ESO's Latest 3I/ATLAS Image is out Reveals The True Size of 3I/ATLAS's Core!

 

In July 2025, astronomers announced the remarkable discovery of an object from outside our solar system, designated 3I/ATLAS. For a third time in history, a visitor from interstellar space has been observed, and this time it is giving us an unusual opportunity to study what may essentially be "alien" material.

Even more curious, the recent imagery from key observatories brings new information about the size of its core, which allowed scientists to reconsider its scale and understand how big the object really is.

1. Discovery & Interstellar Identity

It was first detected on 1 July 2025, by the Asteroid Terrestrial‑impact Last Alert System (ATLAS) telescope in Chile.

Shortly after, orbital determinations indicated that its trajectory through our solar system is hyperbolic, meaning not bound to the Sun, hence interstellar in origin.

That places it in rarified company: only after 1I/'Oumuamua and 2I/Borisov have we seen such visitors.

2. Latest Imaging from the European Southern Observatory ('True Size' Insights)

ESO obtained deep-image observations of 3I/ATLAS using the VLT on 3 July 2025. These images reveal coma and tail form with clarity typical of bright solar-system comets.

Such observations help refine the estimates of the object's nucleus or core size. Though some early estimates were as large as ~20 km across, more recent observations suggest somewhat smaller dimensions—but still large by interstellar standards.

One key result: the coma and dust cloud are substantial, which suggests that the core must be large enough to drive such activity.

3. How Big Is It?: Scaling the Core

Initial reports suggested a diameter up to ~20 km early on—based on brightness and coma size.

More refined analyses (e.g., making use of photometry and multiple filters) now suggest an ≈10–15 km diameter, with some uncertainty depending on the assumed albedo (reflectiveness) and contribution by dust.

In any case, such a size would make 3I/ATLAS considerably bigger than the previous interstellar visitors: 1I/'Oumuamua was measured on a ~100 m scale, and 2I/Borisov ~1 km.

In short: this object is huge for an interstellar traveler, making it a rare—and extremely valuable—target.

4. Why the Size Matters

A larger core would imply that it may have retained ices and other materials long after its origin, possibly back in a distant star system.

Since it's so big-and thus brighter / more active-it gives astronomers better leverage for spectroscopic and compositional studies: what kinds of gases it's releasing, what dust is present.

Knowing its size helps model its origin, age, and the environment in which it formed, which will be key in comparing interstellar bodies to the ones within our solar system.

5. What the Images Reveal Beyond Size

The VLT images show a distinct coma or cloud of gas/dust and an emerging tail structure, evidence for active sublimation of volatile material in spite of this object's having formed elsewhere.

Spectroscopic data, obtained from other telescopes, reveal the presence of CO₂, water-ice grains, and dust ejection-evidences important to its composition and how it differs from typical solar system comets.

Its extremity: highly eccentric orbit, high velocity, large size, and peculiar composition make it an outlier and may suggest a different formation environment. 6. Implications & What's Next Because 3I/ATLAS is large and active, it offers a rare opportunity to study an object from another star system up close (relatively speaking). Future observations-both as it moves through and beyond the inner solar system-will allow researchers to: Track how the tail and coma evolve with solar heating. Compare its dust/gas ratios with known comets. Refine the models of interstellar object populations - how many are there, how often do they pass through? The fact that its size is larger than previous visitors suggests we may have been missing a population of large, faint interstellar objects. 7. Conclusion The newest images of 3I/ATLAS shrink the uncertainty in its size and give us a clearer picture: a substantial body, delivering fresh material from beyond our solar neighborhood. Its core size pushes the limits of what we have seen so far for extrasolar visitors, while vivid images snapped by ESO and other observatories make it more than a curiosity-a window into other planetary systems.