A recent study highlights the intriguing properties of interstellar comet 3I/ATLAS, indicating it may be “anomalously massive” at an estimated weight of about 33 billion tons. This comet was first spotted on July 1, 2025, traveling through our Solar System at nearly double the speed of previous visitors like ‘Oumuamua and Comet Borisov. Unlike these previous comets, 3I/ATLAS has a solid core—its nucleus measures roughly 5.6 kilometers (about 3.5 miles) in diameter.
Measuring comets can be tricky. As they near the Sun, they heat up and release gas and dust, forming a cloudy coma around them. This outgassing not only creates the comet’s tail but can also change its speed and direction. However, it also gives scientists clues about the comet’s mass and structure.
In a preliminary study by astronomers from Harvard, data from 227 observatories worldwide was analyzed. The team found that the comet’s non-gravitational acceleration is quite small—less than 15 meters per day squared. Given that the comet has been actively outgassing at a rate of around 150 kilograms per second, this suggests 3I/ATLAS has a dense nucleus that doesn’t change acceleration significantly.
Interestingly, while 3I/ATLAS is large, it’s not the largest comet we’ve seen. C/2014 UN271 (Bernardinelli-Bernstein) holds that title, weighing in at a staggering 500 trillion tons with a nucleus 128 kilometers (80 miles) across. However, the mystery surrounding 3I/ATLAS lies in the fact that it is much more massive than previous interstellar objects. Avi Loeb, one of the study’s authors, notes that we should have discovered many similar-sized interstellar objects by now, yet we’ve only found a handful.
This raises questions about the processes that create these celestial bodies. According to Loeb, if 3I/ATLAS’s nucleus is over 5 kilometers in diameter, it defies expectations about how often such massive bodies should occur. He even suggested a highly unlikely idea: could this be an alien spacecraft? He pointed out peculiar characteristics in its trajectory and the detection of unusual materials like nickel without accompanying iron, which could suggest an artificial origin. NASA, however, has dismissed these speculative claims, emphasizing that 3I/ATLAS behaves like a comet. “It looks like a comet. It does comet things,” stated Tom Statler, NASA’s lead scientist for small bodies in the Solar System.
As the study progresses, we look forward to observing 3I/ATLAS more closely. The Mars Reconnaissance Orbiter may catch a glimpse with its HiRISE camera on October 3, 2025. However, the comet will be obscured by the Sun at that time, reappearing around December.
Data from cutting-edge observatories like the Vera C. Rubin Observatory will enhance our understanding. In its initial run, the observatory discovered over 2,000 new asteroids in just ten hours, suggesting the potential for finding even more interstellar objects.
For more technical details, check out the full paper from Harvard here.
