Rendezvous with 3I/ATLAS
The interstellar object 3I/ATLAS was discovered on July 1, 2025 to be moving at a speed of about 60 kilometers per second relative to the Sun. As 3I/ATLAS approaches the Sun, it will pass Mars at a distance of 19% of the Earth-Sun separation (AU) on October 3, 2025. After its closest approach to the Sun, it is predicted to arrive at a distance of 1.80 AU from Earth on December 19, 2025, and then at a distance of 0.36 AU from Jupiter on March 16, 2026. From Earth, 3I/ATLAS will not be observable during its closest approach to the Sun because Earth and the object will be on opposite sides of the Sun at that time. The object will be observable from Earth by early December 2025. During the object’s close approach to Mars, it could be observed by some of the probes currently in orbit around Mars.
Could we launch a space probe that will rendezvous with 3I/ATLAS along its path through the inner solar system?
3I/ATLAS is moving fast on a retrograde orbit, opposite to the motion of the Earth around the Sun. As a result, it is impossible with current propulsion technology to launch a spacecraft from Earth which will match the velocity of 3I/ATLAS and land on it. The best we can hope for is to take close-up photographs that resolve the solid surface of the object during a brief passage near it.
An earlier detection of 3I/ATLAS would have allowed a probe to approach it along its orbit at a modest relative speed. However, by now any head-on rendezvous will likely involve a relative speed of order 90 kilometers per second between the probe and the object. The probe will need to carry a camera with a large enough aperture to resolve 3I/ATLAS and collect enough light from its surface during the brief encounter.
The absolute H-magnitude of about 12 measured for 3I/ATLAS suggests that its diameter is 24 kilometers if its reflection coefficient of sunlight (albedo) is 5%, as typical for asteroids. However, this brightness might stem from the cometary plume of gas and dust around 3I/ATLAS, in which case its solid nucleus would be smaller.
To get a resolution of 10 meters across the surface 3I/ATLAS in visible light with a space telescope that is 50 centimeters in diameter requires a close approach to 3I/ATLAS at a distance comparable to the diameter of Earth, 12,756 kilometers. With a relative speed of 90 kilometers per second, this distance will be crossed within 2.4 minutes, allowing to collect a reasonable flux of reflected sunlight from the surface of 3I/ATLAS even when dispersed to thousands of spectral bins. The OSIRIS-REx mission recovered amino acids from the surface of the asteroid Bennu. Similarly, the spectral features from the surface of 3I/ATLAS could inform us about the existence of life’s building blocks near other stars.
The surface spectrum of 3I/ATLAS will complement any spectral measurements by ground-based telescopes or the Webb telescope from its cometary tail. It would be particularly interesting to compare the two spectra, since the coma would be composed mostly of volatile elements or molecules which make only a fraction of the surface materials. For human-made rockets, the two spectra reflect the difference between the composition of the gas coming out of the rocket’s exhaust and the composition of the rocket’s solid surface.
In his science-fiction novel titled `Rendezvous with Rama,’ Arthur C. Clarke tells a story taking place in the 2130s where a cylindrical 50-by-20 kilometers alien spaceship, not far from the maximum size of 3I/ATLAS, is intercepted by humans who wish to unlock its mysteries.
In reality, 3I/ATLAS was flagged as a near-Earth object in the context of a research program to catalog all objects that may pose a risk to Earth. In Clarke’s novel, an asteroid strikes Italy in 2077, prompting the creation of a system to track potentially hazardous space objects. In 2131, this fictional system detected an interstellar object from outside the Solar System, designating it “31/439” before naming it Rama after the Hindu god. A space probe reveals that Rama is a perfectly smooth cylinder, indicating that it was constructed by intelligent beings.
In a paper posted on July 4, 2025, I showed that solid asteroids or comets with a diameter of 24 kilometers must be so rare that they have a very small probability of arriving within the survey volume of our telescopes during the past few decades. Given the mass budget of icy rocks in the Milky Way galaxy, my paper showed that the detection of 3I/ATLAS would only be reasonable if its diameter is smaller than 1 kilometer.
As 3I/ATLAS will get closer to the Sun, our telescopes will be able to constrain the actual size of its solid nucleus. A diameter of order tens of kilometers would be puzzling as it would imply a very massive object. The encounter rate of objects with that mass would require a mass flux beyond the limited supply that the Milky Way can provide in icy-rocks. If indeed 3I/ATLAS is about 20 kilometers in diameter, then the supply rate of such objects must be small, and the only way for them to be consistent with the discovery of 3I/ATLAS is if 3I/ATLAS was aiming at the inner solar system. It is difficult to imagine a natural process that would favor a plunge towards the inner solar system at 60 kilometers per second. The alternative, as in Clarke’s story, is that the object targets the inner solar system by a technological design.
Of course, if 3I/ATLAS is a comet with a nucleus much smaller than 20 kilometers in diameter, then there would be no conflict between the mass flux carried by its population and the mass budget of icy rocks in the Milky-Way galaxy. In that case, it could be a familiar comet of natural origin.
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.
