Moon Hong-kyu
The author is a principal researcher at the Korea Astronomy and Space Science Institute.
In early July, when daytime temperatures hovered around 33 degrees Celsius in Korea, news of a new celestial object spread quickly online. The International Astronomical Union assigned it the temporary name A11pl3Z. Its orbital eccentricity was calculated at 6.1; by comparison, a circle has an eccentricity of zero, and closed, hyperbolic Sun-bound orbits have an eccentricity higher than one. With an eccentricity of 6.1, the object is on a clearly unbound, interstellar trajectory. Astronomers concluded it originated outside the solar system and gave it a formal designation: 3I/ATLAS, meaning the third confirmed interstellar object.
Members of the discovery team urgently contacted the Korea Astronomy and Space Science Institute's exoplanet monitoring network, KMTNet, which operates telescopes in Chile, South Africa and Australia. "An interstellar object has been spotted, and observing conditions are ideal at all locations. Could we use director's discretionary time?" they asked. Permission was granted, and observations began immediately.
Like images captured by other observatories, the KMTNet data showed a hazy coma and tail, confirming it is an interstellar comet. Further analysis by the James Webb Space Telescope revealed its composition differed markedly from comets in the solar system. Although 3I was still beyond the boundary where ice typically begins to sublimate, it emitted jets of vapor comparable to water blasted from a high-pressure hose. The data also showed carbon dioxide levels eight times higher than water. Researchers say this suggests the object formed in a much colder and chemically distinct environment than the one that shaped the solar system.
The object is now moving away from the Sun at a speed of 58 kilometers per second and is not trapped by the Sun's gravity. In late October, about a month after its closest approach to the Sun, 3I appeared briefly in the morning sky before disappearing into deep darkness.
Its exact origin remains unknown. Some astronomers suspect it was ejected from stars in the Milky Way's thick disk, a region populated by older stars containing primordial material. It may have wandered for millions of years before passing through our solar system.
As the interstellar visitor leaves, it raises lingering questions about how different planetary systems form, and how many more such travelers may cross our skies.
This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.