By Joey Roulette and Steve Gorman

WASHINGTON (Reuters) -Members of an independent NASA panel studying UFOs, or what the U.S. government now terms UAP for "unidentified anomalous phenomena," said in their first public meeting on Wednesday that scant high-quality data and a lingering stigma pose the greatest barriers to unraveling such mysteries.

The 16-member body, formed last year among leading experts from scientific fields ranging from physics to astrobiology, held a four-hour session streamed live on a NASA webcast to deliberate their preliminary findings ahead of issuing a report expected later this summer.

The panel's chairman, astrophysicist David Spergel, said his team's role was "not to resolve the nature of these events," but rather to give NASA a "roadmap" to guide future analysis.

NASA officials said several panelists had been subjected to unspecified "online abuse" and harassment since beginning their work in June last year.

"It is really disheartening to hear of the harassment that our panelists have faced online because they're studying this topic," NASA's science chief, Nicola Fox, said in her opening remarks. "Harassment only leads to further stigmatization."

The greatest challenge panel members cited, however, was a dearth of scientifically reliable methods for documenting UFOs, typically sightings of what appear as objects moving in ways that defy the bounds of known technologies and laws of nature.

The underlying problem, they said, is that the phenomena in question are generally being detected and recorded with cameras, sensors and other equipment not designed or calibrated to accurately observe and measure such peculiarities.

"If I were to summarize in one line what I feel we've learned, it's we need high-quality data," Spergel said. "The current existing data and eyewitness reports alone are insufficient to provide conclusive evidence about the nature and origin of every UAP event."

Taboos surrounding the issue also remain.

While the Pentagon in recent years has encouraged military aviators to document UAP events, many commercial pilots remain "very reluctant to report" them due to the lingering stigma surrounding such sightings, Spergel said.

The NASA advisory panel represents the first UFO inquiry ever conducted under the auspices of the U.S. space agency for a subject the government once consigned to the exclusive and secretive purview of military and national security officials.

PENTAGON-BASED INVESTIGATION

The NASA study is separate from a newly formalized Pentagon-based investigation of sightings reported in recent years by military aviators and analyzed by U.S. defense and intelligence officials.

The U.S. military has documented more than 800 cases over the past two decades, said Sean Kirkpatrick, director of the Pentagon's newly formed All-domain Anomaly Resolution Office, or AARO.

But just a few percent are considered beyond relatively simple explanation, while the rest can be attributed to mundane origins such as aircraft, balloons, debris or atmospheric causes, he said.

The parallel NASA and Pentagon efforts highlight a turning point for the government after decades spent deflecting, debunking and discrediting reports of unidentified flying objects, or UFOs, dating back to the 1940s.

But in finally addressing the issue head-on, both NASA and the Pentagon have emphasized the imperative of protecting U.S. airspace, and by extension public safety and natural security.

In a departure from the Pentagon, NASA's panel is examining only unclassified reports from civilian observers, an approach Spergel said permits open sharing of information among scientific, commercial and international entities, as well as the public.

The term UFOs, long widely associated with notions of flying saucers and aliens, has been replaced in official government parlance by the abbreviation UAP.

Recent U.S. law revised the UAP acronym, previously confined to "aerial" phenomena, to stand for "unidentified anomalous phenomena," expanding the NASA study team's research scope to include puzzling events in space or at sea.

Panel members said the majority of their work still focused on aerial phenomena.

Moreover, both NASA and defense-intelligence officials have stressed that while the existence of intelligent alien life has not been ruled out, they have found no evidence suggesting an extraterrestrial origin for UFO sightings.

"To make the claim that we see something that is evidence of non-human intelligence would require extraordinary evidence, and we have not seen that," Spergel said.

(Reporting by Joey Roulette in Washington and Steve Gorman in Los Angeles; Editing by Robert Birsel, Bill Berkrot and Himani Sarkar)