Comet 3I/ATLAS makes its closest approach to the Earth.

Comet 3I/Atlas will make its closest approach to the Earth on Friday 19 December 2025, when it will reach a distance of 1.80 AU (i.e 1.8 times the average distance between the Earth and the Sun), or 269 037 114 km from us. At this time it will be in the constellation of Leo, and have an apparent optical magnitude of 11.4, meaning it will be hard to spot without a fairly good telescope to observe it. Nevertheless, this closest approach falls coincides with the New Moon, so observers with appropriate equipment may be able to see it in the late evening (the comet will set before midnight).

The approximate positions and orbits of the 3I/ATLAS, the Earth, and the planets of the Inner Solar System on 19 December 2025. JPL Small Body Database.

3I/Atlas was discovered on Tuesday 1 July 2025 by scientists at the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile, who observed a body 4.53 AU from the Sun (i.e. 4.53 times as far from the Sun as the planet Earth) between the constellations of Serpens Cauda and Sagittarius, which was given the provisional designation A11pl3Z. This object was travelling towards the Inner Solar System at a speed of 65 km per second, on what appeared to be a more-or-less straight trajectory, highly unusual in a body orbiting the Sun.

Discovery images for object A11pl3Z. ATLAS/University of Hawaii/NASA/Wikimedia Commons.

A series of follow-up observations  by both professional and amateur astronomers confirmed that the body was a comet on a hyperbolic trajectory (a trajectory which will take it straight through the Solar System and out into interstellar space. Most such parabolic comets derive from the Oort Cloud, a vast disc of thinly spread cometry bodies between 2000 and 200 000 from the Sun. These comets are knocked from their orbits be close encounters with other bodies, plunge through the Inner Solar System once, then vanish into the depths of space. 

Full discovery image for 3L/ATLAS. University of Hawaii/NASA/Wikimedia Commons.

However, two previous comets have been found to be on trajectories which cannot be explained in this way, these being 1I/‘Oumuamua and 2I/Borisov, and on Tuesday 2 July it was confirmed that A11pl3Z was a third such body, leading to it being given the designation 3I/Atlas, in which the 'I' stands for 'Interstellar body', the '3' indicates that it was the third such body discovered, and 'ATLAS' refers to the ATLAS asteroid impact early warning system, which discovered the object.

Follow up image of 3I/ATLAS made by the system Las Cumbres Observatory on 2 July 2025. European Space Agency.

However, two previous comets have been found to be on trajectories which cannot be explained in this way, these being 1I/‘Oumuamua and 2I/Borisov, and on Tuesday 2 July it was confirmed that A11pl3Z was a third such body, leading to it being given the designation 3I/Atlas, in which the 'I' stands for 'Interstellar body', the '3' indicates that it was the third such body discovered, and 'ATLAS' refers to the ATLAS asteroid impact early warning system, which discovered the object.

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