A shower of stars and a false heat-shield alarm

During three orbits of the Earth at over 28,000 km/h, Glenn experienced a phenomenon he called “fireflies” — thousands of tiny glittering particles (later found to be ice crystals venting from the spacecraft) drifting past the capsule at sunrise.

During the flight Glenn had to deal with a faulty yaw thruster, which forced him to switch to manual control and burn more fuel than planned. A more serious concern came from Mission Control: a “Segment 51” sensor indicated the heat shield and landing bag might have come loose. Controllers were split on whether to jettison the retrorocket package before reentry or keep it as a safeguard — the more cautious option won, and the package stayed attached.

After the flight it turned out to be a faulty sensor — the heat shield had been fine the whole time. But at the moment of decision no one knew that, so Glenn faced reentry aware he might be flying with damaged protection against the heat of the atmosphere.