Interstellar comet 3I/ATLAS: a cosmic time capsule from beyond the Solar System

For as long as we’ve been able to look up at the stars, we’ve also been waiting for them to fall, bringing disaster upon us. From the Book of Revelation to the modern TikTok prophets, we never seem to tire of anticipating the next Doomsday. So when a wandering comet was spotted making its way toward our Solar System, you could bet that some corner of the internet would proclaim it an omen. However, what doomers perceive as a harbinger of catastrophe, scientists welcome as a rare gift from another part of our galaxy, delivered to their doorstep.

Interstellar Comet 3I/ATLAS: a cosmic time capsule

Fig. 1. Comet 3I/ATLAS passing through the solar system. Digital artwork created in collaboration with ChatGPT (OpenAI).© Physics Core 2025.

This newest visitor, the interstellar comet 3I/ATLAS, is only the third foreign object ever detected passing through our celestial neighborhood. Discovered in September 2024, it comes to us from far beyond the solar system, traveling on a hyperbolic orbit, which will carry it back into interstellar space (Fig. 2). It’s a fleeting visitor. The comet passed Mars on 3 October 2025 and reached its closest approach to the Sun yesterday, completing the inward leg of its hyperbolic trajectory. Now, on its outbound path, it will pass Earth on 19 December, Jupiter by mid-2026, and exit the solar system forever in the early 2027.

Fig.2. The interstellar comet 3I/ATLAS following a hyperbolic trajectory through the inner Solar System, shown near its 2025 perihelion at 1.36 AU from the Sun. Credit: NASA/JPL-Caltech

 What we know so far

For astronomers, even a brief encounter is a treasure, as the interstellar objects represent fragments of other solar systems formed in an earlier universe. Recent studies suggest it may have originated within the thick disk of the Milky Way. That’s the older, more vertically extended population of stars in the galaxy. Trajectory modelling indicates it is arriving from the constellation Sagittarius, near the galactic center, which puts the comet's age at around 8 billion years.. Our solar system, located in the younger thin disk region, is about 4.6 billion years old. This significant age difference makes the comet a time capsule, carrying palpable evidence of what the universe was like 4 billion years before our solar system was formed.

Initial analyses suggest that 3I/ATLAS is similar to an average comet from our solar system, reinforcing the idea of a common chemical heritage throughout the universe. Measuring about eight kilometers wide, it consists of a solid core of ice and dust, followed by a greenish shimmering tail. This emerald hue comes from diatomic carbon (C₂), simple molecules that are released as the comet's surface ices warm in sunlight. The same carbon-rich chemistry is observed in many comets within our solar system, suggesting that organic building blocks might be universal across the stars.

Despite its respectable age, the comet 3I/ATLAS doesn't behave like a frozen relic. Instead, it exhibits the physics of a living comet, spinning and sending jets of vapor into space. These jets act as natural thrusters, subtly altering the comet's rotation and causing it to wobble. Solar radiation and the solar wind draw this material outward, creating a pale, streaming tail. Because the comet is so distant and faint, it is hard to determine its period precisely. Yet, early photometric studies suggest it rotates similarly to most solar comets, probably over tens of hours.

Why the Mars images are special

When the comet swept past Mars, its machine-operated satellites captured high-resolution images of the eight-billion-year-old traveler. It was the first time in human history that an interstellar object was observed from the orbit of another planet, a significant milestone in the exploration of the cosmos. Though the comet will come close to our home planet, Earth, on 19 December, providing us with a new trove of data, these images may remain the best we get.

• The images were taken at an optimal angle for sunlight to illuminate the comet

• Orbiters around Mars have superior cameras and spectrometers compared to those on Earth

• The images captured the comet before it reached perihelion (its nearest point to the Sun), so the comet was probably less active, making its nucleus and jets more visible.

As 3I/ATLAS now begins its long journey back toward interstellar space, telescopes across Earth and in orbit will continue to track the comet's fading light, collecting data in infrared, optical, and radio wavelengths. But the resulting images will be less about sharpness and more about spectral data (its composition, gas emissions, and dust behavior). So these images, captured from the Red Planet, may end up being the sharpest and most detailed visual data we’ll have in the first-of-its-kind interplanetary observation.

The universe sends us surprises, not as omens of doom, but as invitations to wonder. What we see in 3I/ATLAS is a reminder of our cosmic connection: that across the immeasurable distances of other galaxies, the same natural laws are at work, guiding the formation of matter and the evolution of the universe. As the interstellar comet travels through the solar system on its journey back into the darkness, it leaves us with more than just data and images; it provides living proof that in the cosmos, we are not merely spectators. We are active participants in solving its greatest mysteries.