Date: 24 October 2025

Our Latest Visitor from Another Star System: 3I/ATLAS (C/2025 N1)

Doğuş University’s Office of Universal Exploration and Communication (DÜEKİ) and our university’s faculty member in the Department of Electrical and Electronics Engineering, cosmologist Assoc. Prof. Dr. Nihan Katırcı, evaluated the observational and theoretical characteristics of the interstellar comet 3I/ATLAS, which recently visited our Solar System. In recent years, three interstellar objects that have passed through our Solar System have earned a special place in the history of astronomy: 1I/‘Oumuamua (2017), 2I/Borisov (2019), and now 3I/ATLAS (2025). 3I/ATLAS was discovered on July 1, 2025, by the Asteroid Terrestrial-Impact Last Alert System (ATLAS), a NASA-funded project, when it was about 420 million miles from the Sun. The comet’s name reflects its origin as the third interstellar object ever discovered.

Like classical comets, as it approaches the Sun, 3I/ATLAS develops two tails—one of dust and one of ions. Observations from the James Webb Space Telescope (JWST) have revealed that the bright cloud surrounding the comet’s nucleus, known as the coma, is composed of gas and dust and contains approximately 87% carbon dioxide [Cordiner, M. A., et al. (2025)]. The nucleus itself is about 5 kilometers in diameter, making it significantly larger than previous interstellar visitors. The Swift Space Telescope detected an outflow of about 40 kilograms of hydroxyl (OH) per second even when the comet was 2.9 astronomical units (434 million kilometers) from the Sun. Sunlight or other forms of radiation break water (H₂O) into OH radicals. Since comets can transport water and organic matter necessary for life to form on planets, the fact that 3I/ATLAS is still active at such a great distance from the Sun makes it one of the rare comets of its kind. Polarimetric observations have shown that 3I/ATLAS exhibits a deep and narrow negative polarization branch never before observed in any asteroid or comet: a minimum value of –2.7% at a 7° phase angle, and an inversion angle of 17° [Gray, Z., et al. (2025)]. Spectroscopic data from the UVES spectrograph on the ESO Very Large Telescope (VLT) in Chile revealed an unusually high abundance of nickel (Ni I) atoms in the comet’s coma and an exceptionally high Ni I/Fe I ratio [Hutsemékers, D., et al. (2025)]. While these elements are usually found together throughout the galaxy and universe, in ATLAS, nickel dominates and iron appears only in trace amounts, distinguishing it even from Borisov. Previous interstellar objects also displayed distinct characteristics: for example, ‘Oumuamua was dry, whereas Borisov was rich in carbon monoxide, suggesting that Borisov originated from the outermost regions of its parent star system.

3I/ATLAS entered the Solar System on a hyperbolic trajectory at a speed of about 58 km/s. This velocity clearly indicates that it could not have originated in the Oort Cloud and is therefore not gravitationally bound to the Sun. Its orbital eccentricity of about 6.1 confirms its interstellar origin. According to astrophysicist Darryl Seligman (Michigan State University), the comet may have formed about 7 billion years ago [Seligman, D. Z., et al. (2025)]. On October 3, ESA’s ExoMars Trace Gas Orbiter (TGO) and Mars Express spacecraft tracked the comet as it passed within 28 million kilometers of Mars, though only ExoMars managed to observe it. NASA and ESA’s recent silence on the matter has intrigued science enthusiasts, but the real reason is that the comet is currently behind the Sun, hidden by intense solar glare. When it reaches its perihelion on October 30, it unfortunately will not be observable from Earth.

Speculations about NASA’s silence—combined with findings such as the nickel abundance and the comet’s propulsion-like motion (as if guided rather than driven by dust ejection)—have fueled theories suggesting that 3I/ATLAS could be an intergalactic spacecraft or an observation probe from an advanced civilization.

Consequently, 3I/ATLAS (C/2025 N1) has captured the attention of both scientists and the public. With references to Liu Cixin’s “The Dark Forest”, the Fermi Paradox, and Abraham Loeb’s thought experiments [Hibberd, A., Crowl, A., & Loeb, A. (2025)], the discussions have become even more exciting. It is expected that much more will be learned about this object in the coming months. After its close approach to the Sun, its tail will grow even longer, and around December 19, 2025, it will reach its closest approach to Earth—about 270 million kilometers away.

In March 2026, it will pass near Jupiter and then leave the Solar System. Additional data will be collected during the JUICE mission at that time.

This extraordinary visitor seems poised to push the boundaries of science and curiosity once again. You can track the comet’s position in real time at https://theskylive.com/c2025n1-info, and monitor its coordinated observations by Hubble, Webb, TESS, Swift, SPHEREx, Perseverance, Curiosity, Parker Solar Probe, Lucy, Psyche, PUNCH, and SOHO at https://3i-atlas.github.io/coordinated.html. Over 80 academic papers have already been published or are in progress in just four months, which can be accessed at https://3i-atlas.github.io/pubs.html.ESA’s upcoming “Comet Interceptor” mission, scheduled for 2029, will remain in space ready to intercept and study future interstellar visitors. “This and many other visitors—comets, asteroids, and exoplanetary discoveries—are poised to reshape our current understanding of the formation and evolution of celestial bodies.”

Cordiner, M. A., et al. (2025). JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS. The Astrophysical Journal Letters, 991(2), L43. arXiv:2508.18209

Gray, Z., et al. (2025). Extreme negative polarisation of new interstellar comet 3I/ATLAS. arXiv:2509.05181

Hutsemékers, D., et al. (2025). Extreme Ni I/Fe I abundance ratio in the coma of the interstellar comet 3I/ATLAS. arXiv:2509.26053

Seligman, D. Z., et al. (2025). Discovery and preliminary characterization of a third interstellar object: 3I/ATLAS. The Astrophysical Journal Letters, 989(2), L36. arXiv:2507.02757

Hibberd, A., Crowl, A., & Loeb, A. (2025). Is the interstellar object 3I/ATLAS alien technology? arXiv:2509.07409