A Swarm of 35-Million Interstellar Objects Was Just Discovered Within the Earth’s Orbit Around the Sun
Survey telescopes, like the existing NSF-DOE Rubin Observatory for the southern sky or the planned Argus Array for the northern sky, are sensitive to reflected sunlight from interstellar objects at a distance comparable to the Earth-Sun separation, that are larger than our biggest rocket, Starship, namely more than a hundred meters in diameter.
Fortunately, we can also discover much smaller interstellar objects by using the Earth’s atmosphere as the detector. When a 3-meter object collides with Earth, its friction on air generates a meteor fireball with an energy output comparable to the Hiroshima atomic bomb. Such an explosion is easily detectable by the U.S. Government satellites which are monitoring Earth routinely for the heat emitted by the launch of ballistic missiles from adversarial nations. When deemed unclassified, the meteor fireballs detected by these warning systems are reported in NASA’s CNEOS fireball catalog, available here.
Yesterday, I co-authored with my postdoc Richard Cloete a new paper, posted here, that reported the discovery of two meter-scale interstellar meteor candidates in the CNEOS catalog. By exploiting an empirically calibrated uncertainty model from 2025 (reported here), we have found two events that robustly exceed the escape velocity from the Solar System. CNEOS-22 (detected on 2022–07–28 over the eastern tropical Pacific Ocean) exceeds escape by 8.7 standard-deviations and CNEOS-25 (detected on 2025–02–12 over the Barents Sea in the Arctic) exceeds escape by 5.5 standard deviations. The diameters of both objects are 1.8 meters for CNEOS-22 and 1.2 meters for CNEOS-25. For a full account of the discovery details, click here.
Given the detection of two interstellar meteors in the CNEOS fireballs database over a period of 7 years, the inferred collision rate of meter-scale interstellar objects with Earth is about 0.3 per year. This rate equals the product of the number density of the parent population times the Earth’s cross-sectional area: 128 million square kilometers, times the Earth’s orbital speed around the Sun: 30 kilometers per second. The measured collision rate yields a number density of 8.4 million interstellar objects of meter-scale per AU cubed, where 1 AU (Astronomical Unit) is the Earth-Sun separation.
This implies that there are about 35 million meter-scale interstellar objects embedded at any time within the orbit of the Earth around the Sun. Assuming they have a solid density of a few grams per cubic centimeters, each object carries about 3 million metric tons. Altogether, this population totals a hundred trillion (10^{14}) metric tons of interstellar material interior to the Earth’s orbit around the Sun.
For comparison, the estimated number density of the parent population of the interstellar object 3I/ATLAS — which was measured here to have a diameter of 2.6 kilometers, is 0.003 per AU cubed, about 2.8 billion times smaller than the number density of 2-meter-scale interstellar objects. The mass of each of these objects is larger than that of a meter-scale object by a factor of (2.6 kilometer/2 meter)³, namely 2.2 billion.
Multiplying the number per unit volume by the object’s mass, we find that the population of kilometer-scale interstellar objects — like 3I/ATLAS, carries approximately the same mass per unit value as the population of meter-scale interstellar objects, about a hundred trillion metric tons interior to the Earth’s orbit.
The fact that the mass density of kilometer-scale interstellar objects is the same as meter-scale interstellar objects, suggests that the two populations might be related with the smaller objects being fragments of the bigger objects.
As suggested in the recent paper that I co-authored with the brilliant student Oem Trivedi (available here), it would be most efficient to study the population of interstellar objects with a new observational architecture, including discovery by the Rubin and Argus observatories, high-resolution imaging by a lunar optical interferometer, and closer studies of anomalous objects by space interceptors.
A comprehensive campaign for information gathering would alert earthlings of potential threats from impacts by either natural rocks or alien technological gadgets. Ignoring threats from the sky did not work out well for non-avian dinosaurs 66 million years ago. So far, our planetary defense strategy contemplated Solar System rocks, but we should expand our risk assessments to interstellar objects as well.
In the coming months, I will attempt to secure funding for new ocean expeditions to retrieve materials from the two new candidates of interstellar meteors, CNEOS-22 and CNEOS-25. Radioactive dating of their interstellar materials could be used to estimate the durations of their interstellar journeys and constrain their origins. Finding evidence for a Voyager-like meteor would be even more exciting.
Here’s hoping that in a few billion years, after Voyager 1 and 2 will have traverse most of the Milky Way disk of stars — at least one of them will collide with a habitable exo-planet and appear as a meteor to a local population of aliens. Based on its low-altitude explosion, a curious alien astronomer might suggest that Voyager is anomalous in material strength and potentially not a rock. Other astronomers will not only ridicule this proposal but also deny that Voyager is interstellar in origin — by inflating the measurement errors. After leading an expedition to the meteor site, the alien astronomer might find the 12-inch Golden Record of Voyager, with its 115 images, natural sounds, musical selections and greetings in 55 languages from Earth. Figuring out that they are not alone will be the ultimate intelligence test of the aliens. After all, they must have asked numerous times: “Where is everybody?”
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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