page_title: ʻOumuamua
section_title: Trajectory
hierarchical_section_title: ʻOumuamua / Observations / Trajectory
caption_attribution_description: English: en:1'Oumuamua sky trajectory, 2017, 3 day steps
context_page_description: ʻOumuamua is the first known interstellar object detected passing through the Solar System. Formally designated 1I/2017 U1, it was discovered by Robert Weryk using the Pan-STARRS telescope at Haleakala Observatory, Hawaii, on 19 October 2017, 40 days after it passed its closest point to the Sun on 9 September. When it was first observed, it was about 33 million km from Earth, and already heading away from the Sun.
  ʻOumuamua is a small object estimated to be between 100 and 1,000 metres long, with its width and thickness both estimated to range between 35 and 167 metres. It has a dark red colour, similar to objects in the outer Solar System. Despite its close approach to the Sun, ʻOumuamua showed no signs of having a coma, but did exhibit non-gravitational acceleration. Nonetheless, the object could be a remnant of a disintegrated rogue comet, according to a NASA scientist. The object has a rotation rate similar to the average spin rate seen in Solar System asteroids, but many valid models permit it to be more elongated than all but a few other natural bodies.
context_section_description: ʻOumuamua appears to have come from roughly the direction of Vega in the constellation Lyra. The incoming direction of motion of ʻOumuamua is 6° from the solar apex (the direction of the Sun's movement relative to local stars), which is the most likely direction for approaches from objects outside the Solar System. On 26 October, two precovery observations from the Catalina Sky Survey were found dated 14 and 17 October. A two-week observation arc had verified a strongly hyperbolic trajectory. It has a hyperbolic excess velocity (velocity at infinity, ) of 26.33 km/s (94,800 km/h; 58,900 mph), its speed relative to the Sun when in interstellar space.
  By mid-November, astronomers were certain that it was an interstellar object. Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed until 2I/Borisov was discovered in August 2019.  An eccentricity exceeding 1.0 means an object exceeds the Sun's escape velocity, is not bound to the Solar System and may escape to interstellar space. While an eccentricity slightly above 1.0 can be obtained by encounters with planets, as happened with the previous record holder, C/1980 E1, ʻOumuamua's eccentricity is so high that it could not have been obtained through an encounter with any of the planets in the Solar System. Even undiscovered planets in the Solar System, if any should exist, could not account for ʻOumuamua's trajectory nor boost its speed to the observed value. For these reasons, ʻOumuamua can only be of interstellar origin.
  ʻOumuamua entered the Solar System from north of the plane of the ecliptic. The pull of the Sun's gravity caused it to speed up until it reached its maximum speed of 87.71 km/s (315,800 km/h) as it passed south of the ecliptic on 6 September and made a sharp turn northward at its closest approach to the Sun (perihelion) on 9 September at a distance of 0.255 AU (23,700,000 mi; 38,100,000 km) from the Sun, i.e., about 17% closer than Mercury's closest approach to the Sun. The object is now heading away from the Sun towards Pegasus towards a vanishing point 66° from the direction of its approach. This implies that a hypothetical observer near the Sun facing towards ʻOumuamua will eventually rotate through 294 degrees, while the direction of motion of ʻOumuamua (relative to the Sun) will eventually have turned by 114 degrees.
  On the outward leg of its journey through the Solar System, ʻOumuamua passed within the orbit of Earth on 14 October at a distance of approximately 0.1616 AU (15,020,000 mi; 24,180,000 km) from Earth, and went back north of the ecliptic on 16 October and passed beyond the orbit of Mars on 1 November. It passed beyond Jupiter's orbit in May 2018, beyond Saturn's orbit in January 2019, and will pass beyond Neptune's orbit in 2022. As it leaves the Solar System it will be approximately right ascension 23h51m and declination +24°45', in Pegasus. It will continue to slow down until it reaches a speed of 26.33 km/s relative to the Sun, the same speed it had before its approach to the Solar System.
caption: Seen from Earth, the apparent trajectory makes annual retrograde loops in the sky, with its origin in Lyra, temporarily moving south of the ecliptic between 2 September and 22 October 2017, and moving northward again towards its destination in Pegasus.
exif: {}