☄ Oumuamua 1I/Oumuamua

How to follow comet Oumuamua live

The panel above recomputes the position of Oumuamua every second in your browser: its distance from the Sun and from Earth, its position in the sky (right ascension and declination). It runs on the same kind of engine observatories use, a Kepler solver applied to the JPL osculating orbital elements, so the numbers are not a static snapshot, they keep ticking.

Just below, the top-down map of the Solar System shows exactly where Oumuamua is right now among the planets. You can fast-forward time with the day slider, zoom and pan, compare its distance to another body with a click, and press "Next event" to jump straight to perihelion. It is the most direct way to grasp the orbit of Oumuamua with no math at all.

Comet fact sheet

About Oumuamua

1I/Oumuamua was detected on 19 October 2017 by the Pan-STARRS1 system in Hawaii, more than a month after its perihelion on 9 September 2017. It had already passed closest to the Sun and was outbound when telescopes found it. The Hawaiian name means roughly "first messenger from afar." The hyperbolic orbit with an eccentricity of 1.2 immediately confirmed the object did not belong to the Solar System: it was not a stray asteroid, not a local comet, but literally another world in transit.

What made Oumuamua truly puzzling was the simultaneous combination of three characteristics: an extraordinarily elongated shape, a complete absence of any coma or cometary tail, and a non-gravitational acceleration that grew as it moved away from the Sun in the radial direction opposite to what common volatile outgassing would produce. Six years after discovery, with peer-reviewed papers in journals including Nature and The Astrophysical Journal, the mechanism behind that acceleration remains an open question.

History and discovery

Robert Weryk, a Canadian astronomer working with Pan-STARRS1 (Panoramic Survey Telescope and Rapid Response System) data at the University of Hawaii Institute for Astronomy, identified the object on 19 October 2017 while reviewing sky images. It was already 0.21 AU from Earth and receding at hyperbolic velocity when detected. Without the automated survey, it would have passed entirely unnoticed.

The first hours after discovery involved intense verification. The measured orbital eccentricity was 1.2; statistical uncertainty was small enough to rule out instrumental error. No object with such a hyperbolic orbit had ever been found in the entire history of astronomy. The Minor Planet Center published the discovery and within days dozens of observatories worldwide were pointing at it. The observation window was extremely short: by December 2017 Oumuamua had faded beyond detection.

The International Astronomical Union created the "I" (Interstellar) designation class specifically for this object, making it officially 1I/Oumuamua. It was the first time a Solar System object category was created based on a single discovery.

Orbit and physical nature

The eccentricity of 1.2 and entry velocity of about 26 km/s relative to the Sun confirmed interstellar origin. Back-tracing the trajectory places the origin direction vaguely toward the constellation Lyra, but without a match to any known nearby star. Oumuamua did not come directly from any identifiable stellar system and may have drifted through interstellar space for a long time before crossing our path.

The light curve showed brightness variation of 2.5 magnitudes over roughly 7.34 hours, indicating an extremely non-spherical shape. Initial estimates pointed to an axis ratio of at least 5:1 to 10:1, resembling a cigar. An alternative hypothesis proposed in 2020 suggests a flattened "pancake" geometry, which also reproduces the observed light curve and is more dynamically stable during interstellar flight.

What makes it unique

Three characteristics distinguish Oumuamua from every other Solar System object. First, a hyperbolic orbit with eccentricity significantly above 1, incompatible with any known internal gravitational perturbation. Second, an extremely elongated shape with no parallel among the millions of catalogued Solar System objects. Third, and most puzzling, the non-gravitational acceleration.

The non-gravitational acceleration was detected because the actual outbound trajectory deviated systematically and measurably from purely gravitational predictions. The deviation grew as the object moved away from the Sun in the radial direction, consistent with solar radiation pressure or outgassing of some volatile material. The problem is that no coma or tail was detected, and the acceleration magnitude exceeds what radiation pressure would produce on a rocky or icy object of normal density.

In 2023, a team from the University of Chicago and Berkeley published a hypothesis in Nature that gained considerable attention: molecular hydrogen (H2) trapped inside a water-ice matrix could be gradually released by cosmic radiation accumulated during interstellar travel and by solar heating. H2 would sublimate invisibly to conventional spectroscopy, explaining both the acceleration and the absence of a visible coma. The hypothesis is not unanimous but was well received as the most complete natural explanation available to date.

Observations and science produced

The active observation window lasted only 80 days, from October to December 2017. During that period, photometry, spectroscopy and high-precision astrometry data were collected by instruments including the Canada-France-Hawaii Telescope, the ESO Very Large Telescope, and the Hubble Space Telescope. Spectroscopy showed a neutral to reddish featureless spectrum, lacking the absorption signatures of hydrated minerals or water ice, consistent with a surface of organic carbon or silicate processed by cosmic radiation.

Hubble astrometric data published in 2018 provided the most precise measurement of the non-gravitational acceleration. The precision needed to detect the trajectory deviation was in the milli-arcsecond range, a notable technical achievement for such a faint object.

• Photometry: light curve with 2.5 magnitude variation, period of 7.34 hours.
• Spectroscopy: neutral to reddish featureless spectrum, no H2O or hydrated silicate absorption.
• Hubble astrometry: non-gravitational acceleration confirmed with high statistical precision.
• CO and CO2 emission upper limits: none detected, ruling out common volatile outgassing in sufficient quantity.
• Polarimetry: limited data, consistent with a radiation-processed surface.

Debates and subsequent discoveries

In 2018, Shmuel Bialy and Abraham Loeb of Harvard published a paper in the Astrophysical Journal Letters proposing that the acceleration could be explained by solar radiation pressure if Oumuamua were an extremely thin and light artificial solar sail. The paper was explicit in presenting this as a hypothesis to rule out, not a claim. Even so, it generated massive media coverage and polarised public discussion. The hypothesis is considered speculative by the large majority of astrophysicists, who prefer natural explanations even if incomplete.

Natural hypotheses that remained under active discussion include: (a) solid nitrogen iceberg flaked from the surface of an exo-Pluto (Desch and Jackson, 2021); (b) porous cometary dust fragment with geometry that amplifies radiation pressure (proposed in 2023); (c) H2 release from irradiated water ice (proposed in a Nature paper in 2023). No hypothesis has achieved consensus with available data.

In 2024, researchers identified a group of 14 Solar System asteroids with behaviour similar to Oumuamua, unexplained accelerations not attributable to gravity, provisionally classified as "dark comets." Oumuamua began to be considered potentially the first member of this class, which would shift its interpretation from a mysterious extrasolar visitor to a representative of a more common population of objects, though of still uncertain origin.

In July 2025, a third interstellar object, 3I/ATLAS, was discovered with a hyperbolic excess velocity of 58 km/s, surpassing both Borisov and Oumuamua. Its arrival rekindled interest in the entire series and demonstrated that the Solar System is visited frequently enough that modern surveys can detect these objects.

Trivia

• Oumuamua is currently more than 100 AU from the Sun (as of around 2026) and continues to recede at hyperbolic velocity. No existing or planned instrument can detect it any longer.
• The Vera Rubin Observatory (LSST), operational since 2025, was designed to detect interstellar objects days or weeks after entering the Solar System, well before perihelion, providing a much longer observation window than the few days that remained for Oumuamua.
• Project Lyra, proposed by i4is researchers, technically analysed the feasibility of sending a probe to Oumuamua. Calculations showed it would require launch velocities of tens of km/s, impossible with chemical propulsion, demanding concepts such as solar sails or nuclear propulsion to approach an object already dozens of AU away.
• The IAU created the "I" (Interstellar) designation letter in the Solar System body catalogue exclusively following the discovery of Oumuamua. Before that, no category existed for visitors of confirmed extrasolar origin.
• The name "Oumuamua" is pronounced approximately "oh-MOO-ah-MOO-ah" in Hawaiian. The official spelling includes the okina sign (inverted apostrophe before the O) marking a glottal consonant, frequently omitted in popular texts.
• Models published after Oumuamua's discovery suggest there may be up to 10,000 interstellar objects within Neptune's orbit at any given time, far more than previously assumed.

Other comets

See the full comet catalogue.