On the Uncertain Nature of 3I/ATLAS
At a relative speed exceeding 60 kilometers per second, any impact on the surface of the interstellar object 3I/ATLAS would release an amount of energy that is tens of times larger than a TNT explosive with the impactor’s mass. In particular, a collision of 3I/ATLAS with a 20-meter asteroid would release an energy equivalent of 20 megaton of TNT, of order a thousand Hiroshima atomic bombs. The resulting explosion would cause tremendous damage to an interstellar object, even if its diameter is a thousand times larger than that of this impactor, namely as large as 20 kilometers — the estimated diameter of 3I/ATLAS based on its brightness.
If 3I/ATLAS is a rock, then it will shatter to pieces as a result of such a powerful collision in the Solar system. Nature does not mourn devastated rocks, as rock collisions are a common natural occurrence. For example, the Moon is believed to have been created by a collision of a Mars-sized object, Theia, with Earth, about 4.5 billion years ago. This was followed by a Large Heavy Bombardment of asteroids that were demolished on the surface of Earth about 3.8–4.1 billion years ago. But if 3I/ATLAS is a spacecraft, then its makers might have been concerned about its long-term survival as it passes through asteroid belts of planetary systems like the Solar system.
3I/ATLAS is currently passing through the main asteroid belt of the Solar System, a torus-shaped region at a distance of a few times the Earth-Sun separation. There are about a trillion rocks bigger than 20 meters inside this region, posing a potential threat to the integrity of 3I/ATLAS. To protect an interstellar spacecraft of that scale, its creators might have designed it to spray a stream of particles ahead of it, as precursors that would flag any dangerous rock along the path, allowing the craft to navigate away from obstacles. The use of a radar system might have been avoided to eliminate an obvious electromagnetic signature of artificial origin.
Large enough particles (well above a micron, a millionth of a meter) would not be pushed back by Solar radiation pressure or the Solar wind, as is the case for smaller particles in common cometary tails. This is because the repulsive forces scale as surface area or size squared, whereas the particle mass scales as size cubed. Hence, large particles move ahead of the front of 3I/ATLAS and create a buffer zone leading it. If this zone measures roughly the diameter of Earth (12,756 kilometers), it would amount to a few arcseconds on the sky — which happens to be the characteristic angular scale of the glow in front of 3I/ATLAS at its current distance. The interaction of the sprayed particles with potential obstacles would give an advance warning of a few minutes to 3I/ATLAS at its measured hyperbolic speed.
A buffer zone of large particles would show up in images of 3I/ATLAS as a glow of reflected sunlight ahead of the interstellar object. The glow will not be accompanied by any gas particles, as they would be pushed back by the Solar wind and hence be useless for the purpose of flagging dangerous rocks ahead of 3I/ATLAS.
This scenario is consistent with the current data on 3I/ATLAS. The Hubble Space Telescope image shows a glow that extends out to a few arcseconds ahead of the object (see related paper here), and spectroscopic measurements shows no evidence for molecular or atomic gas accompanying this glow (see related papers here, here and here as well as the discussion about water ice here).
On the other hand, what would be the most plausible natural-origin explanation of the current data? The observed forward glow could be produced by a dust-rich interstellar asteroid which releases mostly large dust grains on its Sun-warmed dayside. In this scenario, the observed reddening in the spectrum of 3I/ATLAS (see related papers here, here and here) also implies the existence of small dust grains. But these smaller dust particles should have been pushed back by Solar radiation pressure to trail 3I/ATLAS, yet there is no evidence for a cometary tail of dust and gas behind 3I/ATLAS. Without small dust particles, the reddening must be explained by the red surface of a 20-kilometer object, in analogy with 1I/`Oumuamua. The challenge with this interpretation of the reddening of 3I/ATLAS is that the reservoir of rocky materials in interstellar space can only deliver a 20-kilometer dust-rich rock once per 10,000 years and we discovered 3I/ATLAS within the ATLAS survey in less than a decade. This challenge was discussed in my first published paper on 3I/ATLAS (accessible here).
For now, we cannot assess with any confidence whether 3I/ATLAS is a natural dust-rich comet with no gaseous tail on an extremely rare trajectory, or perhaps a technological object on a path that was designed to align with the ecliptic plane of the planets around the Sun. Irrespective, the observed fact is that 3I/ATLAS is passing through the asteroid belt right now as a result of the rare (0.2% probability) alignment of its retrograde path with the ecliptic plane to within 5 degrees. On top of that, the arrival time of 3I/ATLAS along this path is perfectly timed for a close encounter with Mars, Venus and Jupiter (with a 0.0005% probability, as discussed here). This coincidence would allow a mothership to release mini-probes that will reach planets as they move into the mini-probes, taking advantage of the mothership’s retrograde motion. Unfortunately, 3I/ATLAS will hide behind the Sun at its perihelion on October 29, 2025, and so we will not be able to observe whether it releases any mini-probes into Earth’s orbit.
On the “Loeb scale”, where `0’ refers to a definitely natural comet and `10’ implies a definitely artificial object, I currently rank 3I/ATLAS as a `4’. As we gather more data on 3I/ATLAS in the coming months, my rank might sink to `0’ or rise to `10’. Science is fun because attending to facts allows us to discover the unexpected.
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.
