Glenn’s "fireflies": luminous particles accompany the Mercury flights
On 20 February 1962 John Glenn described luminous particles around his capsule and called them "fireflies". Carpenter, Schirra and Cooper saw the same on later flights. NASA attributed them to frozen condensation separating from the spacecraft.
Background
Where the word comes from
On 20 February 1962 John Glenn became the first American to orbit the Earth aboard Friendship 7. At sunrise he saw the capsule surrounded by countless small, brightly glowing particles. He called them "fireflies". The word stuck within the programme.
What the later flights saw
Mercury-Atlas 7 (24 May 1962): aboard Aurora 7, Scott Carpenter describes white particles moving at "random" that "look exactly like snowflakes". They are reflective, and some seem to move faster than the spacecraft.
Mercury-Atlas 8 (3 October 1962): aboard Sigma 7, Wally Schirra observes "little white objects that tend to come from the capsule itself and drift off" — later calling them "particles" and "lathe shavings". He also sees a burst of light in the window whose source he cannot identify.
Mercury-Atlas 9 (15 May 1963): about one hour and 41 minutes into the flight, Gordon Cooper reports from Faith 7 that he can see "John's fireflies" — explicitly invoking Glenn's term. A second excerpt catches him at sunrise describing small, brilliant white particles drifting away from the spacecraft.
The resolution
NASA later determined the "fireflies" to be frozen condensation separating from the body of the capsule. Their white, green-hued appearance results from sunlight reflecting off the ice.
A separate file — memoranda, correspondence and debriefing transcripts from 1962/63 — documents how seriously the scientific interest in Glenn's and Schirra's observations was taken at the time, including a circa-1955 theoretical analysis of meteoric particles entering the atmosphere.