The Sunrise mission is an adventure: Carried by a giant helium balloon, the unmanned observatory peers at the Sun from an altitude of more than 35 kilometers; several days of flight are followed by a parachute landing. Twice already, the delicate main mirror of Sunrise’s telescope has survived this daring expedition undamaged. “But such a flight does not leave the mirror completely unscathed,” explains Sunrise project manager Dr. Andreas Lagg from MPS. The quality of the outer layer of reflective aluminum is impaired; it must be renewed before every additional flight.
Source:
https://phys.org/news/2019-10-sunrise-telescope.html