Ideas are much easier to have than materials, especially when dealing with outer space. This became evident to me as a theoretical astrophysicist who invested two weeks starting on June 14, 2023 in retrieving tens of milligrams of sub-millimeter spherules from the bottom of the Pacific Ocean, two kilometers deep, at the fireball site of the interstellar meteor, IM1. The fireball flash, detected by sensors onboard U.S. government satellites, implied that IM1 had an impact speed of 45 kilometers per second, a diameter of about 0.5 meters and an explosion altitude of about 19 kilometers. The ocean expedition cost 1.5 million dollars.
For a cost of 800 million dollars, NASA’s OSIRES-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission landed on the near-Earth asteroid Bennu on October 20, 2020 and returned a sample from it to Earth on September 24, 2023, following a seven-year journey. Bennu is likely a rubble pile formed from the disruption of a larger parent asteroid, based on its dynamical association with inner-main-belt asteroid families. The sample collection site was in Hokioi Crater, an impact feature in Bennu’s northern hemisphere measuring 20 meters in diameter. Bennu’s diameter is 500 meters, a thousand times larger than IM1.
By now, both samples of IM1 and Bennu were analyzed in state-of-the-art laboratories. A comparison of the findings illustrates how different the two parent objects are. Whereas OSRIS-REx brought back to Earth 121.6 grams of Bennu, the Galileo Project ocean expedition delivered at most tens of milligrams of IM1.
The Bennu sample was found to have a composition identical to CI chondrites, the primitive materials that made the solar system. CI chondrites have elemental abundances consistent with those of the solar photosphere, except the ice-forming elements (H, C, N, and O), Li, Be, B, and the noble gases. Bennu shows similar composition to the material returned from the asteroid Ryugu, but without refractory element enrichments. In contrast from meteoritic samples where volatile elements are lost in the explosion, the OSIRES-REx sample return mission recovered the full composition of the parent body.
On the other hand, a unique type of differentiated (D-type) spherules from IM1’s fireball site, which we labeled “BeLaU,” showed enhancements by up to a factor of a thousand in many elements including beryllium, lanthanum and uranium, and exhibited a deficiency in volatile elements, as expected from spherules originating in a hot meteor fireball. The unusual chemical composition indicates that IM1 originated from a very different process compared to the solar-system Bennu. The distinct composition is consistent with the speed of the object exceeding the escape threshold from the solar system, implying an interstellar origin. A potential origin for the differentiated BeLaU composition is a magma ocean planet spaghettified by a dwarf star.
In conclusion, the interstellar meteor IM1 and the Solar system rock Bennu have very distinct origins. Finding bigger pieces of IM1 in our next planned expedition will allow us to recover the volatile elements which are missing in sub-millimeter spherules, and to potentially infer the origin and nature of the parent body. This will inspire the search for additional interstellar meteorites in the future.
The difference between theory and material analysis for interstellar objects resembles the difference between using a dating app and an in-person meeting. It is much easier to avoid prejudice and illusions in a physical meeting.
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. His new book, titled “Interstellar”, was published in August 2023.
