A "bright glow" was observed over Kyiv around 10:00 pm (1900 GMT), the head of Kyiv's military administration Sergiy Popko wrote on Telegram.
An air raid alert was activated, Popko said, but "air defence was not in operation" in the besieged country fighting a Russian invasion.
"According to preliminary information, this phenomenon was the result of a NASA space satellite falling to Earth," Popko said.
But a NASA spokesman denied this assessment, telling AFP that the satellite in question was "still in orbit."
The US space agency had announced this week that the retired 660-pound (300-kilogram) Reuven Ramaty High Energy Solar Spectroscopic Imager spacecraft (RHESSI) would reenter the atmosphere on Wednesday.
"However, that reentry has not yet occurred -- RHESSI is still in orbit. NASA and the Department of Defense continue to track RHESSI. No other NASA satellite reentered the atmosphere earlier today," a NASA spokesman told AFP.
The RHESSI spacecraft, used to observe solar flares, was launched into low Earth orbit in 2002 and decommissioned in 2018, NASA said.
The Ukrainian Air Force also said the flash was "related to the fall of a satellite/meteorite."
Speculation and memes abounded on Ukrainian social media after videos posted to several channels showed a powerful flash lighting up the sky over Kyiv.
"While social media is amused by flying saucer memes... please do not use the official symbol of the Air Force to create memes!" the Ukrainian Air Force said.
In a statement on Monday, NASA said it expected most of the RHESSI spacecraft to burn up as it enters the atmosphere.
"But some components are expected to survive reentry," NASA said, adding that the risk of harm to anyone on Earth was low -- approximately one in 2,467.
Decommissioned NASA satellite to re-enter Earth's atmosphere
Washington DC (UPI) Apr 19, 2021 - NASA expects the decommissioned Reuven Ramaty High Energy Solar Spectroscopic Imager spacecraft to re-enter Earth's atmosphere on Wednesday after 21 years in orbit.
NASA said Defense Department projections estimate the spacecraft, abbreviated as RHESSI, will re-enter the atmosphere at approximately 9:30 p.m. EDT on Wednesday but expects there to be an uncertainty window of plus and minus 16 hours.
While most of the spacecraft will burn up due to the intense heat created by friction with the atmosphere, NASA expects some parts to survive re-entry but says the chance of the debris harming people on the ground is low, approximately one in 2,467.
RHESSI was launched in 2002 aboard an Orbital Sciences Corporation Pegasus XL rocket and was decommissioned in 2018.
The spacecraft used an imaging spectrometer to observe flares and corona emanating from the sun. The data helped NASA researchers understand how the sun emits massive bursts of energy.
"During its mission tenure, RHESSI recorded more than 100,000 X-ray events, allowing scientists to study the energetic particles in solar flares," NASA said.
The imaging spectrometer produced the first-ever X-ray and Gamma-ray images of solar flares.
"The imager helped researchers determine the particle's frequency, location, and movement, which helped them understand where the particles were being accelerated," NASA added.
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