ʻOumuamua (1I/2017 U1): The First Interstellar Visitor and the Alien Light Sail Debate

Pan-STARRS1 is a 1.8 m survey telescope at Haleakalā Observatory, near the summit of Haleakalā on Maui. Every night it searches the sky for near-Earth objects. No one saw ʻOumuamua with the naked eye. There was no craft, no landing and no beings. There was only a faint point of light, moving.

October 19, 2017. Rob Weryk, a postdoctoral researcher at the University of Hawaiʻi Institute for Astronomy, spotted the object and sent it to the Minor Planet Center. He then found it in images from the night before, which the automatic software had missed. "Its motion could not be explained using either a normal solar system asteroid or comet orbit," Weryk said. At NASA, Davide Farnocchia called it "the most extreme orbit I have ever seen."

The object had already passed the Sun on 9 September and Earth on 14 October. On 25 October the Minor Planet Center announced it as a comet. Later that day it was reclassified as an asteroid, because no coma could be seen. It was finally named 1I/2017 U1 ʻOumuamua, the first interstellar object. The Hawaiian name is glossed as "a messenger from afar arriving first."

Karen Meech led its characterisation. The dark reddish object's brightness changed by a factor of about 10 every 7.3 hours as it tumbled. Meech described it as "about ten times as long as it is wide." ESO put it at "at least 400 m long." NASA called it "completely inert, without the faintest hint of dust around it."

In June 2018 a team led by Marco Micheli of ESA found something more. The object was being pushed gently away from the Sun, beyond what gravity predicts. "We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets," Micheli said. The team concluded that it was "most likely an interstellar comet."

Avi Loeb, then chair of Harvard Astronomy, was not convinced. He and postdoc Shmuel Bialy asked whether sunlight could explain the push. The paper appeared in ApJ Letters on 12 November 2018. They calculated that a sheet 0.3 to 0.9 mm thick could do it, then went further. ʻOumuamua "may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization."

Loeb set out the case at length in his 2021 book Extraterrestrial. "It didn't look like a comet, yet it behaved some like something that has an extra push," he told CBS News. "We should be open minded and search for evidence rather than assume that everything we see in the sky must be rocks." The SETI Institute and Breakthrough Listen had already listened in. They detected no artificial signals.

Most astronomers favour a natural origin. In 2019 the ʻOumuamua ISSI Team found every observation "consistent with a purely natural origin." The ideas put forward include a hydrogen iceberg, a nitrogen ice fragment from an exo-Pluto, and hydrogen trapped in water ice by cosmic rays, an idea Bergner and Seligman published in 2023. Critics of the sail idea point out that a tumbling sail could not catch sunlight efficiently. Loeb has disputed several of the natural models.

ʻOumuamua is gone, heading out towards Pegasus. One early suggested name, "Rama," was a nod to Arthur C. Clarke's Rendezvous with Rama. The second interstellar object, 2I/Borisov, looked like an ordinary comet. In 2025 Loeb suggested that the third, 3I/ATLAS, could also be technology. The question ʻOumuamua raised is still being asked.