The mesosphere – the atmospheric layer 50 to 80km above the ground – is out of bounds to normal aircraft, but millimetre-sized devices could fly there powered by sunlight
Source:
https://www.newscientist.com/article/2267723-sunlight-could-power-micro-aircraft-flying-above-the-stratosphere/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home