A solar-powered high-altitude drone made by Titan Aerospace is intended to be able to remain in flight for about five years. When manufactured, it would have 3,000 solar panels producing about 7 kW of electricity and would be above the clouds, so it would be exposed to sunlight constantly during daylight hours. One is scheduled for completion next year.
The point of having such a plane would be to keep a payload in flight long-term, presumably in a manner similar to communications satellites. They could be used for surveillance applications such as environmental monitoring, fire monitoring and disaster response, among other things.
Using a long-term solar drone/plane also has the advantage of being a vessel that can be safely brought back down to ground level, so old payloads can be offloaded, or repaired. News ones can be put in place and then the plane can re-ascend to continue its solar-powered flight.
Another potential advantage is cost. Some satellites cost hundreds of millions of dollars. Just getting them up to the desired altitude can be extremely expensive. One they are there, they can be damaged or break down and they become space junk.
Launching satellites into space also produces rocket fuel emissions, which may impact climate change, “The Aerospace study has shown that black carbon particles emitted by hydrocarbon-fueled rockets could play a role in climate change in coming decades. Funded by the Aerospace Research and Program Development Office and other agencies, it is the first study of the effects that rocket exhaust could have on the climate system. Black-carbon particles produced by hydrocarbon-fueled rockets could be significant because rocket exhaust is the only direct source of human-produced compounds in the atmosphere above approximately 20 kilometers (12 miles). Rockets also emit carbon dioxide, water vapor, and other compounds that absorb thermal energy, but soot particles have possibly the greatest potential—on a kilogram-for-kilogram basis— to promote climate change. (Source: Aerospace.org)
In 2009, Japan launched a satellite to monitor greenhouse gases which probably caused the emission of some at the same time.