What were John Glenn's 'fireflies' on Mercury-Atlas 6?

“On February 20, 1962, John Glenn became the first American to orbit Earth aboard Friendship 7 during Mercury-Atlas 6. As the spacecraft passed into orbital sunrise, Glenn reported being surrounded by 'literally thousands of little particles that looked luminous… they looked like they were floating' — and famously named them 'fireflies.'”

On February 20, 1962, U.S. Marine Corps Colonel John Glenn became the first American to orbit Earth aboard the Friendship 7 capsule during Mercury-Atlas 6. As Friendship 7 passed into its first orbital sunrise, Glenn radioed Mission Control to report that the spacecraft was surrounded by 'literally thousands of little particles that looked luminous,' floating alongside the capsule and visible against the dark side of Earth. Glenn coined the term 'fireflies' for the phenomenon in real-time during the flight. Subsequent Mercury astronauts — Scott Carpenter (MA-7, May 1962), Wally Schirra (MA-8, October 1962), and Gordon Cooper (MA-9, May 1963) — reported identical phenomena in the four follow-on Mercury audio recordings declassified under PURSUE Release 02. NASA's later assessment attributes the phenomenon to frozen condensation separating from the spacecraft body and reflecting sunlight, producing the 'fireflies' appearance — the same explanation later applied to the white-green particles described by Carpenter as 'snowflakes.'