The astronomical community is following the latest events in the sky with unconcealed excitement, spurred by the arrival of an extremely rare visitor from deep space. The third confirmed interstellar object, a comet officially cataloged as 3I/ATLAS, has provided scientists with a unique opportunity to study material originating from beyond the boundaries of our Solar System. Thanks to the incredible precision of the Hubble Space Telescope, a team of astronomers has managed to capture the sharpest images to date of this cosmic traveler, revealing new details about its nature and composition. These icy messengers from other star systems represent a tangible link to the rest of the Milky Way galaxy, offering us a window into the chemical and physical processes that occur around other stars.

Hubble's sharp eye specifies the dimensions of the space guest

The Hubble Space Observatory, a joint project of NASA and the European Space Agency (ESA), has played a key role in defining the physical characteristics of comet 3I/ATLAS. Although ground-based telescopes first detected the object and its unusual trajectory, it was Hubble's high-resolution images that allowed scientists to place much stricter limits on the size of its solid, icy nucleus. According to the latest analyses, the diameter of this interstellar comet's nucleus does not exceed 5.6 kilometers, but it could be as small as 320 meters. This wide range of estimates indicates the extreme difficulty of measuring such a small and distant object. Even the powerful Hubble cannot directly resolve the nucleus itself because it is enveloped in a dense cloud of dust and gas, the so-called coma, which forms as the comet approaches the Sun. The Sun's heat causes ice on the nucleus's surface to sublimate, releasing trapped dust and creating a dazzling shroud that obscures the very center of the comet. Hubble managed to record an impressive cloud of dust being ejected from the side of the comet facing the Sun, as well as indications of a dust tail forming and extending away from the nucleus. Interestingly, the data shows that 3I/ATLAS is losing dust in a manner very similar to comets originating from our Solar System, for instance from the distant Oort Cloud. But the fundamental difference lies in its origin – this visitor was born in some other, distant star system.

An interstellar sprinter on its journey through

What makes 3I/ATLAS undoubtedly an interstellar object is its incredible speed and trajectory. The comet is hurtling through our Solar System at a speed of approximately 210,000 kilometers per hour, which is the highest speed ever recorded for a space visitor. Such a speed far exceeds the Sun's gravitational pull, meaning the comet will just pass through our system and continue its journey into interstellar space, rather than being trapped in an orbit around our star. This dizzying sprint testifies to its extremely long journey through the galaxy, which has likely lasted for billions of years. During this journey, the comet passed near other stars and nebulae countless times, with each close encounter acting as a gravitational slingshot, adding to its momentum and gradually increasing its speed. The longer 3I/ATLAS wandered through interstellar space, the faster it became. It is this hyperbolic trajectory, which is not gravitationally bound to the Sun, that is the key evidence of its extra-solar origin. This object is only the third of its kind, after the mysterious object 1I/ʻOumuamua discovered in 2017 and comet 2I/Borisov from 2019.

The search for its origin and future observations

Comet 3I/ATLAS was discovered on July 1, 2025, using the ATLAS (Asteroid Terrestrial-impact Last Alert System) at a distance of about 675 million kilometers from the Sun. The European Space Agency's Planetary Defence Office also reacted promptly to the discovery, which has been tracking the object since early July. The comet is predicted to remain visible to ground-based telescopes until September, after which it will get too close to the Sun to be observed. It is expected to reappear on the other side of the Sun in early December. Although Hubble's observations have provided valuable insights, scientists are hoping for even more detailed data. Great hopes are placed in the James Webb Space Telescope (JWST), which with its infrared sensors will be able to penetrate deeper through the dust cloud and analyze the chemical composition of the gases evaporating from the nucleus. Analysis of the chemical composition, particularly the ratio of different isotopes, could reveal unique characteristics of the star system in which the comet was formed, giving us the first real sample of material from another "solar system." This data will help us to better understand the diversity of planetary systems throughout our galaxy.

The Comet Interceptor mission: Preparing for future visitors

Although a direct visit to comet 3I/ATLAS with existing technology is impossible due to its immense speed and late discovery, its appearance highlights the importance of a proactive approach. In this regard, the European Space Agency is preparing the revolutionary Comet Interceptor mission. Scheduled for launch in 2029, this spacecraft will be placed in a stable "parking" orbit in space, where it will await the arrival of a suitable target. The mission's goal is to intercept and study a pristine comet coming from the distant Oort Cloud or, in an ideal and very rare scenario, a future interstellar object. Comet Interceptor will be the first mission to demonstrate a rapid response capability, waiting in space for its target instead of being launched towards a pre-known one. Although the probability of discovering an interstellar object reachable by Comet Interceptor during the mission's lifetime is low, it will serve as a crucial test and a benchmark for future missions that could intercept these mysterious visitors from other worlds, opening a new chapter in space exploration.