The interstellar comet 3I/ATLAS : a unique visitor in our Solar System

Comets have always fascinated humanity, appearing suddenly in the night sky and provoking both admiration and mystery. Among these celestial visitors, some originate from beyond our Solar System, offering a unique opportunity to study materials formed in other stellar environments. The comet 3I/ATLAS, discovered on July 1, 2025, is the third interstellar object identified to date, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. This article explores the nature of interstellar comets, traces the discovery of 3I/ATLAS, and examines its unique characteristics.

Interstellar comets: Messengers from other worlds

Interstellar comets are celestial objects that traverse our Solar System but originate outside of it. Their hyperbolic trajectory indicates that they are not gravitationally bound to the Sun, which distinguishes them from traditional comets. Before 2017, no interstellar comet had been observed. The first, 1I/ʻOumuamua, discovered in October 2017, displayed an unusually elongated shape and a visible absence of a coma, raising many questions about its nature. In 2019, 2I/Borisov was identified, resembling more traditional comets with a tail and pronounced cometary activity.

The discovery of 3I/ATLAS

On July 1, 2025, the ATLAS system (Asteroid Terrestrial-impact Last Alert System) detected a new object in the sky. Initial observations revealed a hyperbolic trajectory, confirming its interstellar origin. Named 3I/ATLAS, this comet quickly became the subject of particular attention within the astronomical community. Subsequent analyses estimated its age at about 7 billion years, which would make it the oldest comet ever observed, predating the formation of our Solar System by about 3 billion years.

Characteristics of 3I/ATLAS

The nucleus of 3I/ATLAS is estimated to be about 10 kilometers in diameter, which is significantly larger than the previously observed interstellar objects. Its velocity relative to the Sun is about 60 km/s (approximately 215,000 km/h), a speed sufficient to escape the Sun's gravitational pull. As it approaches the Sun, the comet shows increasing activity: its coma, a cloud of gas and dust surrounding the nucleus, expands, and its tail lengthens. These phenomena result from the sublimation of ices present in the nucleus due to solar heat.

Trajectory and visibility from Earth

3I/ATLAS will reach its perihelion, the point in its orbit closest to the Sun, on October 29, 2025, at a distance of about 1.357 astronomical units (203.2 million kilometers). It will pass closest to Earth in December 2025, at a distance of about 167 million miles (269 million kilometers). Although its current magnitude is estimated at 16.7, making it observable only with powerful telescopes, its brightness could increase as it approaches the Sun. In France, amateur astronomers equipped with telescopes might attempt to observe it at dawn in November and December, depending on how its brightness evolves.

Scientific importance of interstellar comets

The study of 3I/ATLAS offers a valuable opportunity to understand the composition and formation processes of planetary systems other than our own. Interstellar comets are regarded as remnants of the formation of their systems of origin, preserving information about the physical and chemical conditions prevailing at that time. By analyzing the composition of 3I/ATLAS, scientists hope to gain insights into the diversity of planetary-formation environments in the galaxy.

Next steps in observation

Astronomers around the world continue to monitor 3I/ATLAS using a variety of ground-based and space-based telescopes. Spectroscopic observations are planned to analyze its chemical composition in detail. In addition, efforts are underway to model its future trajectory and estimate its potential brightness, in order to prepare optimal observing campaigns. International collaboration is essential to make the most of this rare opportunity to study an interstellar visitor.

In conclusion, the comet 3I/ATLAS represents an open window into the mysteries of the formation of planetary systems beyond our own Solar System. Its in-depth study could enrich our understanding of the diversity of worlds that populate our galaxy and, by extension, of our own cosmic origins.