The SPHEREx space mission, operated by NASA, captured a remarkable transformation in the interstellar comet 3I/ATLAS after its closest approach to Sol, an event known as perihelion, on October 30, 2025. The observations, carried out in December of that year, indicate that the object’s chemical composition began to surprisingly resemble that of comets originating from our own Sistema Solar, providing valuable data on the formation of planetary systems.
Detected in July 2025, 3I/ATLAS is the third confirmed interstellar visitor, following ‘Oumuamua and 2I/Borisov. Sua passage represented a unique opportunity for science to closely study primordial materials from another star. Após perihelion, solar heat activated deep layers of ice, triggering an intense release of gases and organic compounds that drastically altered their behavior.
Detailed analyzes revealed a twenty-fold increase in water vapor emission, in addition to the detection of complex molecules such as methanol, ethane and formaldehyde. Essa change from a relatively dormant state to full cometary activity suggests that the chemical and formation processes occurring in other stellar systems may be more similar to our own than previously thought.
Details of the discovery and its interstellar journey
Comet 3I/ATLAS was first identified on July 1, 2025, using the ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope on Chile, a NASA-funded instrument. Confirmation of its extrasolar origin came quickly, when it was discovered that its trajectory was hyperbolic, which means that its speed was high enough to escape the gravitational attraction of Sol, a characteristic of objects that are not linked to our system.
Before its solar approach, the object exhibited moderate activity, driven mainly by the sublimation of more volatile ices, such as carbon dioxide. The first estimates pointed to a core with a diameter between 600 and 800 meters, a mass of frozen material that traveled through interstellar space for billions of years before crossing our path, preserving within it the chemistry of its system of origin.
The chemical transformation after approaching the sun
Observations conducted by SPHEREx between December 8 and 15, 2025 were crucial to understanding the comet’s evolution. During this period, the water vapor flow measured was approximately 20 times higher than that recorded in August of the same year. Esse drastic increase indicates that the perihelion heat wave penetrated the surface layers and reached deeper water ice reservoirs, which until then had remained inactive.
Simultaneously, carbon monoxide emission also increased at the same rate, redefining the comet’s gaseous profile. The proportions between the different gases became remarkably similar to those found in long-period comets of Sistema Solar, an indication that planetary formation processes may be universal.
The release of cyanide also increased significantly, along with the detection of organic molecules rich in carbon and hydrogen. Compostos such as methanol and ethane, which were likely trapped beneath the ice, were released by intense sublimation, offering scientists a direct taste of the building blocks of other worlds.
The new appearance of the comet’s coma and dust
The cloud of gas and dust surrounding the comet’s nucleus, known as the coma, also showed significant structural changes. Enquanto the gas distribution maintained a spherical symmetry, the dust cloud acquired an asymmetrical shape, similar to a pear, with the narrowest part pointing directly towards the Sol. The Essa shape is the result of solar radiation pressure, which pushes dust particles away from the heat source.
The spectrum of light reflected by the comet also changed, becoming dominated by low-albedo dark dust, that is, it reflects little light. The loss of the icy surface layer exposed rockier, carbonaceous materials such as olivine and amorphous carbon, changing the way sunlight was scattered and giving the dust a bluer hue.
Composition revealed and parallels with familiar objects
One of the most notable aspects of 3I/ATLAS was the presence of a vast carbon dioxide coma, which extended for at least 348 thousand kilometers. The detection of this feature, previously carried out by SPHEREx itself, already signaled the abundance of volatile ices preserved in the comet’s nucleus since its formation in a distant star system.
Its behavior contrasted sharply with that of the first interstellar visitor, ‘Oumuamua, which exhibited no visible cometary activity. On the other hand, 3I/ATLAS showed classic activity, being brighter and apparently larger than the second visitor, 2I/Borisov, which allowed for much more detailed and in-depth observations by a variety of instruments.
Data obtained by other cutting-edge observatories, such as the James Webb and Hubble space telescopes, corroborated SPHEREx’s findings, confirming the presence of water and complex organic molecules. Esses compounds are considered the fundamental building blocks for life as we know it, and their detection in an object from another star system reinforces the idea that these prebiotic materials can be transported between stars.
At the end of the period of intense observation, the ratio of carbon monoxide, carbon dioxide and water in the comet’s coma approached typical patterns for Sistema Solar comets. Esse result defied initial expectations and suggests that accretion processes in protoplanetary disks, where comets and planets form, may follow very similar rules in different parts of the galaxy.
Contributions from other space and Earth missions
The 3I/ATLAS observation campaign was a global effort involving multiple instruments. NASA’s Parker Solar Probe probe, for example, captured images of the comet between October and November 2025, as it interacted with the solar wind near perihelion. Esses data was fundamental to mapping the structure of its ionic tail and understanding the dynamics of the coma under the direct influence of solar activity. Complementarmente, the Telescópio Óptico Nórdico, on land, had already recorded the initial changes in its activity in November, providing an essential chronological overview of the object’s evolution. The Agência Espacial Europeia (ESA) XMM-Newton observatory also contributed by detecting X-ray emissions in December, a rare phenomenon associated with the interaction between comet gases and solar radiation, adding an extra layer of information about ionization in the interstellar visitor’s coma.
What the 3I/ATLAS visit means for science
The passage of 3I/ATLAS provided a natural laboratory to study the composition of a body formed in another star system. Activation by solar heat allowed scientists to measure, in real time, the sublimation rates and flows of different gases, revealing the composition hidden beneath their icy surface and offering an unprecedented view into the chemistry of other worlds.
Technical parameters of SPHEREx observations
To perform these measurements, SPHEREx utilized its infrared observation capabilities, capturing light spectrums at wavelengths between 0.75 and 5.0 microns. Essa range of the electromagnetic spectrum is ideal for identifying the chemical “signatures” of gaseous molecules and dust, allowing a detailed analysis of the composition of the coma.
Gas production rates reached an impressive peak, reaching 6 x 10^27 water molecules per second. The Esse volume of activity places 3I/ATLAS on the same level as very active comets like our own Sistema Solar when observed at similar distances from Sol, reinforcing the remarkable similarity between this visitor and our native comets.
