Mariner 6 and 7
First dual Mars flyby mission, 1969.
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Mariner 6 and Mariner 7 were two uncrewed NASA spacecraft that completed the first dual mission to Mars in 1969 as part of the Mariner program. They flew over the equator and south polar regions, analyzing the atmosphere and surface with remote sensors and recording hundreds of pictures. The mission aimed to study Mars during close flybys to establish a basis for future investigations, particularly those relevant to the search for extraterrestrial life.
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, allowing helium pressure gas to escape from the booster's balloon skin.
The Atlas began to crumple, but two pad technicians, Bill McClure and Charles 'Jack' Beverlin, quickly 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 were 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 Atlas-Centaur AC-20; Mariner 7 lifted off from LC-36A on March 27, using Atlas-Centaur AC-19. Both boost phases went according to plan, though a minor LOX leak froze some telemetry probes in AC-20 and BECO occurred a few seconds early due to a faulty cutoff switch, with no serious effect on performance or flight path.
Spaceflight
In late July 1969, just days before its planned flyby, Mariner 7 lost communication with mission control at JPL. The team managed to reestablish contact using a backup low-gain antenna, and the primary high-gain antenna came back online after Mariner 6 completed its own encounter. Engineers suspected that gas leaking from a battery, which later failed, caused the problem. Using data from Mariner 6, controllers reprogrammed Mariner 7 while it was still in flight to target more interesting areas, and despite the battery issue, it ended up sending back more images than its twin.
Mariner 6 made its closest approach on July 31, 1969, passing 3,431 kilometers above the Martian surface. Mariner 7 followed on August 5, 1969, skimming just 3,430 kilometers up. Both probes are now inactive, drifting in orbits around the Sun. They happened to fly over heavily cratered terrain, missing the large northern volcanoes and the massive equatorial canyon that later missions would discover.
Their approach images captured about a fifth of the planet, revealing dark surface markings that Earth-based astronomers had once mistaken for canals. On August 4, Mariner 7 photographed the south polar cap, showing ice-filled craters and the cap’s outline; these images were sharper than those Mariner 6 had taken near the equator. In total, the two spacecraft returned 201 pictures, offering more detail than the earlier Mariner 4 mission. Their infrared radiometers revealed that Mars’s atmosphere is mostly carbon dioxide and detected small amounts of water on the surface.
Spacecraft and subsystems
The two Mariner 6 and 7 probes were built exactly alike. Their main body was an octagonal magnesium frame measuring 138.4 centimeters across and 45.7 centimeters deep. A cone-shaped structure sat on top, holding a one-meter-wide parabolic dish antenna and four solar panels, each 215 by 90 centimeters, attached at the frame’s upper corners. When fully extended, the panels spanned 5.79 meters from tip to tip.
A low-gain antenna stood on a 2.23-meter mast next to the dish. Beneath the frame, a two-axis platform carried the science gear, which weighed 57.6 kilograms total. The whole spacecraft stood 3.35 meters tall. For stability, it locked onto the Sun and the star Canopus using three gyroscopes, two sets of six nitrogen jets on the solar panel ends, a Canopus tracker, and two primary plus four secondary Sun sensors.
A 223-newton hydrazine rocket motor inside the frame provided propulsion, its nozzle protruding from one side with four jet vanes for steering. Power came from 17,472 solar cells covering 7.7 square meters, delivering 800 watts near Earth and 449 watts at Mars, while the craft needed only 380 watts once there. A 1200 watt-hour silver-zinc battery served as backup. Adjustable louvers on the main compartment’s sides managed temperature.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Mariner 6 and 7 (CC BY-SA 4.0).
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