Focusing on aircraft, not balloons, I found this question that led me to some basic research on atmospheric satellites. I imagine the technology has evolved since NASA's research more than 10 years ago.
Given the following points:
- The aircraft cannot stay in flight forever, as it must land for maintenance (as would any aircraft).
- In 2001, NASA planned a 40h long trip based on solar powered technology.
- A fuel-powered UAV can stay up to 33h in flight.
- The Qinetiq Zephyr stayed aloft about 2 weeks.
- Redundancy can be added to continue operation in case of equipment failure (as in any commercial aircraft).
- 'Real' satellite can stay airborne for several years (payloads seem to be able to operate several years without maintenance)
- If the aircraft can land, the payload and equipment do not have to be as reliable as they would on 'real' satellites, as they could be changed or repaired.
For an atmospheric satellite based on solar-powered airplane technology such as the NASA's Helios, operated in normal conditions, what is the most restrictive element that make landing compulsory?