It's a balloon. From Wall Street Journal:
The U.S. tracked what officials described as a Chinese reconnaissance balloon over the continental states this week, in what would be an aggressive act of intelligence gathering over sensitive American national security sites.
The balloon sighting came days before Secretary of State Antony Blinken was to make a planned trip to Beijing, according to U.S. officials, throwing into question efforts to repair relations between the two powers who are at odds over a host of global and regional issues. The balloon was first spotted on Wednesday by civilians in a commercial airliner, U.S. officials said.
Two Air Force F-22 fighters scrambled from Nellis Air Force Base in Nevada to Montana, where the balloon was observed, before the administration decided not to shoot it down.
The U.S. maintains an arsenal of 150 nuclear-armed Minuteman III intercontinental ballistic missiles at Malmstrom Air Force Base in Montana. A senior defense official said the U.S. government was taking steps to shield sensitive sites, but added that the reconnaissance systems on the balloon were presumed to have “limited additive value” beyond what the Chinese could gather from their low-earth-orbit satellites.