There comet 3I/ATLASThe third interstellar object never discovered by man, continues to reserve new surprises as observations accumulate with the most powerful telescopes at our disposal. The latest in chronological order comes from data from the Keck telescope in Hawaii, which shows how 3I/ATLAS appears rich in nickel but devoid of ironin contrast to what is observed in comets originating in the Solar System. Additionally, nickel’s chemical signatures appear to come from compounds, such as nickel tetracarbonylwhich on Earth are normally associated with its industrial processing. This peculiarity adds to a huge rate of water loss (there are those who have spoken of a “cosmic fire hydrant”) and to his foliage particularly rich in CO2.
In its trajectory, 3I/ATLAS also passed close to Mars: taking advantage of the fact that the comet is now currently in conjunction with the Sunthat is, on the opposite side of the star to the Earth, we can exploit this “pause” in the observations of the interstellar visitor to take stock of the peculiarities and nature of this object. Avi Loeba Harvard astrophysicist with an open mind about the possible extra-terrestrial origin of the comet, has been emphasizing for months that the various anomalies presented by this interstellar object would be compatible with its hypothetical artificial origin. For the rest of the scientific community, however, the anomalies indicate that the chemical processes in the formation of planetary disks – which also determine the chemical composition and activity of comets – are not universal but may differ from system to system.
The anomalies on the chemical composition of the comet: nickel, iron and water
Comet 3I/ATLAS was observed with all the most powerful telescopes at our disposalfrom the Hubble and James Webb space telescopes to ground-based ones such as the Very Large Telescope in Chile and the Keck telescope in Hawaii. Each telescope helped reveal a piece of the puzzle of this interstellar visitor.
Observations conducted with James Webb showed how the hair of the comet either unusually rich in carbon dioxidewith a ratio between carbon dioxide and water among the highest ever measured on this type of object. In addition to carbon dioxide they are detected waterfall (also as ice grains), carbon monoxide (CO), carbonyl sulphide (OCS), dust and nitrogenous molecules such as cyanide (CN) and hydrogen cyanide (HCN). The activity is clearly triggered by the Sun, since the heating causes the ice in the nucleus to sublimate and the gases, escaping into space, drag dust and ice with them.
The chemical picture points to a core intrinsically rich in carbon dioxidewhich led scientists to formulate two hypotheses: either the comet formed near the “ice line” of the CO2 in the protoplanetary disk of origin, or has had ice for a long time exposed to more intense radiation on average than those of our Solar System. The identification of water and molecules key to prebiotic chemistry (such as HCN) in an object from another planetary system tells us that the basic ingredients of the chemistry of life are not exclusive to our Solar System.
Observations conducted with the Keck and James Clerk Maxwell Telescope (JCMT) show how in the comet is present nickel in atomic form but not atomic ironsomething almost never observed in “local” comets in which the two elements occur in similar quantities. The concentrations of nickel and cyanide, otherwise, are in line with those of Solar System comets observed at similar distances from the Sun.
The nickel production mechanism accumulated in the coma remains to be deciphered, as does the mechanism that could currently be “hiding” the iron contained in the nucleus. The data seems to favor scenarios in which atomic nickel forms from easy-to-dissociate compounds such as nickel tetracarbonyl or metallic organic molecules that sunlight dissociates near the nucleus. Nickel tetracarbonyl has been the compound that has made the most headlines in recent days, as this compound is produced on Earth through industrial processing.
Further data on the comet also showed how the nucleuswhich would have a diameter of less than 1 km, either surprisingly activelosing water at a rate of 40 kg per secondremarkable for an object of this size.
Where comet 3I/ATLAS comes from: the trajectory
We are certain that the comet prevents the interstellar space due to its very high speed (220,000 km/h), such as to cause any object that was in orbit around the Sun to leave the Solar System. The comet comes from the direction of the constellation of Sagittarius, however tracing it back to its parent star is almost impossible, since the comet has traveled in the Milky Way for billions of years. By accumulating more and more observations on its trajectory, several studies have tried to trace at least the region of the Milky Way in which 3I/ATLAS would have formed. The studies all seem to agree on the fact that the comet is originating in the disk of the Milky Way.
However, there are discrepancies as to which component of the galactic disk we are talking about. In fact, the Milky Way has a thin disk, in which the Sun is also found, and a thick disk, made of older stars. If the comet were to come from the latter, its age could be as much as two billion older than that of the Sun. The latest study in chronological order used data from the Gaia satellite to compare the trajectory of 3I/ATLAS with that of stars near the Solar System, finding that the thin disk is its most probable area of origin.
Loeb considers the comet’s trajectory unusual, as it is particularly close to the ecliptic plane (i.e. the plane on which the Earth’s orbit around the Sun lies) and with close passages to other planets but not to the Earth, with a passage behind the Sun which would prevent us from observing its hypothetical artificial nature. These coincidences were calculated by Loeb as not random and indeed excessively unlikelyalthough calculating the probability of such a trajectory is in many ways a matter of interpretation.
When it will pass by the Earth: it is now behind the Sun
3I/ATLAS is currently projected behind the Sun (in conjunction). Given its small angular distance from our star, it is not currently observable from Earth, but only through instruments in space that normally monitor the Sun, such as the GOES-19 satellite which has in fact managed to record the passage of the comet in its instruments. The point of closest approach to the Sun (perihelion) is instead scheduled for October 29thwhen the comet will pass by 203 million km from the Sun, between the orbit of Mars and that of the Earth. As you will notice, the comet never goes beyond the orbit of our planet and this also partly explains the comet’s non-pronounced cometary activity.
The point of closest approach to Earth it will happen instead December 19 at 270 million km. At that juncture, 3I/ATLAS will have a magnitude of about 14 for which it will be observable only through telescopes with generous opening. In general, by not going too close to the Sun, the comet will never be brighter than magnitude 11, therefore always requiring the use of a telescope to be observed. For comparison, at its maximum brightness the comet will be 120 times dimmer than comet C/2025 A6 (Lemmon) and about 190 times dimmer than comet C/2025 R2 (SWAN) at their closest approach.
The comet will return to being visible from our latitudes in early Novemberwhen it will arise in eastbound around 5 a.m in the company of the planet Venus, thus creating a window of visibility of about an hour before dawn. As the month continues, the comet will rise earlier and earlier, around 3am in the middle of November and around 1.30am at the end of the month. Near the perigee on December 19ththe comet will rise around 10.30pmthus remaining observable for the rest of the night, although very weak.
