When astronomers first spotted the interstellar comet 3I/ATLAS on July 1, 2025, they expected a rare but fairly straightforward study of an alien visitor. Instead, what they found has become one of the strangest mysteries in modern astronomy. Unlike typical comets, 3I/ATLAS breaks nearly every rule, from its unusual chemical makeup to its enormous size and record-breaking speed. Scientists now believe this icy traveler could transform what we know about the nature of interstellar objects.
Table of Contents
Discovery
3I/ATLAS was discovered by NASA’s ATLAS survey telescope in Chile, making it the third confirmed interstellar object after ‘Oumuamua in 2017 and 2I/Borisov in 2019. Almost immediately, observatories around the world, including Hubble, JWST, TESS, and SPHEREx, turned their instruments toward it. Early data revealed something surprising: this was no ordinary comet.
Composition
Perhaps the most shocking trait of 3I/ATLAS is its composition. Observations from JWST on August 6 showed the comet’s coma dominated by carbon dioxide, with far less water than expected. This is virtually unheard of in comets native to our solar system, which usually have water as their primary ice. The unusual ratio suggests 3I/ATLAS may have formed near a CO₂ ice line in its home system or under strong radiation conditions that shaped its ices differently.
| Substance | JWST Detection Strength |
|---|---|
| Carbon dioxide (CO₂) | Strong (4.3 μm) |
| Water (H₂O) | Weak (2.7 μm) |
| Carbon monoxide (CO) | Moderate (4.7 μm) |
Size
Estimating the comet’s size has proven tricky. Hubble’s initial measurements suggested a nucleus as small as 0.3 miles, but later data pointed to a body up to 12 miles wide—possibly making it the largest interstellar object ever observed. Adding to the puzzle, the Vera Rubin Observatory recorded its coma expanding by 3,700 miles in just two weeks. This sudden outburst of activity hints at a dynamic, volatile nature unlike typical comets.
Speed
If its size is surprising, its speed is astonishing. 3I/ATLAS is moving at about 130,000 mph relative to the Sun, far faster than both ‘Oumuamua (58,000 mph) and Borisov (72,000 mph). This makes it the fastest interstellar visitor ever recorded, zooming through our solar system on a nearly straight path due to its extreme orbital eccentricity.
| Object | Speed (mph) |
|---|---|
| ‘Oumuamua (2017) | ~58,000 |
| 2I/Borisov (2019) | ~72,000 |
| 3I/ATLAS (2025) | ~130,000 |
Orbit
Adding another twist, 3I/ATLAS follows a retrograde orbit almost perfectly aligned with the solar system’s ecliptic plane. That means it’s traveling in the opposite direction of the planets but still hugging the same flat disk they orbit in. Astronomers had expected interstellar objects to arrive from random directions, so this alignment is both puzzling and thought-provoking.
Origin
Its path suggests 3I/ATLAS came from the southern sky, nearly opposite the solar apex. Most interstellar debris was expected to arrive from the apex direction, catching up with the Sun’s motion. Instead, this visitor came from the opposite side, raising questions about our assumptions—or even our observational biases—in detecting such objects.
Debate
The comet’s unusual features have sparked both excitement and speculation. Some, like astrophysicist Avi Loeb, have proposed bold ideas about alien technology, though the majority of scientists strongly disagree. Current evidence overwhelmingly points to a natural origin. Still, researchers agree that searching for potential technosignatures in future interstellar visitors remains worthwhile.
Future
3I/ATLAS will reach perihelion on October 29–30, 2025, passing inside Mars’s orbit at 1.4 AU from the Sun. By December, it will make its closest approach to Earth at 168 million miles before vanishing forever into interstellar space. For astronomers, this fleeting opportunity is a reminder that such visitors must be studied quickly, as once they leave, they are gone for good.
3I/ATLAS may never reveal all its secrets, but it has already reshaped how scientists think about interstellar comets. Its strange composition, size, and speed remind us that the universe holds surprises far beyond our imagination.
FAQs
Why is 3I/ATLAS unique?
It has unusual CO₂ levels, huge size, and record speed.
How fast is 3I/ATLAS moving?
About 130,000 mph, the fastest interstellar object yet.
When was 3I/ATLAS discovered?
It was first spotted on July 1, 2025, by NASA’s ATLAS survey.
Will 3I/ATLAS come close to Earth?
Yes, in December 2025, about 168 million miles away.
Is 3I/ATLAS artificial?
Most scientists agree it is natural, not alien technology.
