This question focus on aircrafts (no baloon). I found this question that led me to some basic research on so called atmospheric satellites. I imagine the technology evolved since NASA's research (more than 10 years old). Given the following points:

• The aircraft cannot stay forever in flight, as it should be landed for maintenance (as any aircraft).
• In 2001, NASA planned a 40h long trip based on solar powered technology.
• fulled-power UAV can stay up to 33h in flight
• The zephyr stayed about 2 weeks in flight
• redundancy can be added to continue operation in case of equipment failure (as in any commercial aircraft)
• real satellite can stay airborn for several years (payload seems to be able to operate several years without maintenance)
• if the aircraft can and, the payload and equipement may be less reliable as they can be changed easier than equipement on satellites

For an atmospheric satellite based on solar-powered airplane technology such as the NASA's Helios, operated in normal conditions, what is the most restricitive element that make landing compulsary?