A Request to Jared Isaacman: Intercept 4I/Rubin
The NSF-DOE Rubin Observatory in Chile is expected to discover dozens of new interstellar objects within the next decade. These visitors to our cosmic backyard will be flagged by their speed exceeding the value needed to escape from the pull of the Sun’s gravity. Near the Earth’s orbit around the Sun, the escape speed is 42.1 kilometers per second, just a square-root of 2 larger than the Earth’s orbital speed at the Earth-Sun separation (AU).
3I/ATLAS arrived in our vicinity at about 60 kilometers per second. At that fast speed, which outperforms our fastest rockets, it still takes billions of years to travel throughout the entire disk of the Milky-Way galaxy (as calculated here). Interstellar visitors spend that time traveling and offer us an opportunity to learn about the physical conditions at their origin without us needing to travel for billions of years in order to get there. They already invested that time to get here.
If such objects are on random trajectories, it is natural to expect most of them to be icebergs that shed a cometary tail of gas and dust once they are warmed by sunlight. The reason is simple and can be illustrated through the example of our latest visitor, 3I/ATLAS.
The parent population of 3I/ATLAS was inferred here to deliver a new detectable object within 5 AU every couple of years, implying that there should be about ten trillion such objects right now within the Solar System out to the edge of the Oort Cloud at 100,000 AU. This edge is roughly half-way to the nearest star, implying that each star system in the Milky-Way galaxy needs to produce during its lifespan about ten trillion objects like 3I/ATLAS if what we detected represents the average interstellar abundance of such objects. Given that 3I/ATLAS carried at least a mass 0.1 billion tons, as derived here, the total ejected mass to interstellar space is at least a sixth of the Earth mass per star, a large reservoir which can only be accommodated by the ejection of icebergs during the formation process of a planetary system. A substantial fraction of the building blocks that combine to make rocky planets could be tossed out of their planetary system through gravitational scattering by massive planets or passing stars. Another ejection mechanism is tidal disruption of planets, as I discussed in my paper with Morgan MacLeod, published here.
However, 3I/ATLAS arrived on a trajectory that was aligned to within 4.89 degrees with the orbital (ecliptic) plane of the Earth around the Sun. This alignment is unexpected, given that the ecliptic plane is tilted by 60.3 degrees relative to the plane of the Milky-Way disk of stars. If future interstellar objects will show a preference for an ecliptic orientation, then we would have to entertain the possibility that these trajectories were not drawn randomly but might have been designed technologically. In case of a technological origin, the abundance of visitors near Earth could be much higher than average for the same reason that honey bees cluster around flowers.
The simplest way to figure out whether an interstellar visitor is a natural iceberg or an interstellar Trojan Horse with a technological interior, is to crash on its surface — in the same fashion that the DART spacecraft impacted the asteroid moonlet Dimorphos on September 26, 2022. A close-up photograph just before impact would unravel the nature of future interstellar objects, labeled as XI/Rubin with X=4, 5, 6 ….
In addition to a camera, the interceptor could carry instruments that would probe the composition of the plume of gas or dust around the interstellar object before impact. Even if the object turns out to be a natural iceberg, the instruments onboard the interceptor could check whether the iceberg carries any biological signatures or the building blocks of life-as-we-know-it in the form of organic molecules. This is an entirely new discovery pathway for astrobiology in our search for life beyond Earth.
Obviously, crashing on the hard surface of a spacecraft would be an entirely different experience for a DART-like mission.
Launching an interceptor on a crash course with an interstellar object, say 4I/Rubin, requires detection of 4I/Rubin at a distance of 5–10 AU and a fast response time. 3I/ATLAS was discovered at a distance of 3.5 AU from Earth on July 1, 2025 and arrived closest to Earth at a distance of 1.8 AU on December 19, 2025, nearly half a year later. If 4I/Rubin will be detected at a distance of 10 AU and will take a year to get to within 2 AU, then an Earth-based launch at a reasonable speed of 10 kilometers per second could intercept its path and crash on its surface.
This requires planning for a target-of-opportunity space mission with a billion dollar budget. The total cost of the less ambitious DART mission was a third of that.
The European Space Agency (ESA) plans a mission called Comet Interceptor, to be launched by 2029. The spacecraft will be placed at the second Earth-Sun Lagrange point L2 and wait for up to three years for a long-period Solar system comet or an interstellar object to fly by at a reachable trajectory and speed. The limitation of this mission is that it can propel itself only at a maneuvering speed of up to 1 kilometer per second, equivalent to traversing 1 AU in about 5 years. Unless we are lucky to have an interstellar visitor that arrives very close to this spacecraft, we will not have sufficient time from detection to intercept its path.
NASA could do better, if Jared Isaacman reads this essay.
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/
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https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
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