What critical product flaw was discovered in the Hubble Space Telescope shortly after its launch in 1990 that required a major repair mission?

The Hubble Space Telescope represents one of the most significant product development stories in astronomy. Launched in 1990, it encountered a serious flaw that required an unprecedented repair mission. Since then, it has undergone multiple servicing missions to upgrade its capabilities. This poll tests your knowledge about the product development challenges and innovations behind this iconic astronomical instrument that revolutionized our view of the universe.

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Spherical aberration in the primary mirror
Faulty solar panel deployment mechanism
Overheating in the main imaging sensors
Communication system encryption failure