Our Solar System has once again received a visitor from deep space. Astronomers have confirmed with great excitement the discovery of the third known interstellar object passing through our cosmic neighborhood. This rare celestial traveler, officially cataloged as 3I/ATLAS, is a comet that does not originate from our system, but from the space between stars, carrying with it the secrets of distant worlds. Its discovery followed its famous predecessors, the mysterious object 1I/ʻOumuamua detected in 2017 and the comet 2I/Borisov which visited us 2I/Borisov in 2019.

Confirmation of interstellar origin

The first traces of this space nomad were recorded on July 1, 2025, thanks to the automated Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope located in Río Hurtado, Chile. The very first observations indicated an unusual trajectory that did not match the orbits of objects belonging to our Solar System. Its path was distinctly hyperbolic, a key indicator that the object possesses enough energy to overcome the Sun's gravitational pull and continue its journey back into interstellar space. In the days that followed, astronomers around the world aimed their instruments at the newly discovered object, confirming its extra-solar origin. After official confirmation, the International Astronomical Union assigned it the designation 3I/ATLAS, where "I" indicates its interstellar status.

Following the announcement of the discovery, scientists began searching for older images of the same part of the sky, a process known as "precovery." In the archives of three different ATLAS telescopes and the Zwicky Transient Facility (ZTF) at the Palomar Observatory in California, images dating back to June 14 were found, providing valuable additional data for a more precise calculation of its trajectory.

Characteristics and trajectory of the third interstellar messenger

Comet 3I/ATLAS is currently at a distance of approximately 670 million kilometers from the Sun, hurtling from the direction of the constellation Sagittarius. Its speed is astonishing, about 60 kilometers per second relative to the Sun, which far exceeds the speeds of objects gravitationally bound to our star. The diameter of the comet's nucleus is estimated to be significant, possibly up to 20 kilometers, making it considerably larger than many comets from our system. Fortunately, this celestial visitor poses no threat to Earth. Its trajectory will bring it closest to our planet at a safe distance of about 240 million kilometers, which is more than 1.5 times the distance between the Earth and the Sun.

3I/ATLAS will reach its closest approach to the Sun, known as perihelion, at the end of October 2025. At that time, it will be at a distance of about 210 million kilometers from our star, placing it just inside the orbit of Mars. Astronomers are eagerly awaiting this moment as the increased solar radiation will heat the comet's nucleus.

Global tracking effort and scientific significance

Immediately upon receiving the notification, the Planetary Defence Office of the European Space Agency (ESA) activated its resources. Automated detection systems alerted the agency's astronomers, who are now participating in the global effort to track the comet's path. The ESA uses a network of telescopes located in Hawaii, Chile, and Australia to continuously monitor the comet's movement and behavior. Some of these telescopes are owned by the agency, while others are available through long-standing partnerships, demonstrating the importance of international collaboration in astronomy.

What makes interstellar objects like 3I/ATLAS so extraordinary is their completely alien nature. While every planet, moon, asteroid, and comet that formed in our Solar System shares a common origin and chemical composition, interstellar visitors are true outsiders. They are fragments of other planetary systems, remnants of the processes of world formation around other stars. Studying their composition provides us with a unique opportunity to "touch" material from another solar system and gain invaluable insight into the conditions that prevail far beyond our home.

What can we learn from the icy traveler?

Scientists are now most interested in the composition and behavior of this rare visitor. 3I/ATLAS is classified as an active comet. This means that as it approaches the Sun and heats up, it is expected to begin the process of sublimation. During sublimation, frozen gases on the surface of the nucleus turn directly from a solid to a gaseous state, pulling dust and ice particles with them. This material creates a glowing atmosphere around the nucleus, known as a coma, and a characteristic tail that extends for millions of kilometers in the direction opposite to the Sun. Observing this process with spectroscopy will allow astronomers to determine the chemical composition of the gases, such as water, carbon monoxide, cyanide, and other molecules. These data are a direct fingerprint of the chemical composition of the protoplanetary disk from which the comet originated.

Unfortunately, observation will be difficult. By the time the comet reaches its peak activity and brightness, it will be apparently too close to the Sun in our sky, hiding in its glare. It is expected to become visible to ground-based telescopes again in early December, when it has moved far enough away from the Sun, giving astronomers a second chance for detailed studies.

The future of research: The Comet Interceptor mission

Such rare visits highlight the importance of rapid response missions. To this end, the European Space Agency is preparing a revolutionary mission called Comet Interceptor. The spacecraft is planned to be launched in 2029 and placed in a "parking" orbit at the Sun-Earth Lagrange point 2 (L2), located about 1.5 million kilometers behind the Earth. There, it will wait in a standby state for a suitable target to appear. The primary goal of the mission is to intercept a "pristine" comet coming from the distant Oort cloud, a vast spherical swarm of icy bodies that surrounds our Solar System. Such comets have never before passed through the inner Solar System, and their material is unchanged since the time of its formation.

Although it is unlikely that an interstellar object will be discovered with a trajectory that Comet Interceptor can reach, the mission will serve as a pioneering endeavor. It will demonstrate the ability to react quickly and intercept a target that is not known in advance, paving the way for future, even more ambitious missions that could be aimed specifically at these mysterious visitors from other star systems. Each new interstellar guest, be it 1I/ʻOumuamua, 2I/Borisov, or now 3I/ATLAS, reminds us that we are part of a vast and dynamic galaxy, and that sometimes, the universe comes to us.