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1. For months, scientists tracked 3I/ATLAS as just another fast-moving visitor from outside our solar system, its true nature obscured by distance and dust.
2. That picture has now changed dramatically, with new Hubble and James Webb Space Telescope data revealing an object whose size, mass, and chemistry defy expectations.
3. What emerges is not just a comet, but a city-scale interstellar body whose unusual methane activity is forcing researchers to rethink how such objects form and evolve.

New telescope data shows 3I/ATLAS is as large as a major city and producing methane in ways that challenge existing comet science.

[USA HERALD] - Newly released data from the Hubble Space Telescope and the James Webb Space Telescope has fundamentally reshaped what scientists understand about the interstellar object known as 3I/ATLAS. According to two preprints analyzing post-perihelion observations taken between December 2025 and January 2026, researchers have now directly detected the object’s nucleus and measured it at an effective diameter of approximately 2.6 kilometers, with a margin of error of about 0.4 kilometers.

To put that scale into perspective, the nucleus of 3I/ATLAS is comparable in size to downtown San Francisco. It would cover most of Monaco, or roughly 25 American football fields laid end to end. This is not a minor icy fragment drifting through space. It is a compact, city-sized body moving between stars.

Size matters in space science because mass scales with volume, not length. Since volume increases with the cube of the diameter, this measurement implies that 3I/ATLAS is about 40 times more massive than 2I/Borisov, the interstellar comet detected in 2019. It is also estimated to be at least 20,000 times more massive than 1I/‘Oumuamua, the first known interstellar object detected in 2017.

Data released by the Hubble Space Telescope shows that this size estimate aligns with independent calculations derived from the object’s non-gravitational acceleration. In simple terms, as 3I/ATLAS releases gas into space, it experiences a subtle but measurable push, similar to the recoil of a rocket. The amount of acceleration observed matches what scientists would expect from an object of this size losing mass at the measured rates.

Hubble’s imaging also reveals an important behavioral shift. After reaching its closest point to the Sun on October 29, 2025, 3I/ATLAS faded faster than it brightened on the way in. Its post-perihelion surface brightness profile is noticeably shallower than before perihelion, suggesting changes in surface activity or structure. Light-curve variations further indicate that the nucleus is rotating, exposing different regions over time.

A particularly telling moment occurred on January 22, 2026, when the Sun, Earth, and 3I/ATLAS briefly aligned. During this alignment, dust grains around the object reflected light more efficiently, producing a roughly 20 percent brightness surge. This “opposition surge” had been predicted by prior theoretical work and now appears clearly in the data, adding confidence to the physical interpretations.

While Hubble clarified the object’s size and structure, the James Webb Space Telescope has delivered something even more disruptive: a detailed chemical inventory. Using the MIRI spectrometer, Webb observed 3I/ATLAS in mid-December and again later that month. The spectra show water vapor, carbon dioxide, nickel, and, most notably, methane.

Methane is where the story becomes truly unusual.

The Webb data indicates that methane production on 3I/ATLAS ranges from roughly 14 percent to 27 percent of the water production rate. For comparison, methane on typical solar system comets is either absent or present only in trace amounts.

Earlier Webb observations from August 2025 showed that 3I/ATLAS was dominated by carbon dioxide outgassing, with CO2 accounting for roughly 87 percent of the gas mass loss, compared to just 4 percent for water. Carbon monoxide made up most of the remainder. Methane, despite being more volatile than carbon dioxide, was not detected at that time.

Publicly available data now shows that methane appeared only after perihelion, raising a fundamental puzzle. Methane ice sublimates at lower temperatures than carbon dioxide, meaning it should have been released earlier, not later. Its delayed appearance suggests that methane near the surface had already been depleted, and that the gas now being observed is coming from deeper layers that were only heated once the object passed close to the Sun.

This explanation, however, runs into a contradiction. Carbon monoxide is even more volatile than methane, yet it was detected before methane. That inconsistency challenges existing models of how volatile layers are arranged within comet-like bodies.

According to the researchers, the most conservative conclusion is that 3I/ATLAS has an internal structure and thermal history unlike anything previously observed. Its chemistry does not follow the simple layering seen in typical solar system comets, suggesting formation in a very different stellar environment.

The implications extend beyond this single object. The authors estimate that at any given moment, there is likely more than one 3I/ATLAS-sized interstellar object within about 4.5 astronomical units of the Sun. This is considered a lower bound, since inactive objects of similar size would be far harder to detect. In other words, multiple large interstellar bodies may have passed through the inner solar system unnoticed over the past decades.

USA Herald’s Analysis and Insight

For USA Herald readers, the significance of this discovery is not limited to astronomy. A city-sized object rich in methane and carbon compounds raises questions about how common such bodies are in the galaxy and how well current detection systems are performing. Methane is a molecule often discussed in the context of planetary atmospheres and habitability, but here it appears in a starkly different setting, stripped of any biological assumptions yet still chemically provocative.

As new data continues to arrive, 3I/ATLAS is emerging as the most massive and chemically puzzling interstellar object ever observed. Each layer peeled back by Hubble and Webb reveals deeper inconsistencies with existing models, underscoring how much remains unknown about the material wandering between stars. Rather than resolving the mystery of interstellar visitors, 3I/ATLAS has widened it.

About the Author

Samuel Lopez is an investigative journalist and legal analyst with more than two decades of experience examining complex scientific, legal, and policy-driven issues. As a contributor to USA Herald, Lopez is known for translating highly technical data into clear, accessible reporting while maintaining strict standards of accuracy and ethical journalism. His coverage frequently focuses on interstellar objects, planetary defense, and institutional accountability, blending forensic-style analysis with public-interest reporting to help readers understand why emerging discoveries matter in real-world terms.

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