At really high altitudes air density goes down quickly; this is good for planes but bad for airships because payloads are reduced. Plus, as you say, winds are really really strong: over 200 km/h. https://earth.nullschool.net/#current/wind/isobaric/250hPa/o...
I have seen many high altitude airship studies trying to hover around, but none have been viable in the end. To go around the world it seems better to just float, as in the comment above: https://news.ycombinator.com/item?id=46881679.
Fair points. I am aware that it is a UAV, and that there are some harsh regulations around the world (e.g. drones are completely banned in Morocco). I am also willing to accept my vast ignorance. I don't know exactly what regulations apply to an autonomous airship: balloon, UAV, recreational amateur aircraft? Actual laws for the handful of countries involved in the actual trajectory (Nicaragua, Thailand, Egypt perhaps) are not easy to find online. So I changed the first paragraph to:
"Finally, what are the legal implications of flying an airship drone over foreign air spaces? This very complex area clearly requires in-depth study and careful consideration, and it lies completely out of my depth. The primary objective is just to avoid being shot down since I'm not planning to visit any foreign countries in person, so allow me to just touch over the main issues."
I have seen many high altitude airship studies trying to hover around, but none have been viable in the end. To go around the world it seems better to just float, as in the comment above: https://news.ycombinator.com/item?id=46881679.