Astronomers Discover 3I/ATLAS, Largest Interstellar Comet Yet Detected

Astronomers have discovered the third interstellar comet to pass through our solar system. Named 3I/ATLAS (initially A11pl3Z), it was first spotted July 1 by the ATLAS telescope in Chile and confirmed the same day. Pre-discovery images show it in the sky as far back as mid-June. The object is racing toward the inner system at roughly 150,000 miles per hour on a near-straight trajectory, too fast for the Sun to capture. Estimates suggest its nucleus may be 10–20 km across. Now inside Jupiter’s orbit, 3I/ATLAS will swing closest to the Sun in October and should remain observable into late 2025.

Discovery and Classification

According to NASA, in early July the ATLAS survey telescope in Chile spotted a faint moving object first called A11pl3Z, and the IAU’s Minor Planet Center confirmed the next day that it was an interstellar visitor. The object was officially named 3I/ATLAS and noted as likely the largest interstellar body yet detected. At first it appeared to be an ordinary near-Earth asteroid, but precise orbit measurements showed it speeding at ~150,000 mph – far too fast for the Sun to capture. Astronomers estimate 3I/ATLAS spans roughly 10–20 km across. Signs of cometary activity – a faint coma and short tail – have emerged, earning it the additional comet designation C/2025 N1 (ATLAS).

Studying a Pristine Comet

3I/ATLAS was spotted well before its closest approach, giving astronomers time to prepare detailed observations. It will pass within about 1.4 AU of the Sun in late October. Importantly, researchers can study it while it is still a pristine frozen relic before solar heating alters it. As Pamela Gay notes, discovering the object on its inbound leg leaves “ample time” to analyze its trajectory. Astronomers are now racing to obtain spectra and images – as Chris Lintott warns, the comet will be “baked” by sunlight as it nears perihelion.

Determining its composition and activity is considered “a rare chance” to learn how planets form in other star systems. With new facilities like the Vera C. Rubin Observatory coming online, researchers expect more such visitors in the years ahead. 3I/ATLAS offers a rare chance to study material from another star system.