 The only task left for the crew at this point was to perform the Trans @-@ Earth Injection ( TEI ) , which was scheduled for 2 ½ hours after the end of the television transmission . The TEI was the most critical burn of the flight , as any failure of the SPS to ignite would strand the crew in lunar orbit , with little hope of escape . As with the previous burn , the crew had to perform the maneuver above the far side of the Moon , out of contact with Earth . 
