The “Red Rover” spacewalk and Spider flying alone

On the fourth day of the flight, Schweickart was to make his way along the spacecraft’s exterior from the lunar module to the command module — a rehearsal for a lunar emergency. The day before, however, he had vomited twice from space sickness, and McDivitt shortened the planned excursion to a safer version confined to Spider’s hatch.

Even so, Schweickart became the first in the Apollo program to test the PLSS backpack (Portable Life Support System) outside in open space — the unit that would later supply astronauts with oxygen on the lunar surface. During the excursion he used the call sign “Red Rover,” a nod to his red hair. Scott, meanwhile, performed his own simpler excursion standing in the command module’s hatch.

The next day brought the key test of the whole mission: undocking Gumdrop and Spider. For the first time in history, humans flew in a craft that could not by itself return them to Earth. McDivitt maneuvered Spider independently, fired the descent engine repeatedly at different throttle settings, then redocked with Gumdrop — glare from the Sun forced Scott to help guide the final approach. Mission Control then fired the now-empty Spider’s engine remotely to fuel depletion, testing behaviour that would mirror an ascent stage launching from the Moon.