The team of NASA’s SPHEREx space observatory just reported tantalizing new data on the interstellar object 3I/ATLAS (accessible here). The observations were made between August 8–12, 2025 when 3I/ATLAS was at a distance from the Sun of 3.2 times the Earth-Sun separation (AU) and a distance from Earth of 2.6 AU.
The new observations reveal a cloud of carbon dioxide (CO2) around 3I/ATLAS corresponding to a mass loss rate of about 70 kilograms per second. No water (H2O) cloud was detected with an upper limit of 4.5 kilograms per second on the water mass loss rate. This is an order of magnitude below the previous claims of water detection with a mass loss rate of order 40 kilograms per second at a larger distance from the Sun of 3.5 AU. These early claims from two research teams were unsubstantiated by the reported data, as I argued in a previous essay (accessible here). The excellent SPHEREx report notes that “The lack of a bright water gas coma is puzzling as 3I/ATLAS was not too far outside the Solar system’s “water ice line” at 2.5 AU during the observations.”
Although no water (H2O) in gas form was identified, some absorption features in the reflected spectrum from the surface of 3I/ATLAS were consistent with a mix of water and carbon dioxide ices combined with organics, as often found on the surfaces of Kuiper belt objects in the Solar system which are similarly exposed to interstellar cosmic-rays. Could it be that 3I/ATLAS is not a water-rich comet as envisioned by comet experts when it was discovered?
The SPHEREx images show 3I/ATLAS as a point source. No dust coma was resolved, implying that the glow of scattered sunlight around the object in its Hubble Space Telescope image is compact and amounts to a small amount of dust.
SPHEREx images were taken at specific wavelengths near the characteristic emission lines of water-H2O (3.0 micrometers), carbon dioxide-CO2 (4.26 micrometers) and carbon monoxide-CO (4.7 micrometers). No coma was detected in water or carbon monoxide. However, the CO2 image shows a symmetric cloud around I/ATLAS with a brightness that declines with projected distance to the power of -3/2 out to distances of at least 348,000 kilometers. This corresponds to a steeply declining CO2 density with 3D-radius to the power of -2.5.
Most interestingly, the flux detected at a wavelength of 1 micrometer from 3I/ATLAS suggests a large nucleus with a diameter of 46 kilometers. If this represents a solid body, then the mass of 3I/ATLAS must be a million times bigger than the previous interstellar comet 2I/Borisov. This makes little sense since we should have found of order a million objects of the size of 2I/Borisov before discovering a 46-kilometer interstellar object. Moreover, as I noted in my first paper on 3I/ATLAS (accessible here), the amount of rocky material per unit volume in interstellar space is too small by a factor of ten thousand than the value needed to deliver into the inner Solar system one giant rock of this size over the ATLAS decade-long survey.
Alternatively, 3I/ATLAS may have targeted the inner solar system by technological design. This possibility is consistent with the alignment of its trajectory with the orbital plane of the planets around the Sun, a coincidence of a part in 500 for a random orientation — as observed for 2I/Borisov.
The lack of a cometary tail in the Hubble Space Telescope image is evidence that there is not much dust around 3I/ATLAS with particle size on the scale of the wavelength of sunlight (0.5 micrometers). In that case, the observed reddening in the spectrum of reflected sunlight should originate from the surface of the object, implying that the object is large and dominates the reflected sunlight rather than the dust cloud around it.
The CO2 mass loss amounts to the ablation of a millimeter thick layer from the surface of a 46-km rock over a period of 10 years. This means that a relatively thin outer layer is sufficient to maintain the observed cloud of CO2 gas and dust around 3I/ATLAS. What lies under this outer layer is still unknown. We are all waiting for the release of the first data from the Webb Space Telescope, when it observed 3I/ATLAS on August 6, 2025.
Here’s hoping that as the Sun turns on the heat on 3I/ATLAS in the coming months, it will reveal its true nature.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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