3I/Atlas Status Update 2026

Key Takeaways

• The interstellar comet 3I/Atlas was discovered in July 2025 and reached its closest point to the Sun in October 2025.
• It is a large, active comet with a unique chemical composition, offering a rare sample of material from another star system.
• Observations from a global array of telescopes and spacecraft confirmed its natural origin, finding no signs of artificial technology.

Introduction

On July 1, 2025, telescopes of the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System) survey in Chile spotted a fast-moving, faint object. Quickly designated A11pl3Z, its calculated path immediately stood out: it was moving on a hyperbolic trajectory too fast to be bound by the Sun’s gravity. This object was not from our solar system. Now known as comet 3I/Atlas, it became only the third confirmed interstellar visitor after 1I/‘Oumuamua (2017) and 2I/Borisov (2019).

The discovery triggered one of the most comprehensive astronomical observation campaigns in recent history. As it journeyed through the inner solar system, 3I/Atlas became a target for virtually every major observatory – both on Earth and in space – creating an unprecedented opportunity to study a piece of another planetary system.

Discovery and Initial Analysis

Following its discovery on July 1, 2025, by the ATLAS station in Río Hurtado, Chile, astronomers quickly located earlier “pre-discovery” images of the object dating back to mid-June of that year. Analysis confirmed it was an active comet, not an asteroid, as it displayed a faint coma – a cloud of gas and dust enveloping its nucleus.

Initial observations from NASA’s Hubble Space Telescope in July and August 2025 provided the first size estimates for its icy core. At that time, scientists determined its nucleus had a diameter somewhere between 0.44 and 5.6 kilometers. The comet’s speed was measured at a staggering 221,000 kilometers per hour (137,000 miles per hour) upon discovery, increasing as it fell toward the Sun.

Unlike the first interstellar visitor, ‘Oumuamua, which appeared rocky and showed minimal activity, 3I/Atlas clearly exhibited the classic features of a comet. Early spectral data from telescopes, including the James Webb Space Telescope (JWST), revealed an unusual composition. It was found to be unusually rich in carbon dioxide (CO₂) relative to water vapor, a mix distinct from most comets native to our solar system.

A Coordinated Global Observation Campaign

The journey of 3I/Atlas through the solar system was documented by a remarkable fleet of spacecraft and telescopes. NASA coordinated observations across its heliophysics and planetary missions, creating a multi-perspective view of the comet.

The comet reached its closest point to the Sun, called perihelion, on October 29, 2025, at a distance of about 203 million kilometers (1.36 Astronomical Units), placing it between the orbits of Earth and Mars. It made its closest approach to Earth on December 19, 2025, at a perfectly safe distance of 270 million kilometers (1.8 AU).

Scientific Revelations: Size, Composition, and Natural Origin

As 3I/Atlas moved away from the Sun after perihelion, new observations refined the understanding of this visitor. Crucially, the comet did not disintegrate during its solar passage. Instead, post-perihelion data from Hubble in late 2025 and early 2026 allowed scientists to separate the faint glow of the solid nucleus from its bright coma. These observations suggested a larger, more refined size estimate. The nucleus was calculated to have an effective diameter of approximately 2.6 kilometers (with an uncertainty of about 0.4 km), making it significantly more massive than the previous interstellar comet, 2I/Borisov.

James Webb Space Telescope observations in December 2025 made another major discovery: the robust detection of methane (CH₄) in the comet’s coma. This was significant, as methane is a hyper-volatile ice that sublimates at very low temperatures. Its delayed detection, only after the comet had warmed near the Sun, suggested a layered interior where methane was buried beneath the surface and only exposed later in its journey. This complex outgassing behavior, along with its high CO₂ content, paints a picture of a comet that formed under chemical conditions different from those in our solar system.

The comet’s anomalous properties – including its unusual chemical ratios and its precise orbital alignment – led to scientific discussions about the importance of investigating all possibilities regarding its origin. In response, the scientific community conducted extensive searches for any sign of artificial technology. The Breakthrough Listen initiative used premier radio telescopes like the Green Bank Telescope to scan the comet for engineered radio signals. By late 2025, the conclusion was that no technosignatures had been detected and that 3I/ATLAS continued to behave as expected from natural astrophysical processes. NASA officials and the broader astronomy community consistently described it as a natural, if fascinatingly different, comet.

Legacy and Future Intercepts

3I/Atlas demonstrated that interstellar objects are diverse. Each one – from the rocky ‘Oumuamua to the Borisov comet, and now the large, CO₂-rich Atlas – provides a unique sample of planet-building material from another star. Its study was a real-time test of our global ability to rapidly mobilize and observe unpredictable celestial events.

The comet also underscored the potential of next-generation survey telescopes. The Vera C. Rubin Observatory, which had serendipitously imaged 3I/Atlas during its commissioning phase, is expected to find many more such visitors when its full sky survey begins. Finding these objects earlier is the key goal of future missions like the European Space Agency’s (ESA) Comet Interceptor.

Scheduled for launch in the late 2020s, the Comet Interceptor mission is designed to do exactly what its name implies. The spacecraft will wait at a stable point in space until a suitable pristine comet – or even an interstellar object – is discovered on an incoming path. It will then be dispatched for a rapid-reaction flyby. The study of 3I/Atlas proved the immense scientific value of such a target and validated the mission’s innovative “park and wait” strategy.

Summary

Comet 3I/Atlas, discovered in mid-2025, provided a landmark opportunity for modern astronomy. As the third known interstellar visitor, its intact passage through the solar system allowed for detailed analysis that revealed a large nucleus and a unique chemical makeup dominated by carbon dioxide and methane. A historic, global observation campaign involving dozens of space and ground-based instruments confirmed its natural cometary origin while gathering data that will inform the study of other star systems for years to come. Its visit highlights a future where intercepting and studying such interstellar objects may become a central part of exploring our galactic neighborhood.

Appendix: Top 10 Questions Answered in This Article

When was comet 3I/Atlas discovered?

Comet 3I/Atlas was discovered on July 1, 2025, by the NASA-funded ATLAS survey telescope in Chile. It was the third object ever confirmed to have originated from outside our solar system.

Did 3I/Atlas disintegrate when it passed the Sun?

No, it did not. Contrary to early speculation, the comet survived its closest approach to the Sun (perihelion) on October 29, 2025, intact. Observations from Hubble and other telescopes continued to study it well after this event.

How big is the comet’s nucleus?

Initial size estimates ranged widely from 0.44 to 5.6 kilometers. More refined analysis of post-perihelion data from the Hubble Space Telescope suggests a more precise effective diameter of approximately 2.6 kilometers.

What makes its composition unusual?

Spectroscopic data, primarily from the James Webb Space Telescope, showed that 3I/Atlas is unusually rich in carbon dioxide (CO₂) compared to water. After perihelion, astronomers also made a robust detection of methane (CH₄), which exhibited a delayed outgassing pattern.

Was it ever considered an alien spacecraft?

Some of its anomalous properties led to scientific speculation about all possible origins. However, dedicated searches for artificial radio signals by projects like Breakthrough Listen found no evidence of technology. The scientific consensus is that it is a natural comet.

How fast was it traveling?

When discovered, it was moving at about 221,000 km/h (137,000 mph). Its speed increased to approximately 246,000 km/h (153,000 mph) at its closest approach to the Sun due to the Sun’s gravity.

Which spacecraft observed it?

A fleet of missions observed it, including the Hubble and Webb space telescopes, the Mars Reconnaissance Orbiter, the Perseverance rover, the Psyche spacecraft, and ESA’s Mars Express and JUICE missions, among many others.

What is the Comet Interceptor mission?

Led by the European Space Agency with JAXA, Comet Interceptor is a future mission (launching late 2020s) designed to wait in space for a suitable pristine or interstellar comet. It will then intercept and fly by the target, building on the lessons learned from observing 3I/Atlas.

How close did it come to Earth?

The comet made its closest approach to Earth on December 19, 2025, at a distance of 270 million kilometers (about 1.8 times the Earth-Sun distance). It posed no threat to our planet.

Why was studying 3I/Atlas so important for science?

As only the third confirmed interstellar visitor, it provided a direct, albeit fleeting, sample of material from another star system. Its unique composition and behavior offer clues about how comets and planets form in different stellar environments.