3I/ATLAS: A Month-by-Month Observing Guide

There is considerable scientific interest about Comet 3I/ATLAS, discovered in July 2025. This is the third confirmed interstellar object known to be passing through the inner Solar System. Before its discovery, only two other interstellar objects were known: Asteroid 1I/2017 U1 (Oumuamua) and Comet 2I/Borisov.

As of today we know approximately 1 million minor bodies in the Solar System, a number that is expected to grow exponentially now that the Vera Rubin Observatory is becoming operational. All these objects are gravitationally bound to the Sun and they move on elliptic or almost parabolic orbits: they were born when the Solar System was born and they will not escape into interstellar space. 

Asteroid 1I/2017 U1 (Oumuamua),  Comet 2I/Borisov  and  Comet 3I/ATLAS move instead on hyperbolic orbits, which means that they travel at a speed so high that it exceeds the Sun's escape velocity. They are coming from the interstellar space, they will travel through the inner Solar System, and they will escape in the interstellar space again.

Of the three objects, 3I/ATLAS is the one with the greatest orbital eccentricity (e=6.18), which means that it is moving much faster than the other two. At the beginning of July 2025 3I/ATLAS is moving towards the Sun at the speed of 61 kilometers per second (40 miles per second).

Orbital Characterization and Observing Windows

This animation shows the trajectory of  Comet 3I/ATLAS as it moves across the inner Solar System (you can visualize the orbit of comet 3I/ATLAS using our 3D Solar System Viewer):

The key points to consider are the following:

• The orbital inclination is 175 degrees. This means that the comet's orbit is approximately on the same plane of the Earth's orbit, however the motion is retrograde, which means it moves in the opposite direction to the planets and most other objects in the Solar System.
• From July through the beginning of September, the comet will be positioned as an evening object and it will be setting after the Sun.
• Between late September and the end of October, Comet 3I/ATLAS will not be observable. During this period, when seen from the Earth, the comet will be placed on the opposite side of the Sun. In particular, the comet will not be visible during its perihelion passage, on October 29, 2025.
• November and December 2025 will present the ideal conditions to observe the comet, as the result of both the geometric positioning and the increased brightness caused by the recent perihelion passage.

3I/ATLAS during July 2025

Throughout July, 3I/ATLAS is well positioned for observation. During the first part of the month it will be in the constellation Sagittarius, and during the second half of the month it will be in the constellation Ophiuchus. These two constellations remain visible for the vast majority of the night during the month of July, so the comet will be too.

3I/ATLAS is expected to remain quite faint during this month, starting from magnitude 17.5 at the beginning of the month, and reaching magnitude 16 at the end.

On July 21, the comet will pass about half a degree (about the diameter of the full moon) from the Messier 9 Globular Cluster. The visual magnitude of the cluster is 8.42 while the magnitude of the comet is expected to be around 16.6. You can visualize the position of Comet 3I/ATLAS on that day using our Interactive Star Maps application. As shown in the Digitized Sky Survey image below, that region of the sky is extremely dense with stars, meaning a powerful telescope will be necessary to identify the comet. Our Live Position Tracker tool can be useful aid for this observation.

3I/ATLAS in August 2025

3I/ATLAS will remain visible after sunset during the month of August, however the visibility conditions will become more difficult during the second half of the month because the comet will be approaching to the Sun, decreasing its solar elongation. At the beginning of the month the comet will be in the constellation Ophiuchus, then it will move into Scorpius on August 11 and into Libra on August 19.

An interesting configuration will occur on August 19, when other 3 comets will be visible approximately in the same region of the southwestern sky: C/2025 K1 (ATLAS) (magnitude 12), C/2021 G2 (Atlas) (magnitude 15.6), P/2010 H2 (Vales) (magnitude 14.7). Comet 3I/ATLAS will be approximately of magnitude 14.8.

3I/ATLAS in September 2025

Comet 3I/ATLAS will remain in the constellation of Libra during the whole month of September. During the first part of the month it will still be visible after sunset, setting about two hours and a half after the Sun. The best visibility will be for observers near the equatorial regions, as at higher northern or southern latitudes the comet will be at a very low altitude above the horizon.

The comet's magnitude is projected to increase from about 14 to 11.7, as it continues its approach to perihelion.

3I/ATLAS in October 2025

Comet 3I/ATLAS  will not be visible during the month of October because, as seen from Earth, it will be on the opposite side of the Sun (solar conjunction). As soon as the comet will pass the perihelion on October 29, it will move at its maximum speed, allowing it to become quickly visible again in the early morning sky during the first days of November.

3I/ATLAS in November 2025

November will be the ideal month to observe Comet 3I/ATLAS. It will likely be possible to observe it already on November 3, when it will rise about 2 hour before the Sun. Based on current predictions, it should be around magnitude 10, but there is high uncertainty about this estimate because comets are very dynamic objects and the perihelion transit could dramatically influence the light curve. The proximity with the Planet Venus and with Spica, the brightest star in Virgo will help localize it (see the image below or follow the link to the online star map):

Observing conditions will continue to improve from this point on, however the comet will start becoming fainter. The first half of the month will be ideal to take long exposure images of the comet, to capture the details of the coma and the tail. On November 20 it will rise 4 hours before the Sun, and it will be still brighter than magnitude 11. 

3I/ATLAS in December 2025

The comet will continue to move farther from the Sun and the Earth. It will move through the constellation of Virgo first and Leo. By mid December it will be fainter than magnitude 12 and by the end of the year 2025 it will be fainter than magnitude 13.