There comet 3I/ATLASthe third interstellar object discovered to transit our Solar System, arrives tomorrow 19 December at the fateful moment of closest proximity to Earth: this is a considerable distance (269 million km80% more than the Earth-Sun distance) but it has never been so close to us nor will it ever be in the future, since the comet is destined to leave the Solar System and never return. We reiterate, for the avoidance of doubt, that at that distance the comet it poses no danger to the Earthbut this is the last chance – a “Christmas present”, if we will – for the scientific telescopes from all over the world to capture as much information as possible about this very interesting celestial body, but also for amateur astronomers and enthusiasts to look at it one last time.
How to observe the interstellar object in the Italian skies or live
Luckily for us, the comet is visible from Italy: it rises completely from the eastern horizon at 23 about, and remains in the sky all night, in the constellation of Leoreaching a considerable maximum height in the sky of 51st for about an hour around 5:00 of the morning. Unfortunately the distance is such that the comet will not be visible to the naked eyeand not even with good binoculars: with an estimated magnitude of approx 15.2to observe it you will need a telescope with at least 20 centimeters of opening.
This is actually not the absolute best time to observe 3I/ATLAS, but the comet it is still close to its peak apparent brightness compared to the Earth and, above all, from tomorrow onwards it will move further and further away from us (and from the Sun, from which it has been moving away since October 29th) and therefore will gradually become less and less bright. These are therefore the ideal nights for those who want to try their hand at observing this celestial body. The occasion is also enhanced by the fact that, a few degrees away in a north-east direction (i.e. “to the left” with respect to 3I/ATLAS) another comet will be able to be observed, 24P/Schaumasseof magnitude approximately 12.
If you don’t have the opportunity to observe it in person, you can admire the 3I/ATLAS show at the moment of maximum proximity to the Earth with one of the numerous online live broadcasts prepared for the occasion, for example the one organized by Virtual Telescope Project for December 19 at 5:00 am, which will be conducted by the astrophysicist Gianluca Masi and which you can recover here.
3I/ALTAS will move away forever: the trajectory
Comet 3I/ATLAS entered the Solar System with too high a speed to remain “trapped” by the Sun’s gravity: its trajectory is in fact not elliptical or even closed, in other words it is not destined to become a Solar System body. It was discovered on July 1, 2025 and later entered the inner Solar System, reaching its closest proximity to Mars on October 3 and the Sun on October 29. Now, at its closest to Earth, the comet has recently surpassed the orbit of Mars.
What will happen now? Once you cross the Main asteroid belt between Mars and Jupiter and therefore exiting the internal Solar System, the comet will pass very close to Jupiter in mid-March and then move further and further away until it definitively exits the Solar System and will never return.
