An Etymological Dictionary of Astronomy and Astrophysics

The first known → interstellar object coming from beyond → Solar System. Detected on October 19, 2017 by the → Pan-STARRS sky survey, it was initially classified as an interstellar → asteroid.

‘Oumuamua has a strongly → hyperbolic orbit (→ eccentricity 1.191 ± 0.007), with a very high average velocity with respect to the Sun (v_∞ = 26.32 km s^-1), indicating that it likely originated from beyond our Solar System. Its → inclination with respect to the → ecliptic is 123°. ‘Oumuamua would be approximately 160 m in diameter.
This is the first known asteroid-like energetically unbound from the Sun. By comparison the → NASA Voyager 1 spacecraft travels about 17 km s^-1 through → interstellar medium.

When first detected, ‘Oumuamua was 0.2 → astronomical units (au) from the Earth (30 × 10⁶ km). Initially it was classified to be a → comet and was designated C/2017 U1. But further observations revealed no hint of → cometary activity. The object was therefore reclassified as an asteroid with designation A/2017 U1.

According to orbit calculations, ‘Oumuamua
crossed under the → ecliptic plane just inside of Mercury’s orbit and then went through → perihelion on September 9, 2017, at a distance of 0.25528 au (about 38 million km) from the Sun, speeding up to 87.71 km s^-1.

Pulled by the Sun’s gravity, the object made a U turn below the ecliptic, passing under Earth’s orbit on October 14 at a distance of about 24 million km, that is about 60 times the distance to the Moon. It has now risen above the plane of the planets and is travelling at 44 km s^-1 with respect to the Sun. In 2197 it will be 1,000 au from the Sun.

Recent → outgassing activity observed toward this object suggests that ‘Oumuamua is most likely an interstellar comet and not an asteroid (Michelli et al., 2018, Nature 27 June).

See also: The name ‘Oumuamua approved by the IAU was submitted by the team at the Pan-STARRS telescope who discovered the object. It is a Hawaiian construct combining ‘ou “to reach out” and mua “first” or “in advance of;” the second mua is for emphasis.
‘Oumuamua is loosely translated as “scout” or “messenger.” The 1I prefix indicates the interstellar nature of this first object.