[Illustration: _Two assembly operations and system checkouts are performed separated by a trip to the pad to verify compatibility with the launch vehicle_] [Illustration: _A complete electronic checkout station in the hangar supports the spacecraft to ensure operability_] [Illustration: _Mariner takes form as the solar panels are attached and the final hangar checkout operations are performed before the launch._] [Illustration: _Wrapped in a dust cover, the spacecraft is transferred from Hangar AE at AMR to the explosive safe area for further tests._] [Illustration: _Inside the bunker-like explosive safe area, the powerful midcourse maneuver rocket engine is installed in the center of the spacecraft._] [Illustration: _Final assembly and inspection complete, Mariner is “canned” in the nose shroud that will protect it through the Earth’s atmosphere and into space._] [Illustration: _At the pad, the shrouded spacecraft is lifted past the Atlas ..._] [Illustration: _... and the Agena._] [Illustration: _Twelfth floor: Mariner reaches its mating level._] [Illustration: _The spacecraft is eased over to the top of the Agena ..._] [Illustration: _... and carefully mated to it._] The second launch attempt started at 6:37 p.m., August 26, with the Atlas-Agena B and Mariner II ready on the pad. At 9:52 p.m., T minus 100 minutes, a 40-minute hold was called to replace the Atlas main battery. By 10:37, with 95 minutes to launch, all spacecraft systems were ready to go. A routine hold at T minus 60 minutes was extended beyond 30 minutes in order to verify the spacecraft battery life expectation. At 11:48 p.m., with the count standing at T minus 55 minutes, the spacecraft, the vehicles, the Range, and the DSIF were all given the green light. When good launching weather was reported at 12:18 a.m., August 27, just 25 minutes from liftoff, a cautious optimism began to mount in the blockhouse and among the tired crews. But the tension began to build again. The second prescheduled hold at T minus 5 minutes was extended beyond a half-hour when the radio guidance system had difficulty with ground station power. Counting was “picked up” and the clock continued to move down to 60 seconds before liftoff. Suddenly, the radio guidance system was in trouble again. Fluctuations showed in its rate beacon signals, and another hold was called. Still another hold for the same reason followed at T minus 50 seconds. This time, at 1:30 a.m., the count was set back to T minus 5 minutes. One further crisis developed during this hold—only 3 minutes of pre-launch life remained in Atlas’ main battery. A quick decision was made to hold the switchover to missile power until T minus 60 seconds to help conserve the life of the battery.