As the interstellar object 3I/ATLAS passes through our cosmic backyard, bounded by the orbits of Mars and Earth around the Sun during the month of October 2025, the time is ripe to evaluate its gravity.
A good way to gauge the depth of a gravitational potential well is through the speed needed to escape from it. Consider the Moon for example. The escape speed from the lunar surface is 2.4 kilometers per second, about twice faster than the fastest rifle bullet but 125,000 times slower than light. In contrast, a black hole has an escape speed that exceeds the speed of light, making it the ultimate gravitational prison. Whatever happens inside a black hole stays there, just as is often said about Las Vegas.
In contrast, the escape velocity from a solid-density object, like 3I/ATLAS, is proportional to its diameter. In a recent paper (accessible here), I calculated a lower limit on the diameter of 3I/ATLAS, about 5 kilometers, based on the lack of recoil from its mass loss towards the Sun. An upper limit of 46 kilometers was derived from the SPHEREx data based on the brightness of 3I/ATLAS (as reported here). This range of diameters implies that the escape speed from 3I/ATLAS is between 1.3 and 12 meters per second.
For comparison, the world record time for a 100-meter dash is 9.58 seconds, set by Usain Bolt in 2009. It corresponds to a running speed of 10.44 meters per second, equal to the escape speed from a 40-kilometer asteroid. In other words, motion at the record speed of Usain Bolt would lead to a lift off from the surface of space objects smaller than the diameter of 3I/ATLAS or roughly the size of the state of Rhode Island. Keep in mind that in order to reach his record speed, Usain’s lungs consumed oxygen at atmospheric pressure which can only be supplied within the enclosure of a spacecraft. Without artificial gravity from the centripetal force induced by a rapid spin of the enclosure, his fast body is destined to bounce off the walls of the spacecraft.
3I/ATLAS appears to rotate with a period of 16.16 hours (as reported here). This corresponds to a surface rotation speed in the range between 0.25 and 2.5 meters per second, about a fifth of the gravitational escape speed for a solid density of 0.5 grams per cubic centimeter, characteristic of comets. The binding gravitational force is an order of magnitude larger than the centrifugal force for a comet nucleus with this rotation period. Since both the escape speed and the rotation speed scale with the unknown diameter of the object, their ratio is independent of that diameter. Under these conditions, rotation at a period of 16.16 hours is much slower than the threshold needed for breaking up a comet nucleus bound by gravity. However, for a denser object, gravity is stronger. In particular, the rotation speed for a period of 16.16 hours is only 4% of the escape speed at the density of iron, similar to the ratio between the surface rotation speed and the escape speed for Earth. Of course, unlike the Earth — small solid objects are held together by chemical bonds and not gravity.
All in all, despite its mass of more than 33 billion tons (as derived here), the gravity of 3I/ATLAS is rather weak. The gravitational kick it gave to Mars as it passed at a speed of 67 kilometers per second at a minimum separation of 29 million kilometers on October 3, 2025, was a part in 5 trillion of Usain Bolt’s record speed, amounting to an unmeasurable velocity kick of 2×10^{-12} meters per second.
Despite its weak gravity, 3I/ATLAS attracts a lot of public interest. This is a signature of its attention-grabbing gravitas and not its gravitational mass.
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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