John Glenn's Friendship 7 "Fireflies"

Tuesday, 20 February 1962. One man was aboard Mercury-Atlas 6. John Glenn, a Marine Corps lieutenant colonel and NASA astronaut, was flying Friendship 7 on the first American crewed orbital flight. He made three orbits at about 29,000 km/h and spent just under five hours in space.

About an hour and sixteen minutes in, the capsule was crossing the Pacific toward its first orbital sunrise. Glenn was on the radio to the tracking station on Canton Island. Then the light came up, and he saw that he was not alone. "I am in a big mass of some very small particles, that are brilliantly lit up like they're luminescent. I never saw anything like it."

They looked like little stars, a whole shower of them. They swirled around the capsule and passed in front of the window. "They are bright yellowish-green, about the size and intensity of a firefly on a real dark night," he reported. Thousands seemed to stretch for miles in every direction, moving by at perhaps 3 to 5 miles per hour, but he heard nothing strike the capsule. They came back at each sunrise and faded in full sunlight.

After the flight, Glenn described it this way. "I was riding slowly through them, and the sensation was like walking backward through a pasture where someone has waved a wand and made all the fireflies stop right where they were and glow steadily."

On the ground, the first guess was ice crystals from the capsule's hydrogen peroxide attitude-control thrusters. Glenn could not link the particles to his use of the rockets. Some controllers feared something worse. Were the specks pieces of the heat shield breaking away? On the same flight, a separate heat-shield telemetry alarm was already causing alarm.

NASA wanted a second look. On 24 May 1962, Scott Carpenter flew Aurora 7 under orders to look for the particles and photograph them.

"I have the fireflies," he radioed on the first orbit. At first, facing away from the sun, he missed them, then he yawed about 180 degrees. "Almost like a light snowflake particle caught in an eddy," he said. On the third orbit, he banged on the capsule wall and a fresh cloud broke loose. "They are little tiny white pieces of frost," he concluded, and later called them frostflies.

In the account that followed, moisture condensed on the outside of the spacecraft and froze during orbital night. When the sun returned, the frost flaked off and caught the light. The book Moon Shot said the water vapour came from the astronaut's own body. Wally Schirra blamed the water released by the heat exchanger that cooled the suits, and also urine. In 2012, Glenn himself named moisture from the heat exchanger.

Brian Dunning, host of Skeptoid, cited NASA SP-404 for a wider explanation: every spacecraft carries its own cloud of vented and shed particles. In the 1970s, experiments on Skylab measured such clouds with quartz-crystal microbalances.

No one has ever suggested that Glenn invented or exaggerated what he saw, and that is why the case matters. It is a solved case, one that shows how strange ordinary physics can look from orbit. The original air-to-ground transcript is held at the National Archives at Fort Worth. It still records the voice of a man who had never seen anything like it.