The first interstellar object detected passing through the Solar System.

‘Oumuamua (pronounced oh-MOO-ah-MOO-ah) is a highly elongated, cigar-shaped object that became a topic of significant interest in the field of astronomy when it was discovered on October 19, 2017, by astronomers using the Pan-STARRS1 telescope in Hawaii[1].

The object’s name, ‘Oumuamua, is Hawaiian for “scout” or “messenger from afar arriving first.” ‘Oumuamua is estimated to be about 100 meters long and only about 10 meters wide, giving it a distinct elongated shape (it could fit comfortably inside a football field). It has a red color, similar to objects in the outer Solar System. It was moving through the solar system at a remarkable speed,

reaching a peak velocity of about 315,000 kilometers per hour (196,000 miles per hour) as it passed through the inner solar system. Its trajectory indicated that it came from outside the solar system, entering the solar system from above and passing closest to the Sun on September 9, 2017. ‘Oumuamua’s light curve showed that it was rotating rapidly, with a period of about 7.3 hours. This rapid rotation contributed to its unusual brightness variations.

 It has exhibited non‑gravitational acceleration, potentially due to outgassing[2] or a push from solar radiation pressure[3].

On March 22, 2023, astronomers proposed the observed acceleration was “due to the release of entrapped molecular hydrogen that formed through energetic processing of an H2O-rich icy body”, consistent with ‘Oumuamua being an interstellar comet, “originating as a planetesimal relic broadly similar to solar system comets.”

Controversy and Speculation
  • Cometary Activity: Initially, ‘Oumuamua was thought to be a comet due to its trajectory and high speed. However, no cometary activity was observed, such as the development of a visible coma or tail when it approached the Sun.
  • Asteroid: Subsequent observations and analysis suggested that ‘Oumuamua might be more like an asteroid, lacking the typical signs of cometary activity.
  • Alien Probe: One of the most controversial speculations was that ‘Oumuamua could be an artificial object sent by an extraterrestrial civilization. This idea garnered significant media attention but lacked substantial evidence and was largely dismissed by the scientific community.
  • Hydrogen Iceberg: Another proposal suggested that ‘Oumuamua could be a hydrogen iceberg, which might explain its lack of observable cometary features. However, this hypothesis also faced challenges in explaining its trajectory and behavior.

For decades we’ve theorized that such interstellar objects are out there, and now―for the first time―we have direct evidence they exist

Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington

Observations and conclusions concerning ʻOumuamua’s trajectory were primarily obtained with data from the Pan-STARRS1 Telescope, part of the Spaceguard Survey, and Canada–France–Hawaii Telescope (CFHT), and its composition and shape from the Very Large Telescope and the Gemini South telescope in Chile, and the Keck II telescope in Hawaii. These were collected by Karen J. Meech, Robert Weryk, and their colleagues and published in Nature on November 20, 2017. After the announcement, the space-based telescopes Hubble and Spitzer joined in the observations. The SETI Institute’s radio telescope, the Allen Telescope Array, examined ʻOumuamua but detected no unusual radio emissions.

By the time ‘Oumuamua was detected, it was already on its way out of the solar system. Given the object’s incredible speed and steep angle relative to the rest of the system, ‘Oumuamua was not gravitationally bound to the sun. 

So what is ‘Oumuamua? The debate continues; speculation is ripe in such a data-poor environment. Harvard astronomer Avi Loeb has used the lack of data to propose that ‘Oumuamua is an alien spaceship. But considering intelligent aliens may be capable of doing whatever they want, this could be an explanation for pretty much any observation. Ultimately, we’ll never know. And maybe, like that strange visitor, we should just move on.

The Challenges
  • It was discovered after it had already passed its closest point to the Sun, limiting the time available for observations.
  • Its rapid speed made it difficult to gather detailed data, and its small size made it faint and challenging to observe.
  • The limited data available led to uncertainties in determining its exact nature and origin.

  1. The Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) is a wide-field astronomical observatory located on the summit of Haleakalā in Hawaii. It is equipped with a 1.8-meter diameter telescope and a 1.4-gigapixel camera, which collectively allow it to conduct deep and comprehensive surveys of the night sky. Pan-STARRS1’s primary mission includes mapping the entire visible sky multiple times, enabling the discovery and tracking of various astronomical objects, including near-Earth objects, asteroids, comets, and transient phenomena like supernovae. The discovery of ‘Oumuamua, the first known interstellar object to pass through the solar system, was made possible by Pan-STARRS1’s capabilities. [Back]
  2. Outgassing refers to the release of volatile substances, such as gases or vapors, from the surface of a solid or liquid material. This process typically occurs when the material is exposed to a decrease in pressure, increase in temperature, or a combination of both, causing the trapped gases within the material to escape. Outgassing is commonly observed in various contexts, including space exploration, where spacecraft and equipment can release gases when exposed to the vacuum of space. It is also relevant to comets and certain types of asteroids, where the sublimation of ice (such as water, carbon dioxide, or methane) due to solar heating results in the emission of gas and the formation of a coma and tail. [Back]
  3. Solar radiation pressure is the force exerted by the photons (particles of light) from the Sun as they collide with and are absorbed or reflected by objects in space. This pressure is responsible for exerting a gentle force on objects, such as spacecraft or celestial bodies, causing them to accelerate away from the Sun. The pressure is higher for objects with larger surface areas and lower for objects with high reflectivity. Solar radiation pressure plays a crucial role in the dynamics of small objects in space, including asteroids and comets, affecting their trajectories and motions. In the context of ‘Oumuamua, solar radiation pressure was considered as a potential explanation for its peculiar acceleration. [Back]

Further Reading


Author: Doyle

I was born in Atlanta, moved to Alpharetta at 4, lived there for 53 years and moved to Decatur in 2016. I've worked at such places as Richway, North Fulton Medical Center, Management Science America (Computer Tech/Project Manager) and Stacy's Compounding Pharmacy (Pharmacy Tech).

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