Welcoming a New Interstellar Object: A11pI3Z or 3I/ATLAS
On the early morning of July 2, 2025, I woke up at 2AM for a two-hour interview on Coast to Coast radio to discuss my research on interstellar objects. This was a fortunate timing as rumors just appeared overnight on X and Bluesky about the discovery of a new interstellar object, A11pl3Z.
Objects originating from outside the solar system are flagged by having a speed above the minimum value needed to escape from the Solar system, which is 42 kilometers per second — a thousand times over the speed limit on a highway — in the vicinity of Earth. The first reported interstellar object, `Oumuamua, was not a familiar comet nor a familiar asteroid. It was inferred to have a disk-like shape and to exhibit non-gravitational acceleration, raising the possibility of an artificial origin. The second reported interstellar object, Borisov, appeared like a familiar natural comet. Will the next object on the interstellar roulette table be anomalous or familiar?
The IAU Minor Planet Center added A11pl3Z to the Near-Earth Object confirmation list on July 1, 2025, with a similar listing appearing on the NASA/JPL CNEOS website. The preliminary fit for the trajectory of A11pl3Z suggests an eccentricity of about 6, a hyperbolic velocity of about 66 kilometers per second and an interstellar origin. Interestingly, this is the typical velocity of interstellar objects that we predicted in a paper that I published with Morgan MacLeod a year ago about the production of interstellar objects from the spaghettification of rocky planets by common dwarf stars.
An extrapolation of A11pl3Z’s orbit implies that its closest distance to Earth would be 2.4 times the Earth-Sun separation (=astronomical unit, abbreviated as AU) on December 17, 2025 and that it will pass much closer to Jupiter around March 10, 2026. This is because the Earth happens to be on the other side of its orbit around the Sun when A11pl3Z gets close to the Sun. Its closest distance from the Sun is expected to be about 1.4 AU on October 27, 2025, three weeks after its passage within 0.4 AU from Mars. The current distances of A11pl3Z are about 3.8 AU from Earth and 4.8 AU from the Sun as of July 2, 2025.
The amateur astronomer Sam Deen identified earlier images of A11pl3Z in the ATLAS Survey from June 25–29, 2025, implying that it is almost certainly interstellar in origin. The amateur astronomer Filipp Romanov stacked 5x20 second images from the iTelescope.Net T72 (0.51-m f/6.8 reflector + CCD) in Chile and detected a G magnitude of 17.5. The H-magnitude of 12 listed by NASA/JPL suggests a diameter of about 20 kilometers, somewhat bigger than the Chicxulub impactor that killed the non-avian dinosaurs on Earth 66 million years ago. Better data will refine the size estimate as well as the orbital parameters of A11pl3Z.
Observing A11pl3Z also with the Webb telescope, located a million miles away from an Earth-based telescope, will allow us to detect via parallax any non-gravitational acceleration to exquisite precision, as shown in a recent paper I wrote with my student Sriram Elango. The Webb telescope can also measure the emitted infrared flux and surface temperature of A11pl3Z — which when combined with its known parallax distance, can be used to infer its surface area. If the object is tumbling, it would be possible to use the evolution of the surface area projected along the line-of-sight in order to map the shape of the object in three dimensions. The known size and shape and the reflected flux of sunlight from the object will allow us to infer its surface albedo (reflection coefficient) for sunlight. We did not have direct measurements of the area, surface temperature or albedo for `Oumuamua.
If A11pI3Z’s brightness stems from reflecting sunlight at the typical albedo of order 10%, then its diameter of 20 kilometers is about 100–200 times larger than the estimated length of `Oumuamua (and over a thousand times larger than `Oumuamua’s width) and about 50–100 times larger than the core of the comet Borisov. If all three objects are rocks, then A11pI3Z’s mass is over ten million times larger than that of `Oumumua and at least a hundred thousand times larger than the core mass of Borisov.
This is surprising because one expects high mass objects to be much rarer. Based on data about the Main Asteroid Belt in the Solar system, one would expect millions of objects like `Oumuamua for each object on the mass scale of A11pI3Z.
How come we did not observe millions of `Oumuamua-scale objects before discovering A11pI3Z?
Of course, we may have missed many of them but probably not millions of them. It is also possible that the size distribution of interstellar rocks is bimodal with a peak around a diameter of 20 kilometers. The most likely possibility is that A11pI3Z is a comet and we are detecting the reflection of sunlight from its cometary plume of gas and dust, whereas its mass is contained in a much smaller core. Indeed, a new notice on July 2, 2025 by the Minor Planet Center which labels the object as 3I/ATLAS, officially interstellar, suggests tentative evidence for a coma. Comets populate the outskirts of planetary systems and can be easily dislodged into interstellar space by passing stars because they are loosely bound gravitationally to their host star. Other, more speculative, possibilities are that 3I/ATLAS’s albedo is much larger than usual or that it generates its own light. In the comet case, 3I/ATLAS’s nature will be closer to that of the comet Borisov and very different from that of `Oumuamua, which did not show a coma and had an extreme disk-like shape, given that the amount of sunlight it reflected changed by a factor of ten as it was tumbling every 8 hours.
Based on its direction of motion, 3I/ATLAS appears to be coming at a retrograde orbit with an inclination of 175 degrees relative to the Earth’s orbital plane from the thin disk of stars in the Milky-Way galaxy. In the coming months, we will learn much more about 3I/ATLAS’s properties based on data from multiple ground-based telescopes including the new Rubin Observatory in Chile, as well as possibly from the Webb space telescope. Stock your popcorn.
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.
