Three interstellar objects are known, and each was faster than the one before
| Object | Found | Discovered by | Perihelion | Perihelion AU | Eccentricity | Speed in interstellar space km/s | Size | Nature |
|---|---|---|---|---|---|---|---|---|
| 1I/'Oumuamua | 2017-10-19 | Pan-STARRS1, Haleakala, Hawaii (Robert Weryk) | 2017-09-09 | 0.26 | 1.2 | 26.3 | about 100 to 400 m long, very elongated | No coma; sped up slightly as if pushed by gas |
| 2I/Borisov | 2019-08-30 | Gennady Borisov, an amateur, with his own 0.65 m telescope in Crimea | 2019-12-09 | 2 | 3.35 | 32.2 | nucleus under 1 km | Active comet, rich in carbon monoxide |
| 3I/ATLAS | 2025-07-01 | ATLAS survey telescope, Rio Hurtado, Chile | 2025-10-29 | 1.36 | 6.14 | 58 | nucleus likely under 1 km | Active comet with a carbon dioxide-rich coma |
Speed in interstellar space is the speed far from the Sun, before its gravity sped the object up. Orbits from the Minor Planet Center.
An eccentricity above 1 means an open path: the object came from outside and will not return
Every comet and asteroid bound to the Sun moves on an ellipse, with an eccentricity below 1. A body falling in from the Oort Cloud arrives at almost exactly 1. All three interstellar objects arrived on hyperbolas, with eccentricities of 1.2, 3.4 and 6.1, moving tens of kilometers per second faster than the Sun's gravity alone could make them. That excess speed is their motion relative to the Sun as they drifted in from interstellar space. The I in their names stands for interstellar.
1I/'Oumuamua tumbled through in 2017, showed no tail, and sped up without one
Pan-STARRS found 'Oumuamua, Hawaiian for scout, on October 19, 2017, more than a month after it had passed the Sun and as it was leaving. It was small, red and changed brightness tenfold as it tumbled, implying a cigar or pancake shape several times longer than wide. It showed no coma, yet it moved slightly faster than gravity predicts, as comets do when venting gas; the most likely explanation is gas such as hydrogen escaping from ice without dust. It is now beyond Neptune's orbit.
2I/Borisov, found by an amateur in 2019, looked like an ordinary comet from another star
Gennady Borisov found it on August 30, 2019, with a telescope he built himself, while it was still inbound. It had a coma and tail like any solar-system comet, but carried far more carbon monoxide than almost any comet of ours, suggesting it formed in the cold outer disk around a small, cool star. Its nucleus was under 1 km, and it shed fragments after perihelion.
3I/ATLAS, at 58 km/s, was watched by about 25 spacecraft and may be older than the Sun
The ATLAS survey found it on July 1, 2025, near Jupiter's orbit, on a retrograde path close to the plane of the planets. It reached perihelion on October 29, 2025, at 1.36 AU, between the orbits of Earth and Mars, on the far side of the Sun from Earth, so spacecraft got the best views: the Mars orbiters of ESA, NASA and China's Tianwen-1 in early October as it passed Mars, then NASA's Psyche and Lucy, ESA's Juice, which measured water and carbon dioxide streaming from it, Europa Clipper, and finally Juno at Jupiter in March 2026. Webb found an unusually carbon dioxide-rich coma. Its speed suggests it formed around an old star in the Milky Way's thick disk, perhaps 3 to 11 billion years ago.
The Rubin Observatory should find several a year; ESA plans a spacecraft to meet one
The Vera C. Rubin Observatory in Chile, which began its survey in 2025, sweeps the whole southern sky every few nights and should find interstellar objects far earlier and more often than ATLAS and Pan-STARRS could. ESA and JAXA's Comet Interceptor, due to launch around 2029, will park beyond Earth and wait for a target, possibly an interstellar one. Meteors from interstellar dust have been claimed, but none is confirmed. Comets · Asteroids