Interstellar visitors

The first three objects known to have entered the solar system from interstellar space and left again were 1I/'Oumuamua in 2017, 2I/Borisov in 2019 and 3I/ATLAS in 2025.^[1] Each formed around another star, and each was seen only once, on a single pass.

How astronomers know they came from outside

An object bound to the Sun follows a closed, oval orbit. An interstellar object follows a hyperbolic path that is not closed, and when its path is traced back in time it clearly comes from outside the solar system.^[1] The letter I in the designation stands for interstellar, and the number gives the order of discovery: 3I/ATLAS is the third.^[1]

1I/'Oumuamua (2017)

The first, 1I/'Oumuamua, was discovered on 19 October 2017 by the Pan-STARRS1 telescope in Hawaii, part of NASA's near-Earth object search.^[2] Its name is Hawaiian for a messenger from afar arriving first.^[2] It is up to 400 m long and perhaps 10 times as long as it is wide, a more extreme shape than any asteroid or comet seen in the solar system; its brightness changed by a factor of 10 as it spun every 7.3 hours.^[2]

'Oumuamua showed no gas or dust, so it was first classed as a comet, then as an asteroid, until measurements showed it was accelerating slightly, a sign that it behaves more like a comet.^[2] Scientists are still not certain which it was.^[3] It passed closest to the Sun on 9 September 2017, moving at 87.3 km per second.^[2]

2I/Borisov (2019)

The second visitor, 2I/Borisov, was an unmistakable comet.^[3] Gennady Borisov, an amateur astronomer in Crimea, discovered it on 30 August 2019, and a week of observations around the world confirmed that it came from outside the solar system.^[3] Its nucleus was estimated at about 975 m across, about the length of nine football fields, and it travelled at about 177,000 km per hour, too fast for the Sun's gravity to capture it.^[3] Like comets that form around the Sun, it developed a coma of gas and dust as it warmed, and in March 2020 the Hubble Space Telescope saw signs that a fragment had broken away from it.^[3]

3I/ATLAS (2025)

The third, comet 3I/ATLAS, was first reported on 1 July 2025 by the NASA-funded ATLAS survey telescope in Rio Hurtado, Chile, and earlier images found in survey archives extend back to 14 June 2025.^[1] It passed closest to the Sun around 30 October 2025, at about 1.4 au, just inside the orbit of Mars, and came no closer to Earth than about 270 million km.^[1] Observations by the Hubble Space Telescope put the diameter of its nucleus between about 440 m and 5.6 km.^[1]

NASA followed 3I/ATLAS with many spacecraft, including Hubble, the James Webb Space Telescope, orbiters and a rover at Mars, and Parker Solar Probe, and its SPHEREx telescope detected dust, water, organic molecules and carbon dioxide in the comet's coma.^[1]

How often they come

Astronomers estimate that an object like 'Oumuamua passes through the inner solar system about once a year, but such objects are faint and were missed until survey telescopes became powerful enough to find them.^[2] Comets in general are covered in Comets: what they are and how to see one.