A Harvard astrophysicist behind the theory that a mysterious object speeding through the solar system could actually be a lost Soviet spacecraft is pushing back on NASA's top official, saying the object is "not an asteroid."Known as 1998 KY26, the roughly 36-foot-wide object is believed to be a near-Earth asteroid but is accelerating in a way gravity alone cannot explain. Harvard astrophysicist Avi Loeb and his colleagues have examined whether it could instead be Phobos 1, a Soviet spacecraft that stopped communicating while traveling to Mars in 1988.NASA Administrator Jared Isaacman weighed in on the mysterious object Thursday during an appearance on "Fox & Friends."EIFFEL TOWER-SIZED ASTEROID APOPHIS TO PASS CLOSER TO EARTH THAN MANY SATELLITES IN 2029, NASA SAYS"I've not seen any evidence to suggest this is...anything other than an asteroid," Isaacman said.Loeb challenged that assessment during an interview with Fox News Digital on Friday.HARVARD ASTRONOMER TAPPED TO LEAD WHITE HOUSE UFO COUNCIL SAYS US GOVERNMENT 'BAFFLED BY WHAT THEY ARE SEEING'"Well, first of all, it's not an asteroid because 1998 KY26 appears to have non-gravitational acceleration, meaning it's being pushed by some force other than gravity, and asteroids are just rocks," Loeb said.Scientists have been trying to explain what is causing the additional acceleration.SCIENTISTS DISCOVER POTENTIALLY MASSIVE DWARF PLANET CANDIDATE HIDING BEYOND PLUTOOne theory has been that KY26 is a "dark comet," a rocky object behaving like a comet without producing a visible cloud or tail.New research from Georgia Tech has cast doubt on whether KY26 could retain enough ice to produce that effect, while other research has suggested pressure from sunlight and heat radiating from its surface could account for the acceleration.GIANT ROBOTIC CLAWS WILL GRAB DEAD SATELLITES IN SPACELoeb told Fox News Digital that his team found KY26's trajectory closely resembles that of the failed Soviet mission and calculated that sunlight reflecting off Phobos 1 could produce a push consistent with the nongravitational acceleration observed in KY26.He also pointed to recent observations indicating the object is highly reflective and rotates about once every five minutes.NASA'S CHANDRA TELESCOPE REVEALS MILKY WAY'S OUTER REACHES MAY STRETCH FARTHER THAN PREVIOUSLY KNOWNThe lack of a visible cometary tail or evidence of material escaping the object is central to Loeb's skepticism of the dark-comet explanation."It could be a comet shedding some gas backward that is pushing it forward, but in that case we should have seen the gas," Loeb said. "We should have seen the dust because for comets we see a tail and the dust particles are being pushed away from the sun by the solar radiation.""We don't see a cometary tail. We don't see any evidence for outgassing around this object. That's the puzzle," he added.Loeb compared the debate over how to classify KY26 to encountering an unfamiliar animal at a zoo.FOLLOW THE FOX TRUE CRIME TEAM ON X"If you are an expert on zebras and you see an elephant, you would say the elephant is a zebra without stripes," Loeb said. "But that's an oxymoron because it's a completely different animal."He also argued that scientists should be willing to consider explanations outside the familiar categories of asteroids and comets when observations do not neatly fit either one.THE NEXT FRONTIER: WASHINGTON GRAPPLES WITH ITS LATEST SPACE ODDITY"Innovation in science requires some imagination," Loeb said. "It requires you to allow for the possibility that our knowledge is incomplete."SIGN UP TO GET THE LATEST TRUE CRIME NEWSFox News Digital reached out to NASA for a response to Loeb's comments and asked whether Isaacman stands by his assessment of KY26.CLICK HERE FOR MORE FROM THE FOX TRUE CRIME HUBLoeb acknowledged that his team's theory remains unproven, but said a definitive answer could come in 2031, when the Japan Aerospace Exploration Agency's Hayabusa2 spacecraft is expected to rendezvous with KY26.SEND US A TIPLoeb predicts the encounter could show that scientists have been looking at a piece of Soviet space history rather than an unusual natural object.GOT A TIP?"We predict that when they land on it, instead of seeing an iceberg, they will find the label ‘Made in the USSR,’" Loeb said.LISTEN TO THE NEW 'CRIME & JUSTICE WITH DONNA ROTUNNO' PODCASTLoeb also acknowledged that the 2031 encounter could settle the question once and for all.GET BREAKING NEWS BY EMAILAsked whether the Phobos 1 theory would be completely over if Hayabusa2 reaches KY26 and finds a rock or piece of ice, Loeb didn't hesitate."Yes," he said.