Mariner 6

Launch

Three Mariner probes were built for the mission, with two intended to fly and one as a spare. The spacecraft were shipped to Cape Canaveral with their Atlas-Centaur boosters between December 1968 and January 1969 for pre-launch testing. On February 14, during a simulated countdown on LC-36A with electrical power running but no propellant loaded, a malfunctioning electrical relay in the Atlas opened two pneumatic valves, releasing helium pressure gas from the booster's balloon skin.

The Atlas began to crumple, but pad technicians Bill McClure and Charles 'Jack' Beverlin activated a manual override switch to close the valves and pump helium back in. Although Mariner 6 and its Centaur stage were saved, the Atlas sustained structural damage and could not be reused. The spacecraft was removed from that booster and placed atop Mariner 7's launch vehicle on adjacent LC-36B, while a different Atlas was used for Mariner 7. Mariner 6 lifted off from LC-36B on February 25, 1969, using the Atlas-Centaur AC-20 rocket.

A minor LOX leak froze some telemetry probes, registering a drop in sustainer engine fuel pressure, but the engine performed normally. BECO occurred a few seconds early due to a faulty cutoff switch, causing a longer sustainer burn, but this did not seriously affect performance or flight path. AC-20 was launched at a 108-degree azimuth.

Spaceflight

In late July 1969, just days before its planned flyby, Mariner 7 lost contact with mission control at JPL. Engineers managed to reestablish communication using a backup low-gain antenna, and the high-gain antenna was restored shortly after Mariner 6 completed its own encounter. The problem was likely caused by gas leaking from a battery, which later failed entirely. Using data from Mariner 6, the team reprogrammed Mariner 7 mid-flight to target areas of interest, and despite the battery issue, it ended up sending back more images than its twin.

Mariner 6 made its closest pass on July 31, 1969, coming within 3,431 kilometers of the Martian surface. Mariner 7 followed on August 5, 1969, at a distance of 3,430 kilometers—less than half the altitude of the earlier Mariner 4 flyby. Both probes are now inactive and orbiting the Sun. By coincidence, they both flew over heavily cratered terrain, missing the large northern volcanoes and the equatorial canyon system discovered later.

Their approach images captured about 20 percent of the planet, revealing dark surface features that Earth-based astronomers had once mistaken for canals. On August 4, 1969, Mariner 7 passed over the south pole and returned pictures of ice-filled craters and the polar cap’s outline, which were sharper than Mariner 6’s equatorial shots. In total, the two spacecraft took and transmitted 201 photos, providing more detail than Mariner 4. They also studied the Martian atmosphere.

Because the flybys occurred just a week after Apollo 11, they received less media attention than usual. The engineering model of both spacecraft remains at JPL, on loan to LASP and displayed in its lobby. Their infrared radiometer data revealed that Mars’s atmosphere is mostly carbon dioxide and detected small amounts of water on the surface.

Spacecraft and subsystems

The Mariner 6 and 7 spacecraft were identical, with an octagonal magnesium frame base 138.4 cm diagonally and 45.7 cm deep. A conical superstructure held a 1 m diameter parabolic high-gain antenna and four solar panels, each 215 cm x 90 cm, affixed to the top corners.

Tip-to-tip span of deployed solar panels was 5.79 m. A low-gain omnidirectional antenna was mounted on a 2.23 m mast next to the high-gain antenna. Underneath was a two-axis scan platform holding scientific instruments, with total science instrument mass 57.6 kg.

Total spacecraft height was 3.35 m. Attitude stabilization was three-axis, referenced to the Sun and Canopus, using 3 gyros, 2 sets of 6 nitrogen jets on the solar panel ends, a Canopus tracker, and two primary and four secondary Sun sensors.

Propulsion was a 223-newton hydrazine mono-propellant rocket motor with 4-jet vane vector control. Power came from 17,472 photovoltaic cells covering 7.7 m², providing 800 watts near Earth and 449 watts near Mars, with a 1200 watt-hour silver-zinc backup battery. Thermal control used adjustable louvers on the main compartment sides.