Small bodies · Space Codex
1I/ʻOumuamua
Discovered in October 2017 already leaving, this elongated or flattened object came from another star system, showed no tail, yet accelerated slightly as it left.
The first known interstellar visitor
Artist's concept. 1I/ʻOumuamua has never been imaged in enough detail to show its true shape or surface. Any depiction of it is an illustration based on measurements, not a photograph.
At a glance
Key points
- First confirmed interstellar object
- No tail, but a small non-gravitational push
- Never resolved: all images are artist's concepts
A puzzle
It tumbled and changed brightness about tenfold, implying an extreme shape. It showed no visible coma, yet its path bent slightly away from the Sun as if pushed by outgassing too faint to see.
Seen only on its way out
Pan-STARRS1 on Haleakalā picked it up on 19 October 2017, about six weeks after it had swung around the Sun inside Mercury's orbit and five days after its closest pass to Earth. Telescopes had less than three months before it faded beyond reach in early January 2018. It crossed Saturn's orbit in January 2019 and is heading out of the Solar System towards Pegasus.
Explaining the push
In 2018 a team led by Marco Micheli showed its path could only be fitted with a small extra push away from the Sun, the kind outgassing gives comets. Yet no gas or dust was seen, and Spitzer failed to detect it in infrared, which limits a sphere of the same area to at most about 440 m across and possibly 100 m. Proposed explanations include a shard of nitrogen ice, or water ice releasing hydrogen created by cosmic-ray damage; each has critics.
What we still don't know
Its true shape and composition. It was never resolved; every image of it is an artist's concept.
Missions
- 1990–
Hubble Space Telescope
NASA / ESAActiveUltraviolet to near-infrared imaging from low Earth orbit.
Related entries
- 2I/BorisovThe first interstellar comet
- 3I/ATLASThe third interstellar object, and the fastest
Sources
- Micheli et al. 2018, non-gravitational acceleration of ʻOumuamua, Nature (opens in a new tab)
- Trilling et al. 2018, Spitzer observations of ʻOumuamua, AJ (opens in a new tab)
- Bergner & Seligman 2023, acceleration from radiolytic H2, Nature (opens in a new tab)
- Jackson & Desch 2021, ʻOumuamua as an N2 ice fragment, JGR Planets (opens in a new tab)
- ESA/Hubble heic1813: Hubble sees ʻOumuamua getting a boost (opens in a new tab)
- NASA Science: ʻOumuamua (opens in a new tab)
- JPL Small-Body Database: 1I/ʻOumuamua (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.