Yesterday, we learned that the comet 3I/ATLAS is moving differently than we expected. According to data from the Atacama Large Millimeter Array (ALMA), it was off by 4 arcseconds from its anticipated position on October 29, 2025. This unusual movement suggests it’s experiencing non-gravitational acceleration, which could indicate major changes in its structure.
When I explored this, I found that for 3I/ATLAS to show this level of acceleration, it would need to have lost a significant amount of its mass—around one-sixth—due to cometary outgassing. This can happen when the sun heats up the comet, causing gas and dust to escape.
Here’s a breakdown of the calculations:
- The non-gravitational force acting on the comet equals its mass times its acceleration.
- Using the formula: ( M \cdot a = \left( \frac{dM}{dt} \right) \cdot v ).
- Where ( a ) is the non-gravitational acceleration, ( M ) is the mass, ( \frac{dM}{dt} ) is how fast the mass is being lost, and ( v ) is the speed of gas leaving the comet’s surface.
By observing a non-gravitational acceleration of about 0.02 millimeters per second squared, I calculated that the evaporation process must last less than six months. 3I/ATLAS crossed a distance of 203 million kilometers in roughly a month, leading to my conclusion that it has lost a considerable amount of its mass.
Notably, data from the Webb telescope indicated a mass loss rate of 150 kilograms each second. Yet, this didn’t show any non-gravitational acceleration for the five months prior. This implies that the total mass of 3I/ATLAS is at least 33 billion tons, surrounded by a dense gas cloud weighing about 5.5 billion tons.
If the current theories hold true, we should see this gas cloud around 3I/ATLAS in November and December 2025. The comet’s closest approach to Earth will be on December 19, 2025, at a distance of 269 million kilometers. Both ground-based telescopes and advanced instruments like the Hubble and Webb will have an excellent chance to study it.
The International Asteroid Warning Network (IAWN) will monitor 3I/ATLAS from November 27, 2025, to January 27, 2026. If no massive gas cloud is detected, it may challenge the cometary evaporation explanation for its unusual acceleration. This would echo past debates such as those surrounding 1I/`Oumuamua, the first interstellar object, which also displayed strange acceleration without noticeable gas emissions.
Recent commentary from public figures, like Brian Cox, who assert 3I/ATLAS is purely a natural phenomenon, raises questions. While public interest is essential, scientific conclusions should rest on data rather than popularity.
In a field filled with mysteries, the ongoing observations of 3I/ATLAS could lead to remarkable discoveries about our universe. Whether it’s a comet or something engineered, its study will enhance our understanding of interstellar objects.
For further details, you can explore the NASA/JPL orbital predictions regarding 3I/ATLAS.
Avi Loeb is a notable figure in astronomy. As the head of the Galileo Project and a former chair of the astronomy department at Harvard, he actively explores questions about extraterrestrial phenomena. His extensive research and publications add a valuable perspective to discussions on objects like 3I/ATLAS.
