Mariner 7

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. During a simulated countdown on February 14, an electrical relay in the Atlas on LC-36A malfunctioned, opening two valves that released 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 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. Mariner 6 was then placed atop Mariner 7's launch vehicle on LC-36B, while a different Atlas was used for Mariner 7. Mariner 7 lifted off from LC-36A on March 27, 1969, using the Atlas-Centaur AC-19 rocket. The boost phase proceeded without serious anomalies.

Spaceflight

Just days before its closest approach to Mars, on July 29, 1969, the Jet Propulsion Laboratory lost contact with Mariner 7. The signal was recovered using a backup low-gain antenna, and the high-gain antenna came back online after Mariner 6’s flyby. Engineers suspected that gas leaking from a battery, which later failed, caused the problem. Using data from Mariner 6, Mariner 7 was reprogrammed mid-flight to target specific areas, ultimately returning more images than its twin despite the battery issue.

It made its closest pass on August 5, 1969, at 5:00:49 UT, coming within 3,430 kilometers of the Martian surface. The day before, on August 4, Mariner 7 flew over the south pole and sent back pictures of ice-filled craters and the polar cap’s outline. Despite its earlier communication trouble, these images were sharper than those from Mariner 6. The spacecraft transmitted 201 photos in total.

Both probes also studied the Martian atmosphere. Because the flyby occurred a week after Apollo 11, it received less media attention than usual. Infrared radiometer data revealed that Mars’s atmosphere is mostly carbon dioxide, with trace amounts of surface water detected.

Spacecraft and subsystems

The Mariner 7 spacecraft was identical to Mariner 6, consisting of 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, with a tip-to-tip span of 5.79 m. A low-gain omnidirectional antenna was mounted on a 2.23 m mast. Underneath was a two-axis scan platform holding scientific instruments with a total mass of 57.6 kg.

The spacecraft was three-axis attitude stabilized, referenced to the Sun and Canopus, using gyros, nitrogen jets, a Canopus tracker, and Sun sensors. Propulsion came from a 223-newton hydrazine rocket motor. Power was supplied by 17,472 photovoltaic cells covering 7.7 m², providing 449 watts near Mars, with a 1200 watt-hour silver-zinc backup battery. Thermal control used adjustable louvers on the main compartment sides.