The luminous particles reported by astronaut John Glenn during the Friendship 7 orbital flight in February 1962, which he described as bright specks drifting outside the capsule. Similar particles were seen on other Mercury flights.
The phenomenon was explained as frozen condensation, small flakes of ice shedding from the spacecraft and catching sunlight. It recurs in the NASA spaceflight transcripts in PURSUE as a worked example of a striking in-flight observation with a mundane, well-supported cause, useful as a baseline when reading astronaut debriefs.