second, more than twice ʻOumuamua’s speed. Its path is also the most open of the three. A closed orbit has an eccentricity below 1; ʻOumuamua’s was about 1.2, Borisov’s 3.4, and 3I/ATLAS’s is 6.1.
It passed closest to the Sun on 29 October 2025, at 1.36 AU, about a week after slipping behind the Sun as seen from Earth. Mars had the best seat: on 3 October the comet passed about 29 million kilometres from the planet, and ESA’s ExoMars Trace Gas Orbiter photographed its coma. Earth’s closest approach came on 19 December 2025, at a safe 1.8 AU. On 16 March 2026 it passed Jupiter, and ESA’s Juice spacecraft, on its way there, collected data.
Possibly older than the Sun
The way 3I/ATLAS moves through the galaxy suggests it may come from the Milky Way’s thick disk, a population of old stars. Two studies in July 2025 put its likely age at 7.6 to 14 billion years and 3 to 11 billion years. If the higher numbers are right, it formed before the Sun and the planets, which are 4.6 billion years old. Which star it came from is unknown.
What it is made of
In August 2025 the James Webb Space Telescope measured the gases streaming off it: about 129 kilograms of carbon dioxide per second against only 6.6 of water. That ratio of about eight to one is among the highest ever seen in a comet. The Very Large Telescope found nickel vapour but no iron, which is also unusual.
The comet itself is small, a few kilometres at most and perhaps under one. Its coma is not: in early July it was already about 26 400 kilometres across, twice the width of Earth. By mid-September its dust tail stretched about 100 000 kilometres.
Not a spaceship
In July 2025 Avi Loeb and colleagues posted a paper suggesting that 3I/ATLAS could be alien technology, pointing to its apparent size and a trajectory lying close to the plane of the planets. Other astronomers pointed out flaws in the statistics, and the gas, dust and tails are textbook comet behaviour. The overwhelming consensus is that it is a comet. Loeb himself wrote on his blog that the most likely answer is “probably a comet”.