Encyclopedia Astronautica
1969.05.21 - Apollo 10 lunar orbit insertion


At 4:49 p.m. EDT on May 21 the spacecraft was inserted into a lunar orbit of 110.4 by 315.5 kilometers. After two revolutions of tracking and ground updates, a maneuver circularized the orbit at 109.1 by 113.9 kilometers. Astronaut Cernan then entered the LM, checked all systems, and returned to the CM for the scheduled sleep period.

When Apollo 10 reached the lunar vicinity on 21 May, the controllers informed the crewmen that at one time or another more than a billion persons had watched their televised activities. But interest now focused on the exact moment when their craft would shoot around the moon and lose communications with the earth. At 74 hours 45 minutes into the mission, flight control predicted that loss of signal would come at 75 hours, 48 minutes, 24 seconds. The controllers had already determined that the ship would reach the moon 11 minutes later than scheduled, since there had been only one midcourse correction, rather than two. Its trajectory would be 110 kilometers above the lunar surface.

The crew was impressed by the lunar landscape, although Stafford insisted it looked like a big plaster of Paris cast. The three found it almost incredible that someone back on earth had been smart enough to place them within 110 kilometers of the moon - but there they were. They caught just a glimpse of the surface a minute before they fired the service module engine to go into lunar orbit, an activity that required all their attention. The six-minute retrograde maneuver seemed interminable, just as it had to Borman's group, but the engine kept firing and their confidence in it kept growing. When the engine finally shut down and they were sure that it had done its job, Stafford and Cernan had time to look at the lunar surface. They likened one area to a volcanic site in Arizona. Finally Stafford forced his attention back inside the cabin and told his crewmates that he thought the best thing to say when they got back in radio contact was, "Houston, tell the earth we have arrived."

Stafford, Young, and Cernan were fascinated by how much more slowly they seemed to travel around the moon than they had around the earth. They liked the slower pace, because on the first circuit they would pass directly over the area where Apollo 11 was due to land two months later. They had barely rounded the corner before Stafford and Cernan began describing the physical features down the highway they called "U.S. 1," leading to the landing site. By the third circuit, the world was sharing the view on color television. Watchers could see the gray, white, black, and brownish tints of the landing site, which seemed to be free from boulders, providing a smooth landing field.

Six hours after reaching the moon, Cernan and Stafford began getting the lander ready. The hatches, probe, and drogue were easily removed. As he entered the lunar module, Cernan was greeted by a snowstorm of mylar insulation, apparently sucked into the vehicle through a vent from the tunnel. The insulating material had come loose in the tunnel, and the crewmen had spent some time capturing and cleaning it up in the command module. Now they had the same job to do in the lunar module.

Cernan had floated head down through the tunnel into the lunar module. Because the two spacecraft were locked together from top to top, his own private world had a new orientation. He later commented that the best way to handle this psychologically was to slide through the hatch, look around, and then mentally assign an arbitrary up and down. Once he had accepted the new environment, he had no problems in checking, hauling in equipment, and getting things in order. The crew had intended to leave the passageway to the lander open after returning to the command ship, but the hardware was too bulky. It was simpler, and quite easy, to put the probe and drogue back into place.

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