What We Know About Interstellar Comet 3I/ATLAS
The interstellar comet 3I/ATLAS is only the third confirmed object and the second comet from outside our Solar System. While it was first identified by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) on July 1, 2025, newly uncovered images from NASA’s Transiting Exoplanet Survey Satellite (TESS) reveal evidence of the object as early as May.
This Manhattan-sized object, with a nucleus size estimated to be between 5.6 and 15 kilometers, has an orbit that is the most dynamically extreme of any object yet recorded, arriving from the direction of the constellation Sagittarius.
It has also taken a path through the inner solar system with a likelihood of only 0.02%. Therefore, it has provided astronomers with a rare opportunity to study a body that originated around another star.
Interstellar Overhype: Alien Claims
Initial observations of 3I/ATLAS revealed some perplexing behaviors. For starters, it appeared to be emitting its own light even when it was very far from the sun.
Usually, comets are extremely dark until solar radiation starts to melt their reflective ices and volatile compounds as they get closer.
This unusual activity, in addition to an unorthodox trajectory, led Harvard professor Avi Loeb to suggest it could be a “technological artifact” with a “malign intent.” He even wrote that the consequences, if his hypothesis were correct, “could potentially be dire for humanity.”
It Behaves Like a Comet
NASA has formally dismissed Loeb’s theory. “It appears to be a comet,” said Tom Statler, NASA’s chief scientist for tiny things in the Solar System. It does comet things.” The evidence, therefore, overwhelmingly points to this object being a natural body.
It is also the fastest object ever seen traveling through our solar system, with a recorded speed of more than 130,000 mph (210,000 km/h). Furthermore, Loeb himself has stated that the simplest hypothesis is that 3I/ATLAS is a comet.
What is it?
Based on imagery from the Hubble Space Telescope, its nucleus is estimated to be up to 3.5 miles (5.6 km) wide. However, newly released data from the Zwicky Transient Facility (ZTF) suggests the nucleus could be much larger.
This is due to a “plateau” in the comet’s brightening that could be interpreted as the bare nucleus. In that case, the nucleus diameter could be approximately 15 kilometers, which would make it approximately 20 times larger than the nucleus of 2I/Borisov.
A Little Gassy
The comet’s unique chemistry has been revealed in other ways. For instance, data from the James Webb Space Telescope has revealed a carbon dioxide ice-to-water, ice ratio of 8:1, which is among the highest ever recorded.
In addition, carbon dioxide is said to make up a whopping 87% of its mass. Its polarization, or the way light comes off of it, is very different from what has been observed with other comets.
Why is it amazing?
The object’s coma has recently transitioned from giving off red light to green. Photos captured by amateur astronomers Michael Jäger and Gerald Rhemann during a total lunar eclipse in Namibia on September 7, 2025, revealed this new glow.
The dark skies provided an unprecedented opportunity for deep imaging. This unexpected color has raised new questions about the comet’s “alien chemistry.” It is likely due to the presence of cyanide and nickel, which have been detected with a dramatic rise in outgassing.
The increase is a “super-linear” process that is likely “switched on” by a minimum temperature threshold. The ATLAS team also interprets this as a shift from sunlight scattering off dust to the production of small, optically bright icy grains.
The Question of Size
The latest data from the Zwicky Transient Facility (ZTF) shows a “plateau” in the comet’s brightening between May 15 and the beginning of June 2025. This may represent the bare nucleus before substantial mass loss was initiated.
If this is the case, the nucleus diameter could be approximately 15 kilometers, which would make it approximately 20 times larger than the nucleus of 2I/Borisov. However, the plateau could also be spurious due to larger error bars in the early data.
What’s Next for 3I/ATLAS?
It is still unclear where precisely 3I/ATLAS originated, but it is likely that it came from the Milky Way bulge. The alignment of its path with the ecliptic plane brings it into close proximity to Mars.
Gladly, a high-resolution image from the HiRISE camera onboard the Mars Reconnaissance Orbiter will be taken on October 3, 2025.
In addition, NASA’s Jupiter-orbiting Juno satellite will be in a good position to observe the comet after it crosses Jupiter’s orbit. The comet is not expected to come closer to Earth than about 170 million miles.
Want to learn more?
Understanding other solar systems helps put humankind in perspective. As stated by astronomer Adina Feinstein, “One of life’s greatest questions is ‘are we alone in the Universe?”
The unique composition of 3I/ATLAS can consequently help scientists refine their models of what solar system formation looks like. Ultimately, this helps us understand if our own solar system is common or rare, which in turn helps us to quantify that.
Conclusion
The interstellar comet 3I/ATLAS is providing scientists with an extraordinary window into other star systems. By studying its unique properties and unexpected behaviors, we are gaining new insights into the diversity of the cosmos and our place within it.
