Comet 3I/ATLAS’ predecessors, 1I/ʻOumuamua, and 2I/Borisov: a comparison of the only 3 interstellar objects which passed our Solar System

The discovery of interstellar objects passing through our solar system has opened a new chapter in astronomy. To date, only three confirmed interstellar visitors have been detected: 1I/ʻOumuamua in 2017, 2I/Borisov in 2019, and 3I/ATLAS in 2025. Each provides unique insights into the composition and dynamics of material from beyond our solar system.

1I/ʻOumuamua: Detected in October 2017 by the Pan-STARRS telescope in Hawaii, ʻOumuamua was the first known interstellar object. Its unusual elongated shape and fast hyperbolic trajectory confirmed its origin outside the solar system.

2I/Borisov: Spotted in August 2019 by amateur astronomer Gennadiy Borisov in Crimea, this comet resembled solar-system comets in appearance but traveled at interstellar speeds, providing the first opportunity to study an interstellar comet’s composition directly.


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3I/ATLAS: Discovered in July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, 3I/ATLAS is the third interstellar object. Unlike ʻOumuamua’s rocky nature, ATLAS is a comet with a visible coma and tail, offering a rare chance to study interstellar ice and dust.

Physical Characteristics

Object	Shape & Size	Composition	Trajectory & Speed	Closest Approach to Earth
1I/ʻOumuamua	Elongated, ~230–400 m long	Likely rocky or metallic	Hyperbolic, ~196,000 mph	~24 million km (2017)
2I/Borisov	Cometary nucleus, ~0.4–1 km diameter	Water ice, CO, CN, dust	Hyperbolic, ~100,000 mph	~300 million km (2019)
3I/ATLAS	Cometary nucleus, 0.44–5.6 km	Water ice, CO, CO₂, CN	Hyperbolic, ~130,000 mph	~27 million km (Oct 2025)

Scientific Significance

1I/ʻOumuamua: Its unusual cigar-like shape and acceleration beyond gravitational effects sparked debates about non-gravitational forces, including outgassing or even exotic explanations. It provided the first evidence that objects routinely travel between star systems.

2I/Borisov: The first observed interstellar comet with an active tail, Borisov’s chemical composition closely matched comets in our solar system, suggesting similarities in comet formation across star systems.

3I/ATLAS: With its visible coma and tail, ATLAS offers a chance to study interstellar dust and volatile compounds in real-time. Its passage near Mars in October 2025 allows detailed observation of its trajectory, chemical makeup, and potential meteor showers if Earth intersects its debris.

Impact on Astronomy

Together, these three objects highlight the diversity of interstellar visitors:
ʻOumuamua challenged existing models of asteroid formation.
Borisov confirmed that comets form similarly in other star systems.

ATLAS provides both chemical and orbital data, enhancing planetary defense models and expanding knowledge of interstellar material interacting with the solar system.

Also read: Interstellar comet 3I/ATLAS first image from Mars Perseverance Rover sparks debate over alien mothership tEach interstellar object has contributed uniquely to astronomy. While 1I/ʻOumuamua raised questions about shape and acceleration, 2I/Borisov confirmed interstellar chemical similarities. Now, 3I/ATLAS adds the ability to study dust, ice, and gravitational interactions in detail.