Rio Hurtado, Chile · 2 July 2025 — A NASA-funded survey telescope in Chile has detected a comet originating from beyond our solar system, making it only the third confirmed interstellar object ever observed. The comet, officially designated 3I/ATLAS (also catalogued as C/2025 N1), was first reported to the Minor Planet Center on 1 July 2025 by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Rio Hurtado, Chile.
The object arrived from the direction of the constellation Sagittarius and was approximately 420 million miles (670 million kilometres) from Earth when discovered — within the orbit of Jupiter. Its hyperbolic trajectory, meaning it is moving too fast to be gravitationally bound by the Sun, confirmed its interstellar origin. 3I/ATLAS is simply passing through our solar system and will never return.
Discovery and Pre-Discovery Observations
Since the initial report, astronomers have traced the comet's path back through archival data. Pre-discovery images were found in the archives of three different ATLAS telescopes around the world and the Zwicky Transient Facility at the Palomar Observatory in San Diego County, California, extending the observation record back to 14 June 2025. Numerous telescopes worldwide have since reported additional observations.
The ATLAS survey telescope is part of NASA's planetary defence network, which continually monitors the skies to detect asteroids and comets that could pose a threat to Earth. The system's wide-field survey capability makes it well-suited to serendipitous discoveries such as this one.
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Physical Characteristics and Trajectory
Based on observations from NASA's Hubble Space Telescope on 20 August 2025, astronomers estimated the diameter of 3I/ATLAS's nucleus to be not less than 1,400 feet (440 metres) and not greater than 3.5 miles (5.6 kilometres). This makes it larger and brighter than either of its interstellar predecessors, providing an extended window for scientific study.
When discovered, the comet was travelling at approximately 137,000 miles per hour (221,000 kilometres per hour). Pulled by the Sun's gravity, its speed increased to about 153,000 miles per hour (246,000 kilometres per hour) at perihelion — its closest approach to the Sun — which occurred around 30 October 2025 at a distance of approximately 1.4 astronomical units (about 130 million miles or 210 million kilometres), just inside the orbit of Mars.
| Property | Value |
|---|---|
| Designation | 3I/ATLAS (C/2025 N1) |
| Discovery date | 1 July 2025 |
| Discoverer | ATLAS survey telescope, Rio Hurtado, Chile |
| Origin direction | Constellation Sagittarius |
| Nucleus diameter | 440 m – 5.6 km (estimated from Hubble) |
| Speed at discovery | ~221,000 km/h (137,000 mph) |
| Speed at perihelion | ~246,000 km/h (153,000 mph) |
| Perihelion date | ~30 October 2025 |
| Perihelion distance | 1.4 AU (inside Mars's orbit) |
| Closest approach to Earth | 1.8 AU (no threat) |
No Threat to Earth
3I/ATLAS posed no danger to Earth. At its closest approach, the comet remained approximately 1.8 astronomical units (about 170 million miles or 270 million kilometres) from our planet — nearly twice the Earth-Sun distance. During perihelion, Earth was on the opposite side of the Sun.
The comet was expected to remain visible to ground-based telescopes through September 2025, after which it would pass too close to the Sun to observe. It was expected to reappear on the other side of the Sun by early December, allowing for renewed observations. Astronomers reported that 3I/ATLAS could be observed even with a small telescope in the pre-dawn sky, remaining visible until spring 2026.
A Multi-Mission Observation Campaign
NASA coordinated observations across multiple space-based and ground-based observatories to study 3I/ATLAS during its passage. The Hubble Space Telescope and the James Webb Space Telescope (JWST) both observed the comet, along with the SPHEREx observatory. Spacecraft at Mars — including NASA's Perseverance rover, the Mars Reconnaissance Orbiter, and MAVEN — were positioned to potentially observe the comet as it passed the Red Planet on 3 October 2025.
Scientific Significance
3I/ATLAS is scientifically important because its differences from comets in our solar system may provide insight into the composition and conditions of other planetary systems. Each interstellar object serves as a messenger from a distant star system, carrying chemical and physical information about environments humanity may never otherwise observe directly.
The growing catalogue of interstellar objects — from 'Oumuamua's puzzling acceleration to Borisov's carbon monoxide-rich coma and now 3I/ATLAS's methane signatures — suggests that such visitors may be more common than once thought. Improved survey capabilities, particularly with the upcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time, are expected to detect many more interstellar objects in the coming decade.
As 3I/ATLAS continues its journey out of the solar system and into interstellar space, never to return, the data collected during its brief passage will be studied for years to come — a reminder that the universe delivers samples from distant worlds to our cosmic doorstep, if only we are watching.
Sources:
- NASA Science, "NASA Discovers Interstellar Comet Moving Through Solar System," 2 July 2025,
- NASA Science, "3I/ATLAS Facts and FAQs," 13 November 2025,
- Space.com, "Interstellar comet 3I/ATLAS is blasting out a bunch of methane — here's why that's weird,"
- Sky & Telescope, "Interstellar Comet 3I/ATLAS Is Ancient,"
- Smithsonian Magazine, "The 7 Most Important (and Weirdest) Discoveries of 2025,"
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