To pull it off, Apollo 8 needed to accelerate from its current speed of about 17,400 miles per hour to nearly 24,250 miles per hour. That boost would be accomplished by the single J-2 engine on the Saturn’s third stage, which would be reignited and burned for nearly six minutes. Doing this would not, as many believed, cause the spacecraft to leave Earth orbit; rather, it would simply change the shape of Apollo 8’s orbit around Earth from a near circle into a highly elongated ellipse, one that would stretch all the way from Earth to the Moon and back.
