Comet Comet 3I/ATLAS approaches perihelion.

Comet 3I/ATLAS (also known as C/2025 N1 (Atlas)) will reach perihelion (its closest approach to the Sun) on Wednesday 29 October 2025, when it will reach 1.36 AU from the Sun (1.36 times the average distance between the Earth of the Sun). Since the comet will be on the other side of the Sun it will not be visible from Earth, and it would be highly unwise to look for it with a terrestrial telescope.

The trajectory of 3I/ATLAS and the orbits of the planets of the Inner Solar System, and their positions on 29 October 2024. 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/Wikipedia.

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. 

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.

3I/ATLAS will make its closest approach to the Earth on 19 December, when it will be 1.80 AU from us. Unfortunately, this will also happen while the comet is on the far side of the Sun, preventing observations during this period. The comet passed the planet Mars at a distance of 0.19 AU on 3 October, and will pass Jupiter at 0.38 AU on 16 March 2026. 3I/ATLAS is apparently a weekly active comet with an absolute magnitude of about 14.9 (a measure of its brightness), in the constellation of Virgo.

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