We will not know until the DoD provide the public report on this information. Everyone is speculating so I would wait for the official report.
A drone of some sort, something with an engine if they used a heat seeking missile.
"Heat seeking" is a bit of a misnomer. It uses thermal imaging to determine contrast against the background and then heads to the center of the detected object. No engine required. This is the same missile they used last week.
Didn't they use a heat seeking missile on the balloon in Carolina?
The US doesn't have any heat-seeking missiles. They use multi-spectral imagers that can see in the infrared spectrum among others.
Maybe it was a balloon but Kirby is being curmudgeonly and thinks it should be called an "Unmanned Buoyant Aircraft (UBA)" or something like that. These national security types love wordy terms and acronyms. Maybe he thinks calling a balloon a balloon makes it sound too trivial or something.
Medium Altitude Long Endurance UAV drone.
(based on further reporting it sounds a lot more balloon-like)