A First Glimpse of the Interstellar Comet 3I/ATLAS

Cyrielle Opitom and collaborators report on their first observations of the third-known interstellar object (ISO), 3I/ATLAS, taken just two days after its discovery. Using the MUSE spectrograph on the Very Large Telescope (VLT), the team captured a detailed early look at this visitor from another star system as it was still far from the Sun, offering a baseline to compare against future changes as it heats up and becomes more active.

Introduction

The study of interstellar objects in our Solar System began only recently, with the discovery of 1I/‘Oumuamua in 2017, which showed no signs of a coma, and then 2I/Borisov in 2019, which resembled a typical comet. Each provided tantalizing but very different clues about the diversity of ISOs. Scientists have been eagerly waiting for a third ISO to appear so they could build a clearer picture of these objects and their origins. 3I/ATLAS, discovered in July 2025 by the ATLAS survey, is the long-awaited third. Unlike ‘Oumuamua, it showed activity immediately, with a visible coma (a cloud of dust and gas) even at a distance of almost 5 astronomical units (au) from the Sun. The object is expected to reach its closest point to the Sun in October 2025, at about 1.35 au, giving astronomers a year-long opportunity to study it as it becomes more active.

Observations

Opitom’s team quickly secured observing time on the VLT just two days after the discovery. They used the MUSE instrument, which allows both spectroscopy (splitting light into its component colors to study material composition) and imaging. At the time, 3I was about 4.47 au from the Sun and crossing a crowded star field. To minimize interference from background stars, the team took eight short exposures, rotating and shifting the camera between them. They then used standard data-reduction techniques to clean and calibrate the data. The team also performed complementary observations with the TRAPPIST-North telescope in Morocco, taking images through different color filters to measure the object’s brightness and color.

Results

The combined VLT images revealed that 3I has a small but clear coma extending slightly westward, confirming its cometary activity. The extracted spectrum showed sunlight reflected from the dust, but no detectable gas emissions such as from C₂, CN, or [OI], which are common in active comets. This is consistent with other Solar System comets observed at similar distances, where the coma is often dominated by dust rather than gas. By comparing the reflected light to the Sun’s spectrum, the authors measured how “red” the coma appears -- meaning it reflects more red light than blue -- finding a relatively steep reddening slope of about 18% per 1000 Å. This makes 3I redder than most comets in our Solar System but similar to some Trans-Neptunian Objects (TNOs) and Centaurs, bodies that orbit far beyond Neptune.

Discussion and Conclusions

The red color of 3I’s coma suggests its dust resembles the surfaces of distant Solar System bodies, possibly shaped by radiation during its interstellar journey. Compared to 2I/Borisov, 3I is even redder, more like some Trans-Neptunian Objects or Centaurs than typical comets. The absence of gas emissions at this distance is expected, but future observations as it nears the Sun may reveal gas activity and test predictions of a high water-ice content. With its low perihelion and year-long observability, 3I offers an exceptional chance to study how an interstellar comet compares to those from our own Solar System.

Source: Opitom