lifted off from LC-36A on March 27, using the Atlas-Centaur AC-19 rocket. The boost phase for both spacecraft went according to plan and no serious anomalies occurred with either launch vehicle. A minor LOX leak froze some telemetry probes in AC-20 which registered as a drop in sustainer engine fuel pressure; however, the engine performed normally through powered flight. In addition, occurred a few seconds early due to a faulty cutoff switch, resulting in longer than intended burn time of the sustainer engine and Centaur, but this had no serious effect on vehicle performance or the flight path. AC-20 was launched at a 108-degree azimuth. The Centaur stage on both flights was set up to perform a retrorocket maneuver after capsule separation. This served two purposes, firstly to prevent venting propellant from the spent Centaur from contacting the probe, secondly to put the vehicle on a trajectory that would send it into solar orbit and not impact the Martian surface, potentially contaminating the planet with Earth microbes. Spaceflight On July 29, 1969, less than a week before closest approach, Jet Propulsion Laboratory (JPL) lost contact with Mariner 7. The center regained the signal via the backup low-gain antenna and regained use of the high gain antenna again shortly after Mariner 6's close encounter. Leaking gases from a battery (which later failed) were thought to have caused the anomaly. Based on the observations that Mariner 6 made, Mariner 7 was reprogrammed in flight to take further observations of areas of interest and actually returned more pictures than Mariner 6, despite the battery's failure. Closest approach for Mariner 6 occurred July 31, 1969, at 05:19:07 UT at a distance of above the martian surface. Closest approach for Mariner 7 occurred August 5, 1969 at 05:00:49 UT at a distance of above the martian surface. This was less than half of the distance used by Mariner 4 on the previous US Mars flyby mission. Both spacecraft are now defunct and in heliocentric orbits. Science data and findings By chance, both spacecraft flew over cratered regions and missed both the giant northern volcanoes and the equatorial grand canyon discovered later. Their approach pictures did, however, photograph about 20 percent of the planet's surface, showing the dark features long seen from Earth – in the past, these features had been mistaken for canals by some ground-based astronomers. When Mariner 7 flew over the martian south