A former American intelligence agent made explosive claims that the US government has covered up knowledge of UFOs.
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A whistleblower told US Congress hearings the government had possession of "intact and partially intact" alien vehicles and "non-human bodies". The Pentagon has denied the claims.
Key witnesses shared encounters with strange objects in the sky and said the government had conducted a "multi-decade" program of collecting and studying crashed UFOs.
Cosmic questions
The story generated enormous engagement globally with interest in UFOs surging in recent years.
University of Melbourne astrophysicist Michele Trenti said these stories resonate with people on an existential level.
"It's mentally connected to one of the most fundamental questions of humanity, understanding our cosmic origins. Where do we come from? Are we alone in the universe?," he said.
"I think it's very natural that that people are super interested and excited about UFOs and aliens."
However, professor Trenti said within current understandings of physics it is highly unlikely something of a large size and weight could travel from distant stars.
"Going to the closest star at the speed of light would take more than four years," he said.
"The distances involved and the laws of physics and specifically the fact that there is a maximum speed at which anything can travel, which is the speed of light, makes travel between stars very difficult."
But that did not mean we are alone in the universe.
"There are 100 billion stars in our own galaxy and there are about 100 billion galaxies in the universe with a similar number of stars. It would be very unusual if we're the only one where the conditions were right for life to thrive," he said.
Weather balloons, drones, satellites
Astronomy Australia programs director James Murray said UFO sightings generally have a simple explanation.
"I get phone calls and emails every couple of weeks from people seeing things in the sky, and they think because they're doing weird things they could be aliens, but they're often Elon Musk's Starlink satellites," he said.
"The other classic one is weather balloons and people flying drones at night."
In early 2023 the US government shot down a Chinese spy balloon which travelled over Alaska, Canada and mainland US states.
National security
Dr Murray said a key reason governments may not disclose all known information about UFOs is for reasons of security.
"When they get a report of a UFO sighting and deploy whatever equipment that they have to try and work out what's going on, they won't necessarily want foreign powers to know what it was that they deployed," he said.
Shrouding UFO research in secrecy could spawn conspiracies, according to professor Trenti.
"I think this is understandable in the context of protecting national security, but this is contributing to a bit of mystery and that can very easily give rise to conspiracy theories," he said.
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