This Friday will mark a milestone for the interstellar object; it will be shortly before it begins its final journey out of the Solar System, when it will leave the last image for the rest of history. 19 December will not be just any day for the interstellar comet 3I/ATLAS. This date has been established as ‘D-Day’ in the astronomical calendar, as it will mark the point of maximum approach of the enigmatic object to Earth. After this event, the comet will begin an irreversible journey to the far reaches of space, leaving our solar system forever.
This cosmic traveller, only the third of its kind confirmed to come from outside our star system, will pass at a distance of approximately 270 million kilometres from Earth, according to NASA measurements. Although this distance guarantees total safety for our planet, 19 December offers the best opportunity for detailed observation. To be able to appreciate it, those interested will need to use optical instruments, such as small telescopes or astronomical binoculars, since its low brightness, estimated at an apparent magnitude of about 10, will prevent it from being visible to the naked eye. The ideal time for observation will be during the hours before dawn, when the sky is darkest, looking towards the south-eastern horizon, near constellations such as Virgo and Leo.
3I/ATLAS was discovered in July and has since attracted the attention of scientists for a number of unusual characteristics. After its closest approach to the Sun on 29 October, the comet exhibited considerable activity. Recent NASA images, taken by the Hubble telescope, revealed its nucleus, the surrounding gas cloud known as the coma, and an even more impressive phenomenon: the presence of two tails. The European Space Agency (ESA) explained that one of them is the ‘plasma tail,’ formed by electrically charged gas, and the other is a fainter ‘dust tail,’ composed of solid particles. This double structure is a sign of intense activity in its trajectory.
In addition to the tails, astronomers have detected unusual behaviour that they have described as a ‘heartbeat’. New observations have revealed jets of gas and dust that shoot out in rhythmic bursts every 16.16 hours, generating an intermittent glow that increases and decreases by between 20 and 40%. The official explanation from NASA and ESA is that the comet’s nucleus rotates once every 16.16 hours, exposing areas of ice that, when heated by the Sun, turn directly into gas, creating these jets that act ‘like a clock’. The ejected material can reach speeds of up to 1,585 kilometres per hour and travel 25,000 kilometres, which causes the luminous pulse.
However, astrophysicist Avi Loeb pointed out that this glow is too intense to be explained solely by the rotation of the nucleus. In his view, if only the nucleus generated the jets, the large coma should soften these flashes, which would limit the brightness variation to 5%, rather than the 20-40% observed. Loeb also highlighted other anomalies in previous observations, such as an exceptionally large non-gravitational acceleration that cannot be explained by normal ice outgassing and an atypical chemical composition with unusual levels of carbon dioxide and nickel.
These characteristics, together with the absence of a large gas cloud to justify its acceleration, have led some researchers, such as Loeb, to consider more speculative hypotheses, including the possibility that it is a structure of artificial or technological origin. However, NASA has completely ruled out this hypothesis and insisted that 3I/ATLAS is a natural object. Despite scientific discussions about its exact nature, what is certain is that 3I/ATLAS represents a unique opportunity to study interstellar objects, as it allows for improved surveillance of celestial bodies that may pose risks in the future. After 19 December, its hyperbolic trajectory will take it beyond Jupiter’s orbit in the northern spring of 2026, and its brightness will gradually diminish until it disappears from view, never to return to the solar system.