Free Astronomy Magazine September-October 2025

13 SEPTEMBER-OCTOBER 2025 ASTRO PUBLISHING connection to other star systems. They carry information about the chemical elements that were present when and where they formed, which gives scientists insight into how planetary systems form at dis- tant stars throughout our galaxy’s history — including stars that have since died out. 3I/ATLAS, formally designated Com- et C/2025 N1 (ATLAS), is only the third interstellar object ever discov- ered after 1I/ ‘ Oumuamua in 2017 and the comet 2I/Borisov in 2019. While astronomers think many inter- stellar objects exist, and likely pass through our Solar System on a regu- lar basis, they are exceptionally dif- ficult to capture since they are only visible when they’re close enough to see and when our telescopes are pointing in the right place at the right time. Multiple teams of astronomers around the globe are using a wide variety of telescopes to observe 3I/ATLAS during its temporary visit to our Solar System, allowing them to collectively determine some of the comet’s key characteristics. Al- though much remains unknown, it is already clear that 3I/ATLAS is unique compared to the two other known interstellar objects. Observations so far suggest that 3I/ATLAS has an ap- proximate diameter of at most 20 kilometers (12 miles), compared to ‘ Oumua- mua’s diameter of 200 meters and Borisov’s of less than one kilometer. 3I/ATLAS’s larger size makes it a better target for scientists to study. The comet also has an exceptionally eccentric orbit, where eccentricity describes how much an object’s or- bital pathway is ‘stretched out.’ An eccentricity of 0 is a perfectly circular orbit, while an eccentricity of 0.999 is a very stretched-out ellipse. An object with an eccentricity above 1 is on a path that does not loop back around the Sun, implying it comes from — and will return to — interstellar space. 3I/ATLAS has an eccentricity of 6.2, which is highly hyperbolic and ensures its classifica- tion as an interstellar object. In com- parison, ‘ Oumuamua had an eccen- tricity of about 1.2, and Borisov about 3.6. Right now, 3I/ATLAS is within Jupiter’s orbit at a distance of about 465 million kilometers (290 million miles) from Earth and 600 million kilometers (370 million miles) from the Sun. The closest 3I/ATLAS will come to Earth is approximately 270 million kilometers (170 million miles) on 19 December, though it will pose no threat to the planet. It will reach its closest approach to the Sun around 30 October at a distance of 210 million kilometers (130 million miles) — just inside the orbit of Mars. During this close approach, it will be traveling almost 25,000 kilo- meters (15,500 miles) per hour. Although 3I/ATLAS is only the third interstellar object ever discovered, the astronomical community ex- pects many more to soon come into focus once NSF–DOE Vera C. Rubin Observatory, jointly funded by NSF and the U.S. Department of En- ergy’s Office of Science, begins its decade-long Legacy Survey of Space and Time (LSST). By repeatedly scanning the entire southern hemisphere sky every few nights, Rubin will capture millions of objects moving throughout our Solar System, including an unpre- dictable number of never-before- seen interstellar objects. C omet 3I/ATLAS streaks across a dense star field in this image captured by the Gemini North telescope’s Gemini Multi-Object Spectrograph (GMOS-N). The left panel captures the comet’s colorful trail as it moves through the Solar System. The image was composed of expo- sures taken through three filters, shown here as red, green, and blue. The right inset zooms in to reveal the comet’s compact coma — a cloud of gas and dust sur- rounding its icy nucleus. [International Gemini Observatory/NOIRLab/NS F/AURA/K. Meech (IfA/U. Hawaii) − Image Process- ing: Jen Miller & Mahdi Zamani (NSF NOIRLab)] !