The latest findings from NASA’s SPHEREx space observatory provide intriguing information about the interstellar object 3I/ATLAS. These observations, conducted between August 8 and August 12, 2025, reveal significant data about the object’s composition and behavior as it traveled through our Solar System. At the time of observation, 3I/ATLAS was located 3.2 times the average distance from the Earth to the Sun (AU), or 2.6 AU from Earth.
The new data indicates the presence of a cloud of carbon dioxide (CO2) surrounding 3I/ATLAS, with a mass loss rate estimated at approximately 70 kilograms per second. Surprisingly, no significant cloud of water (H2O) was detected, with the upper limit for its mass loss rate set at just 4.5 kilograms per second. This figure is significantly lower than earlier claims suggesting a water mass loss rate of around 40 kilograms per second when the object was at a distance of 3.5 AU from the Sun. These earlier assertions from two research teams lacked substantiation, as detailed in previous analyses.
The SPHEREx report notes a curious absence of a bright water gas coma, especially given that 3I/ATLAS was not far from the Solar System’s “water ice line” at 2.5 AU during the observations. While no gaseous water was identified, some absorption features in the reflected spectrum suggest a composition that includes a mix of water and carbon dioxide ices, alongside organic materials—similar to what is found on the surfaces of objects in the Kuiper Belt.
These findings raise questions about the nature of 3I/ATLAS. Is it truly a water-rich comet, as initially believed by experts upon its discovery? The SPHEREx images depict 3I/ATLAS as a point source, with no discernible dust coma. This suggests that the glow observed from the object in Hubble Space Telescope images is compact, indicating a minimal amount of dust surrounding it.
SPHEREx captured images at specific wavelengths near the characteristic emission lines of water (3.0 micrometers), carbon dioxide (4.26 micrometers), and carbon monoxide (4.7 micrometers). Notably, no coma was detected in either water or carbon monoxide. Nevertheless, the CO2 image reveals a symmetric cloud around 3I/ATLAS, with brightness declining with distance to the power of -3/2, indicating a steep decrease in CO2 density with distance.
Interestingly, the flux detected at a wavelength of 1 micrometer suggests that 3I/ATLAS has a large nucleus measuring approximately 46 kilometers in diameter. If this measurement is accurate, it implies that the mass of 3I/ATLAS could be a million times greater than that of the previous interstellar comet, 2I/Borisov. This raises further questions, as the likelihood of finding such large objects in interstellar space is extremely low.
One theory posits that 3I/ATLAS may have been directed toward the inner Solar System through technological means, given the alignment of its trajectory with the orbital plane of the planets—a coincidence with a probability of 1 in 500. The lack of a visible cometary tail in Hubble Space Telescope images suggests there is minimal dust, which may explain the observed reddening in the spectrum of reflected sunlight. This reddening likely originates from the object’s surface, indicating that the object itself is large enough to dominate the reflected sunlight.
The mass loss of CO2 corresponds to the ablation of a millimeter-thick layer from the surface of a 46-kilometer rock over a decade, indicating that a relatively thin outer layer can sustain the observed cloud of CO2 gas and dust surrounding 3I/ATLAS. The true nature of what lies beneath this outer layer remains a mystery. As we await the release of data from the Webb Space Telescope, which observed 3I/ATLAS on August 6, 2025, hope remains that the increasing heat from the Sun will reveal more about this fascinating interstellar object.
