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What caused John Glenn to see what he described as "fireflies"? I've heard three different explanations. If it's true that it was pee and sweat, could you explain how and why it was collected, and how it was expelled from the ship?

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Googling the subject produces results which all seem to quote each other: the fireflies were produced by condensation on the outer walls of the spacecraft. When Carpenter knocked on the wall, he could produce fireflies at will. The question really boils down (sorry) to what produced the condensation?

The Mercury capsules rejected heat to space from their cooling loops through what we called in shuttle a "flash evaporator". Water is sprayed over a heat exchanger, picks up heat from the cooling loop(s), and is then exhausted into space. This diagram shows the Mercury cooling loops including the evaporators (the exhaust is denoted as "overflow").

enter image description here

This diagram is from the wonderfully detailed "Project Mercury Familiarization Manual" but it contains not one word about liquid waste disposal. Solid waste is mentioned to be managed by astronaut diet!

The 1969 NASA publication "A review of Spacecraft Waste-Management Systems" describes the Mercury liquid waste disposal system:

During the first manned Mercury missions, spacecraft waste-management system requirements were nominal, primarily because of the limited duration of the flights. For subsequent Mercury flights of somewhat longer duration, a simplified waste-management system, which consisted of an in-suit urination bag, was the only requirement. However, on the extended flight of Mercury-Atlas 9 (MA-9), a more complex waste-management system was required. The system (fig. 1) consisted of two units: (1) the urination bag affixed to a quick disconnect and (2) a storage bag, a syringe-type pump, a hose assembly, and a quick disconnect.

The referenced diagram shows only a storage tank with no provision for dumps.

enter image description here

Based on all NASA publications I can find, it appears that there was in fact no provision for dumping urine from the Mercury spacecraft. To address sweat, note that the condensate tank has no provision for being dumped. Water was certainly emitted and it is plausible that it would condense on the spacecraft. I vote for condensed cooling water as the source of the fireflies.

You mention a third explanation - if I have not addressed that one, please edit your question and say what it is.

Also please note - the Gemini system did contain provision for dumping urine overboard. Hence the famous Schirra quote about the Constellation Urion. But your question asked about Glenn so I have limited my discussion to the Mercury capsule.

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    $\begingroup$ "Solid waste is mentioned to be managed by astronaut diet!" All but one of the Mercury missions lasted under 10 hours. Gordon Cooper's Mercury-Atlas 9 mission was a day and a half; he ate a high-protein, "low-residue" diet for three days prior. $\endgroup$ Jan 2, 2017 at 19:59
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    $\begingroup$ A sample menu is here: history.nasa.gov/MR-4/chap05.htm#tab5-I $\endgroup$ Jan 2, 2017 at 20:09
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    $\begingroup$ Shepard didn't even have a diaper, and due to delays on the pad he eventually had to relieve himself in his flight suit. Grissom had a makeshift collection bag. $\endgroup$ Jan 2, 2017 at 21:50
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    $\begingroup$ Back in the day (mid '80s) when I was flying A-10s (which featured slow cruise speeds for a fighter), we were counseled by our flight surgeons to ingest a "low-residue" diet starting several days before long (12 hours plus) ferry flights. IIRC, the recommended diet looked a lot like the one in the link provided above by @RussellBorogove $\endgroup$
    – Digger
    Jan 3, 2017 at 19:32
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    $\begingroup$ @Digger how far from the taxiway to the restroom at the far end of the flight? ;) $\endgroup$ Jan 3, 2017 at 21:34

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