NASA is set to release on Thursday the findings of a long-awaited study on unexplained flying objects in Earth's skies.
The US space agency announced last year it was reviewing evidence regarding unidentified anomalous phenomena, or UAPs -- which has replaced the term "UFO" in official parlance.
The subject has long fascinated the public but was shunned by mainstream science.
An independent team of 16 researchers shared their preliminary observations in May, finding that existing data and eyewitness reports are insufficient to draw firm conclusions, while calling for more systematic collection of high-quality data.
It's unlikely Thursday's report will change that bottom line -- but it could eventually usher in the start of a new mission for the agency.
While NASA's probes and rovers scour the solar system for any fossils of ancient microbes, and its astronomers look for signs of intelligent civilisations on distant planets, its historic posture has been to "debunk" sightings on our home planet.
There have been more than 800 "events" collected over 27 years, of which two to five per cent are thought to be possibly anomalous, the report's authors said during the May meeting.
These are defined as "anything that is not readily understandable by the operator or the sensor," or "something that is doing something weird," said team member Nadia Drake.
The US government has begun taking the issue of UAPs more seriously in recent years, in part due to concerns that they are related to foreign surveillance.
NASA's work, which relies on unclassified material, is separate from a Pentagon investigation, though the two are coordinating on matters of how to apply scientific tools and methods.
In July, a former US intelligence officer made headlines when he told a congressional committee he "absolutely" believes the government is in possession of unidentified anomalous phenomena -- as well as remains of their alien operators.
"My testimony is based on information I've been given by individuals with a longstanding track record of legitimacy and service to this country -- many of whom also shared compelling evidence in the form of photography, official documentation and classified oral testimony," David Grusch told lawmakers.
Earlier this week, the alleged bodies of two "non-human" beings were presented during a congressional hearing in Mexico, generating a mixture of surprise, disbelief and ridicule on social media.
The purported mummified remains, which had a grayish colour and a human-like body form, were brought by Jaime Maussan, a controversial Mexican journalist and researcher who reported finding them in Peru in 2017.