Flashes on the Dark Side of the Moon: A New Fishing Net for Interstellar Objects
During one of the dozen television interviews on Artemis II that I had in recent days (as summarized here, here, here and here), I made the trivial prediction that its four astronauts will witness meteor flares on the surface of the dark side of the Moon. Since the Moon has no atmosphere, objects on a collision course with it would impact the lunar surface directly rather than burn-up and disintegrate inside a fireball, as they do due to friction in the Earth’s atmosphere.
This prediction came true near the end of Orion’s closest approach to the Moon on April 6, 2026, when the Sun was eclipsed by the Moon for almost an hour. During that time, the astronauts witnessed with their naked eyes six micrometeor impacts on the dark side of the Moon.
It is widely believed that the Moon formed out of material ejected from the surface of Earth as a result of an impact by a Mars -size proto-planet named Theia. Upon formation the lunar surface was smooth, but as evident from the latest high-resolution images — transferred by laser communication from Artemis II to Earth — the lunar surface is scarred with numerous impact craters. The Moon serves as a museum for the material collected from these impact events, some of which was delivered by interstellar objects from outside the Solar System.
What is the chance that an interstellar object like 3I/ATLAS impacted the Moon over the 4.5 billion years of its history?
The calculation is straightforward. The probability for an interstellar impact equals the product of the number of interstellar objects per unit volume, N, times the typical velocity of 3I/ATLAS-like objects, V=60 kilometers per second, times the cross-sectional area of the Moon, A=9.5 million square kilometers, times the age of the Moon, T=4.5 billion years. The number density was estimated as N=0.007 per astronomical unit cubed in the latest analysis of the Hubble Space Telescope data, reported here. Altogether this product yields an impact probability of, P=(N*V*A*T) = 17%.
In other words: the chance of a 3I/ATLAS-like interstellar object to impact the Moon during its entire history equals to the probability that a six-sided die cube will land on a specific face. This is a significant probability. Adopting a mass of order a billion tons for 3I/ATLAS (as estimated here), suggests that a single impact could have cover the entire lunar surface with a thin layer of interstellar dust. By now, this layer was mixed with a much thicker dust layer from accumulated impacts of numerous Solar System asteroids of the type observed by the Artemis II astronauts.
Since Earth has a surface area that is larger than that of the Moon by a factor of 13.5, there should have been a few impacts of interstellar objects like 3I/ATLAS on Earth. These events were likely separated in time by about a billion years from each other, implying that their imprint was buried underground by geological mixing over these long periods of time.
Calibrating the statistics of lunar impacts by millimeter-size micrometeorites would have important implications for the long-term survival of astronauts or any delicate infrastructure on the lunar surface.
In 2019, I co-authored a paper (accessible here) with my undergraduate student at that time, Amir Siraj, suggesting a systematic search for interstellar impacts on the Moon. The paper proposed to send a new telescope in lunar orbit in order to study in real-time interstellar impacts and to serve as a laboratory for hypervelocity collisions. We calculated that a telescope with a diameter larger than 2 meters should be able to detect at least one impact of an interstellar object with a diameter of a few centimeters among hundreds of Solar System meteoroid impacts, every year. For each interstellar object, measurements of the reflected sunlight and shadow, as well as the impact’s optical flash and crater, would allow for the determination of the velocity, mass, density, and composition of the impactor.
This proposal might be realized in the coming years as part of the Artemis program to establish a human base with technological and scientific infrastructure on the Moon. Pursuing a comprehensive survey of meteor flashes would provide a new opportunity to use the Moon as a fishing net for interstellar material. Perhaps among the numerous rocks that constitute most lunar impactors, humanity will also identify crash sites of technological debris from extraterrestrial civilizations. This will constitute a new tool in the toolbox of the 66-year-long search for extraterrestrial technological civilizations.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, former director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics (2005–2026), 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.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
Social media:
https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
https://x.com/ProfAviLoeb
