The biggest uncertainty about the interstellar object 3I/ATLAS involves the diameter of its solid-density nucleus. The flux detected by the SPHEREx space observatory at a wavelength of 1 micrometer from 3I/ATLAS on August 8–12, 2025 suggests a huge nucleus or alternatively an opaque dust cloud that scatters sunlight with a diameter of 46 kilometers (as reported here). The limited resolution of the Hubble Space Telescope image does not provide a robust constraint on the fraction of sunlight reflected by the nucleus relative to a surrounding dust cloud. The theoretical inference drawn from the data (accessible here) is highly model-dependent and does not resolve the existing uncertainty about the size of 3I/ATLAS.
If the reflecting region has solid density, then its 46-kilometer diameter implies a nucleus mass of about 10^{20} grams, a million times bigger than the estimate for the previous interstellar comet 2I/Borisov.
Since the nucleus mass scales as diameter cubed, measuring the mass of 3I/ATLAS would tightly constrain its size. What are the possible ways to measure the mass of this intriguing interstellar object?
One way to gauge the nucleus mass is through the rocket equation. The force acting on the object equals the excess of its mass loss rate towards the Sun times the outflow speed relative to its surface. Dividing this non-gravitational force by the object’s non-gravitational acceleration gives its mass. In principle, all three parameters: the mass loss rate, the outflow velocity and the non-gravitational acceleration, can be measured. The mass loss rate of CO2 from 3I/ATLAS was inferred from the recent Webb telescope data to be 129 kilograms per second, and the outflow speed was estimated at 0.44 kilometers per second (both discussed here). The product of these measured quantities yields for a 46-kilometer solid a non-gravitational acceleration of order 6×10^{-11} centimeter per second squared (or equivalently 3×10^{-14} Earth-Sun separations (AU) per day squared). This level of acceleration is an order of magnitude below the lowest levels measured for solar system objects (reported here). Hence, the non-gravitational acceleration will be detectable if the mass loss rate increases as 3I/ATLAS approaches the Sun, or if the diameter of its nucleus is smaller. A sub-kilometer diameter is required to reconcile the discrepancy between a high mass for 3I/ATLAS and the reservoir of rocky material in interstellar space, as I noted in my first paper on 3I/ATLAS (accessible here). In that case, the reduced diameter would imply a nucleus mass below 10^{15} grams and a non-gravitational acceleration above 6×10^{-6} centimeters per second squared (or equivalently 3×10^{-9} AU per day squared), only 50 times smaller than the large value measured for 1I/`Oumuamua (as reported here).
Since the mass loss rate scales with area and the non-gravitational acceleration scales inversely with volume, the rocket equation is a good approach for measuring the mass of small objects. In the opposite limit of large objects, gravity offers a better gauge.
On October 3, 2025, 3I/ATLAS will pass within a distance of 29 million kilometers from Mars. As a result of its gravitational influence, it will give Mars a kick as if the two objects were fuzzy billiard balls. The magnitude of the velocity kick is given by the gravitational acceleration that its mass, M, exerts at the distance of closest approach to Mars, b, namely: (GM/b²) with G being Newton’s constant, times the period of time over which 3I/ATLAS acts strongly on Mars, (2b/v), given their relative velocity v. For M~10^{20} grams, b=29 million kilometers, and v~90 kilometers per second, one gets a velocity kick of ~3×10^{-7} centimeters per second. Unfortunately, this kick is unmeasurable given the uncertainties in the orbit of Mars or any other Solar system planet that 3I/ATLAS will interact with.
Of course, the kick would have been larger if 3I/ATLAS were to maneuver and get closer to Mars. The so-called Minimum Orbit Intersection Distance (MOID) of 3I/ATLAS from Mars, namely the closest that 3I/ATLAS gets to the complete path of Mars around the Sun is remarkably short, just 0.018 AU or 2.7 million kilometers. This by itself constitutes another rare anomaly of 3I/ATLAS. If 3I/ATLAS is a technological mothership, this proximity makes it easy for it to release a mini-probe that would reach Mars easily with the appropriate ejection velocity. In addition, a small orbit correction by 3I/ATLAS could shrink this MOID of Mars to zero.
But as Francis Bacon noted: “If the mountain won’t come to Muhammad, then Muhammad must go to the mountain.” NASA should use all the fuel available to bring the Juno spacecraft as close as possible to 3I/ATLAS when it passes within 34 million kilometers from Jupiter on March 16, 2026 as discussed in my paper with Adam Hibberd and Adam Crowl (accessible here). The gravitational deflection that might be introduced by 3I/ATLAS to the path of Juno can later be used for an exquisite mass measurement of 3I/ATLAS.
In the coming months, we might have the privilege of measuring the mass of 3I/ATLAS by applying the rocket equation to its mass loss or measuring the gravitational kick it gives to various objects along its path.
Following the advice of basketball coaches to their team players, we must keep our eyes on the ball and not on the audience. The nature of 3I/ATLAS will be decided by better data and not the number of likes or premature Nobel Prize promises on social media.
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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