„Since the AIM-9L model, which made its debut in the late 1970s, Sidewinders have been “all-aspect" missiles, which means they can pick out the heat of the skin

„Since the AIM-9L model, which made its debut in the late 1970s, Sidewinders have been “all-aspect" missiles, which means they can pick out the heat of the skin of target against an ambient background instead of just hot exhaust parts of the engine“ [1] Apparently, against the cold background of the sky, a slightly warmed balloon is visible enough. [1]: https://www.quora.com/The-Chinese-spy-balloon-was-shot-with-a-Sidewinder-missile-a-missile-that-locks-on-to-the-heat-from-the-engines-of-aircraft-Are-high-altitude-balloons-really-hot-enough-for-that-Can-the-F-22-even-reach-high-altitude https://www.quora.com/The-Chinese-spy-balloon-was-shot-with-...