Astronomers have confirmed two interstellar objects that entered the Solar System before the recent 3I/Atlas, discovered in July 2025. The first, 1I/Oumuamua, appeared in 2017, detected by telescopes in Hawaii. The second, 2I/Borisov, appeared in 2019, identified by an amateur observer in Crimea. These hyperbolic passages highlight the external origin of these bodies, coming from other stellar systems.
These events occur because the Solar System attracts travelers from the Milky Way due to its gravity. Scientists estimate that hundreds of similar objects arrive annually, but only three have been monitored so far. Detection depends on advanced telescopes, which calculate orbits to confirm the interstellar nature.
- Oumuamua speed: about 26 km/s upon entry.
- Estimated diameter of Borisov: up to 1 km.
- Orbital eccentricity of both: greater than 3, indicating a trajectory not linked to the Sun.
Oumuamua: The First Stellar Messenger
The 1I/Oumuamua arrived in October 2017, with an elongated shape that resembled a cigar. Telescopes at the Pan-STARRS observatory spotted it near Venus.
Its hyperbolic orbit, with an eccentricity of 1.2, confirmed an interstellar origin. The object measured 400 meters in length and rotated on itself, varying the observed brightness.
Astronomers noticed non-gravitational acceleration, possibly due to the release of gases. Later studies ruled out artificial origins, attributing the phenomenonor natural processes.
Borisov’s unique features
Astronomers detected 2I/Borisov in August 2019, thanks to a homemade telescope. The comet exhibited a typical coma and tail, unlike Oumuamua.
Its speed of 32 km/s and composition rich in carbon monoxide suggested it formed in a distant protoplanetary disk. Hubble captured detailed images during the flyby.
The object moved within 2 astronomical units of the Sun, allowing prolonged observations for months. Analysis revealed similarities to solar comets, but with exotic chemical traits.
Why so few are detected
The rarity of detections is due to the small size and low luminosity of these objects. Many measure less than 1 km and travel quickly, evading conventional telescopes.
Collisions with giant planets like Jupiter alter trajectories, destroying or diverting visitors. Telescopes like Vera Rubin, in future operation, promise to identify dozens annually.
Studies indicate that interstellar dust and cosmic radiation degrade these bodies over billions of years.
Trajectories and speeds compared
Oumuamua came from the direction of Lyra, with an initial speed of 26 km/s. Its passage lasted weeks, limiting data collected.
Borisov, originating from Cassiopeia, accelerated at 44 km/s near the Sun. Observations from Hubble and ground-based telescopes provided detailed spectra.
- Oumuamua: No visible coma, 8 hour rotation.
- Borisov: 160 thousand km tail, CN and H2O outgassing.
- Comparison with 3I/Atlas: Speed of 61 km/s, greater brightness.
Both left the Solar System without returning, reinforcing their nomadic nature.
Implications for modern astronomy
These objects carry material from exoplanetary systems, offering clues about planetary formation. Chemical compositions differ from those of the Solar System, indicating galactic diversity.
Missions like ESA’s Comet Interceptor prepare for future interceptions. Observations of Oumuamua and Borisov have inspired models of stellar ejection.
Frequency of annual arrivals
Scientists estimate that the Solar System receives 10 to 100 interstellar objects per year above 100 meters. Most pass in the outer regions, far from telescopes.
Factors such as the density of the Oort Cloud interfere with detections. Advances in AI fororbital analysis will increase discoveries in the coming years.
