The Quiet Wanderer: Tracking Interstellar Comet 3I/ATLAS Before Its Solar Flyby
When astronomers spot an object hurtling through space on a path that will never return to the Solar System, it becomes something extraordinary, an interstellar object (ISO). Comet 3I/ATLAS is just the third confirmed ISO, following in the footsteps of 1I/‘Oumuamua and 2I/Borisov. Discovered in July 2025, 3I/ATLAS immediately sparked interest. In their paper, Santana-Ros and collaborators present an early look at this visitor, using telescopes around the world to study its spin, color, dust activity, and light over the month following its discovery.
Spinning in the Dark
The team began by observing 3I/ATLAS with a mix of ground-based telescopes to collect photometry (brightness over time) and spectroscopy (light broken into colors to reveal composition). These observations showed that the comet spins once every 16.16 hours, this was determined by tracking changes in brightness as the comet rotated. The lightcurve, or graph of brightness over time, had a small peak-to-peak change of 0.3 magnitudes, a clue that the shape of the comet may not be very elongated, or that activity from dust clouds was smoothing out those variations.
A Comet Changing Color
Color observations revealed that the comet is becoming redder as it gets closer to the Sun, especially in the visible wavelengths between 0.4 and 0.7 microns. This reddening might suggest changes in the dust being ejected from the comet's surface or from evolving material in the coma, the cloud of dust and gas surrounding its nucleus. Interestingly, the colors measured by the team differ from earlier results but are in line with those of distant comets and 2I/Borisov, hinting that despite its interstellar origin, 3I/ATLAS may have dust properties similar to Solar System comets.
A Tail That Isn’t There
Although the comet was active, with a noticeable coma, it never showed a clear tail in images, likely due to a combination of observational angle and a lack of very fine dust particles that are typically pushed away by sunlight to form tails. Using digital image processing, the researchers searched for hidden structures like jets or filaments but found none, reinforcing the view that 3I is only weakly active. The coma did show a slight elongation over time, suggesting subtle dust outflows.
How Much Dust is Escaping?
The study also measured how much material the comet was losing. Using a standard dust production metric called Afρ (pronounced “A f rho”), the team estimated dust mass loss rates between 0.3 and 4.2 kilograms per second. These values are modest, less than some Solar System comets, but still significant for an object so far from the Sun. The team also used brightness to estimate the comet's size, suggesting it could be up to 11 kilometers across, although this is just an upper limit since active comets can appear brighter than they really are.
What We Know So Far
Santana-Ros and colleagues point out that 3I/ATLAS looks much like an outer Solar System comet in terms of color, brightness, and activity, despite its clear interstellar origin. This supports the idea that planetesimal formation (the process that builds comets and asteroids) might be similar in other star systems. The researchers plan to continue monitoring 3I/ATLAS as it swings around the Sun in October 2025. The hope is that further changes in brightness, color, and dust activity will reveal more about how material from another star system reacts to our Sun’s radiation.
Looking Ahead
This study of 3I/ATLAS is a powerful example of rapid-response astronomy and offers a valuable baseline to compare with future interstellar visitors. For now, 3I continues its journey, quietly active, subtly changing, and carrying secrets from a star system far beyond our own.
Source: Santana-Ros
