The mysterious "fireflies" that John Glenn saw during his first trip into space were caused by frozen condensation on the skin of his spacecraft. As the capsule moved from light into dark, the condensation froze into tiny snowflakes, and these ice crystals sparkled in the sun when dislodged.
The mystery of the fireflies was solved during the next Mercury mission, which was flown by Scott Carpenter. When he noticed the same phenomenon, he theorized that they came from the spacecraft itself. To test his hypothesis, he struck the capsule bulkhead with his fist. The blow triggered a shower of icy sparkles and proved the origin of the mysterious fireflies.