In The Right Stuff, John Glenn is reported to have seen his capsule surrounded by tiny sparkling lights. The movie depicts Glenn as having a very strong and loud reaction to these. Searching Google, I’ve seen them accounted for as everything from Glenn’s imagination to ice crystals from his urine.
Does anybody have the Straight Dope on this?
After reading 5 well-written sites, a few of which were from NASA?, you can only conclude that it was waste water from the ship.
This site summarizes it well while shooting down some hoax proponents at the same time.
I’m fairly certain NASA confirmed the phenomena as ice crystals flaking off the capsule.
In Tom Wolfe’s “The Right Stuff” it is stated that Scott Carpenter confirmed that the “fireflies” emanated from the surface of the capsule. During his multiple orbit flight he could make a cloud of them appear by banging on the hatch.
I’ve heard reports of some sort of radiation (cosmic rays?), against which we on earth are largely protected causing Cherenkov actually within the aqueous humour of the astronauts’ eyes, this being visible as flashes or streaks of light that kept them awake, but it looks as though this has little connection with the OP and of course they would be visible all the time, not just when you looked out of the window.