The interstellar comet 3I/ATLAS arrived in our solar system last July, but will reach its minimum distance from Earth on December 19th.
Last July 1, 2025, a new visitor arrived in our solar system: the third interstellar object ever observed, cataloged as 3I/ATLAS.
Since that moment, a truly global network of terrestrial and space observatories has been mobilized to study it in the most complete way possible. Soon after the automatic sky surveillance systems were reported, astronomers from the European Space Agency (ESA) activated telescopes in Hawaii, Chile and Australia, followed by observations with leading space instruments, including the Hubble Space Telescope and the XMM-Newton X-ray observatory.
Ever closer. When 3I/ATLAS reemerged from behind the Sun in early November, cometary activity became more evident. On November 30, Hubble observed the object again from a distance of about 286 million kilometers, using its Wide Field Camera 3.
The images showed a comet in full outgassing, approaching its closest approach to Earth, scheduled for December 19, when it will pass at about 269 million kilometers, just under twice the average distance. Earth–Sun.
On X-rays. But one of the most surprising looks at 3I/ATLAS came from Space. On 3 December, ESA’s XMM-Newton observatory tracked the comet for around 20 hours, when it was at a distance of between 282 and 285 million kilometers from the spacecraft. Thanks to the EPIC-pn camera, the most sensitive to X-rays on board the satellite, astronomers detected a faint but clear glow of low-energy X-rays.
In those images, the surrounding space appears almost empty, while the comet emerges as a bright source. The phenomenon was expected: the gases that escape from the cometary nucleus, colliding with the solar wind, produce X-rays through interaction processes between charged particles. However, observing this mechanism in an object of interstellar origin represents a result of great scientific value.
Out of the ordinary. As observations accumulate, 3I/ATLAS is revealing itself to be a unique object. In fact, spectroscopic data collected by ground-based and space telescopes indicate an unusual chemical composition, quite different from that of most comets in our solar system. In particular, the foliage is exceptionally rich in carbon dioxide, while the presence of water is scarce. Such a high CO₂/H₂O ratio suggests that the comet may have formed in a very cold or chemically different environment from that in which “local” comets were born.
Alongside the most common gases, astronomers have also identified traces of carbon monoxide and, quite unexpectedly, atomic nickel vapours. The latter is an element rarely observed in gaseous form in comets and could indicate particular physical processes or a different primordial composition, linked to the star system of origin of 3I/ATLAS.
An ancient traveler. According to analyses, 3I/ATLAS could be an extremely ancient object, probably formed billions of years ago and subsequently expelled from its home planetary system. During its very long journey among the stars, the comet’s surface would have been exposed to the bombardment of galactic cosmic rays for ages, creating a sort of altered and chemically evolved crust. The current cometary activity could therefore result from the fracture of this surface layer, which allows “fresher” materials to emerge from within.
Powders, jets and rotation. The physical behavior of the comet also provides valuable clues. Polarimetric observations indicate that dust in the coma reflects light anomalously, suggesting grains of different sizes or structures than those typical of Solar System comets.
Furthermore, high-resolution images show the presence of localized jets of gas and dust, probably linked to active regions on the nucleus. These jets appear to vary over time in a manner consistent with a rotation of the nucleus of about 15–16 hours, a value compatible with that of many comets, but surprising for an object that has spent most of its existence in interstellar space.
A natural laboratory. Taken together, these features make 3I/ATLAS a natural laboratory. Its properties recall some hypotheses advanced to explain the nature of 1I/’Oumuamua, the first interstellar object discovered in 2017, which according to some models may have been composed largely of exotic ices such as nitrogen or hydrogen. Today ‘Oumuamua is unattainable, but 3I/ATLAS is providing a rare opportunity to test these ideas with direct data.
X-ray, infrared and visible observations, combined, are allowing scientists to reconstruct not only the composition of the comet, but also the physical and chemical conditions of the distant planetary systems from which these enigmatic travelers came.
