Astronomers around the world have been fascinated by a particular comet for weeks. It is called 3i/ATLAS, and it is the third interstellar object to reach our solar system from another part of the universe. Before the visit of this exotic stranger, only the asteroid 1I/’Oumuamua and the comet 2I/Borisov had been detected.
It was detected by the ATLAS system in Chile on July 1, 2025—hence the other half of its name. This comet goes its own way; its orbit is clearly hyperbolic, it is not influenced by the gravity of any planet, moon, or even our sun, making it a true intergalactic nomad. And it seems to be in a hurry when it travels, maintaining a speed of approximately 60 km/s when it is outside the direct influence of the sun. But why is it so fascinating to the space science community?
Being a comet from another galaxy, its chemical composition is totally different from the comets we are used to. Its anomalous composition causes it to change colors and leave a totally different and colorful radio signature. It is so different from other native comets that there has even been speculation that it has a technological origin. However, a telescope in South Africa has been able to investigate it more closely.
The MeerKAT telescope in South Africa
On October 24, 2025, the radio telescope was able to detect a very curious signal from the comet before it reached its closest approach to the sun (perihelion). This occurred five days later, on October 29, 2025.
When comet 3i/ATLAS was only 3.76º from the sun in the sky, the Radioscope was able to detect and track it despite intense interference from solar energy—absorption lines at specific frequencies, which are the fingerprint of the hydroxyl radical. Since hydroxide is a byproduct of water ice decomposed by solar radiation, this ruled out the possibility that the comet was some kind of spacecraft created by advanced technology. Comet 3i/ATLAS is simply a comet with ice and a high proportion of carbon dioxide and nickel. This complex composition indicates that it was formed in a unique environment in the early Milky Way. Scientists date it to 7 billion years ago, while our own subsoil is 4.6 billion years old.
This has been considered irrefutable proof of the object’s frozen nature. It turns out that the main driver of the comet is the sublimation of water ice. Ultimately, it exhibits the physical behavior expected of any frozen body that is heated. Although the technological zone hypothesis was fascinating to alien enthusiasts, the water activity of comet 3i/ATLAS places it squarely within the realm of natural celestial objects.
Radio astronomy: tracking comets
Believe it or not, this is the first time that an interstellar object has left such a clear radio trail. Low-frequency radio astronomy allows us to study the chemical composition of comets near the sun. From now on, we will be able to monitor the chemical activity and water production rate in real time of every comet that approaches the center of our solar system.
Comet 3i/ATLAS became observable last month while it was so close to the sun, although it is expected to reappear in the sky between the end of this month and early December 2025. In the meantime, we will wait for new spectroscopic observations to be made to continue investigating this curious comet. Scientists are also waiting for high-resolution images from HiRISE (Mars Reconnaissance Orbiter) to be released, which will shed more light on the nature of this interstellar object.
Comet 3i/ATLAS will not remain in our solar system, but will continue on its journey, exploring the rest of the universe. It will gradually leave the system, but is expected to pass close to Jupiter in March 2026. By the early 2030s, it will have left the region of the giant planets and will continue on its way into interstellar space.