3I/Atlas: Third interstellar comet discovered.

On Tuesday 1 July 2025 scientists at the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile, 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 cometary 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. 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.

The trajectory and current position (on 5 July 2025) of interstellar comet 3I/ATLAS. The Sky Live.

3I/ATLAS is predicted to reach its perihelion (closest point on its trajectory to the Sun) on 29 October 2025, when it will be 1.36 AU from the Sun. It will make its closet approach to the Earth on 19 December, when it will be 1.80 AU from us. Unfortunately, these events will happen while the comet is on the far side of the Sun, preventing observations during this period. The comet will pass the planet Mars at a distance of 0.19 AU on 3 October, and Jupiter at 0.38 AU on 16 March 2026. 3I/ATLAS is apparently a weekly active comet with an absolute magnitude of about 12 (a measure of its brightness), which implies a nucleus with a diameter of 3-5 km.

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