Is 3I/ATLAS a Comet or Something Else?
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Diverging Reports Breakdown
Is 3I/ATLAS a Comet or Something Else?
3I/ATLAS was discovered on July 1, 2025 to be moving at a speed of about 60 kilometers per second towards the inner Solar system. The Minor Planet Center labeled it a comet, based on preliminary reports about hints of cometary activity. Stacked images show a limited fuzz around the object but it is difficult to tell whether the elongation of the fuzz results in part from smearing of the image as a result of the motion of the object. At present, the limited cometary fuzz appears to be entirely dusty. There could be some undetectable gas in between the solid nucleus of 3I/ ATLAS and the dust surrounding it. The surface of 3i/ATlAS could be predominantly a non-evaporating crust that provides thermal insulation and blocks the heat transfer and sublimation of volatile ices under it. Future observations will provide a key opportunity to witness its activity, infer the size of its solid nucleus, study the abundance and composition of its population, and test predictions for its velocity dispersion.
Composite image of 3I/ATLAS in the R-band. The faint fuzz around it appears to be elongated along the direction of motion. (Image credit: Seligman et al. 2025)
The interstellar object 3I/ATLAS was discovered on July 1, 2025 to be moving at a speed of about 60 kilometers per second towards the inner Solar system.
The Minor Planet Center labeled it a comet, based on preliminary reports about hints of cometary activity. Stacked images show a limited fuzz around the object but it is difficult to tell whether the elongation of the fuzz results in part from smearing of the image as a result of the motion of the object. The elongation is along the direction of motion with a spatial extent comparable to the product of the object’s speed of 60 kilometers per second times the cumulative exposure time which is typically hundreds of seconds. The total brightness of 3I/ATLAS remained nearly constant over a period of a few days, suggesting that either the object is hidden beyond the veil of dust or it is nearly spherical if its rotation period is shorter than that.
Today, the first spectroscopic data on 3I/ATLAS was shared publicly in a new preprint by Cyrielle Opitom and collaborators. Such data offers a unique opportunity to study the cometary activity and composition of 3I/ATLAS, which may only get more prominent as the object is heated along its path to closest approach from the Sun (perihelion), expected to occur on October 29, 2025.
The reported observations were conducted on July 3, 2025, using the Multi Unit Spectroscopic Explorer (MUSE) spectrograph on the European Very Large Telescope (VLT), when 3I/ATLAS was at a distance of 4.47 times the Earth-Sun separation (au) from the Sun and 3.46 au from the Earth. The data reveal a red coma with a spectral slope indicating extreme reddening of reflected sunlight, redder than most Solar System comets but similar to the surface color of some Trans-Neptunian Objects or Centaurs in the outer Solar system. Opitom’s team searched for the spectral fingerprints of gas emission from various molecules, such as C_2, NH_2, CN, as well as neutral oxygen atoms, but did not detect any. The limit they set on the gas content is consistent with the non-detection of volatiles for Solar System comets at the same distance from the Sun. At present, the limited cometary fuzz appears to be entirely dusty.
Kuiper belt objects within the outer Solar system are reddened when organics on their icy surface are exposed to ultraviolet light or cosmic rays for billions of years. This is caused by so-called Tholins (after the Greek tholós which means “muddy”), a wide variety of organic compounds formed by Solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide (CO_2), methane (CH_4) or ethane (C_2H_6), often in combination with nitrogen (N_2) or water (H_2O).
There could be some undetectable gas in between the solid nucleus of 3I/ATLAS and the dust surrounding it. Only a small fraction of the surface of comets is typically observed to be active. The surface of 3I/ATLAS could be predominantly a non-evaporating crust that provides thermal insulation and blocks the heat transfer and sublimation of volatile ices under it. The brightness of the object implies a reflecting surface that is about 20 kilometers in diameter, assuming a typical asteroid albedo of 5%. The estimated diamter scales inversely with the square root of the assumed albedo value.
Future observations of 3I/ATLAS as it comes closer to the Sun will provide a key opportunity to witness the evolution of its activity, infer the size of its solid nucleus, study its composition, test predictions for the abundance and velocity dispersion of its population, and compare 3I/ATLAS to Solar System comets.
The fundamental question is whether 3I/ATLAS is a comet with a kilometer-scale nucleus or a solid object that is 20 kilometers in diameter which shows very limited evaporation?
In the latter case, the large size of 3I/ATLAS is puzzling. In a new paper, I showed that
interstellar objects with that radius would amount to an interstellar mass density that is well above the expected mass budget of interstellar comets or asteroids. Given this budget, the detection rate of objects like 3I/ATLAS implies that it is a comet with a small core diameter below 1.2 kilometers, or a member of a rare population with a number density. Both of these possibilities ease the tension of not detecting many more interstellar objects with smaller radii than 3I/ATLAS, as sub-km objects are expected to be much more numerous than 20 km objects based on their statistics in the solar system.
The second possibility would suggest that the rare population of 3I/ATLAS objects favors plunging orbits towards the inner solar system to accommodate their inferred detection rate.
If future spectroscopic data from state-of-the-art ground-based or space-based telescopes -like the Webb or Hubble space telescopes, will demonstrate that 3I/ATLAS has a solid core with a diameter of order 20 kilometers or more, then the limited interstellar reservoir of rocky materials would suggest that its trajectory favored a plunging orbit towards the inner Solar system, perhaps by technological design. To paraphrase Forrest Gump: “Science is like a box of chocolates, you never know what you’re going to get.”
ABOUT THE AUTHOR
(Image Credit: Chris Michel, National Academy of Sciences, 2023)
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.
Source: https://avi-loeb.medium.com/is-3i-atlas-a-comet-or-something-else-7c37f8e095bd