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?

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?

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?

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?