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3I/ATLAS was spotted on July 1, 2025, by the ATLAS network, via an instrument in Chile, and then reported to the Minor Planet Center. The MPC subsequently formalized its designation, as a comet, under the name C/2025 N1 (ATLAS), while acknowledging its hyperbolic trajectory. This orbital detail is crucial, as it indicates an origin outside the Solar System. Astronomers thus classify it as the third identified interstellar visitor, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019.
The interstellar comet 3I/ATLAS photographed on November 11, showing a bright nucleus and a well-defined ion tail
Credit: The Virtual Telescope Project
The event of December 19 does not correspond to a passage 'near the Sun', but to a favorable geometry with Earth. The perihelion of 3I/ATLAS occurred in late October 2025, which altered its visibility and tracking pace. Observers are particularly interested in the gases and dust released when the nucleus is heated. These spectral signatures are used to compare the chemistry of a body formed around another star with that of local comets. The distance remains significant, but the alignment improves the signal-to-noise ratio.
To refine predictions, the European Space Agency indicated that it has used data acquired from Mars to better constrain the trajectory. The goal is practical: reduce pointing uncertainties when the comet is most accessible. Several imaging campaigns have already been conducted, including shots by Hubble in late November, when the object was still several hundred million kilometers (hundreds of millions of miles) away. This tracking also serves to measure the evolution of activity and to estimate the size of the nucleus.
Regarding amateur observation, specialists warn that 3I/ATLAS will not be a naked-eye spectacle. Recommendations converge towards a telescope, with an aperture of at least 15 cm (about 6 inches), and preferably 30 cm (about 12 inches) to facilitate detection. Around December 19, the comet is located in the region of Leo, near the star Regulus, with easier spotting before dawn.
For the public far from dark sites, an alternative is to follow live observations. The Virtual Telescope Project has announced an online broadcast around the closest approach, subject to favorable weather conditions. This type of retransmission relies on robotic instruments and long-exposure cameras. The images are also useful to researchers, as they document the overall brightness and morphology of the coma and tail. The challenge is to collect as much data as possible before the object moves away again, with decreasing brightness.
After December 19, the trajectory of 3I/ATLAS will lead it towards the outer regions of the Solar System. A next milestone involves a close passage to Jupiter in March 2026, on the scale of planetary distances, with no risk to the planet. This step should not be confused with a 'capture', as the object retains a speed and orbital energy characteristic of an interstellar object. Astronomers will continue to observe it as long as its magnitude allows. Each measurement helps to better understand the frequency and diversity of these cosmic intruders.