Credit: Russian Academy of Sciences/Don P. Mitchell
The 1973 launch window was dominated solely by the Soviet Union, which launched four spacecraft, Mars 4 through 7. NASA, at this time, was busy preparing for the Viking missions, which lifted off in the next launch window, in 1975.
The four Mars spacecraft was the Soviet Union's political attempt to steal the United State's thunder by soft-landing two landers onto Mars two years prior to Viking. To accomplish these plans, the four Mars spacecraft each carried separate pieces needed for the whole mission. Mars 4 and 5 were designed to orbit Mars and return information on the composition, structure, and properties of the Martian atmosphere and surface. The spacecraft were also designed to act as communications links for the Mars 6 and 7 landers. These latter two landers were carried by orbiter-style buses that didn't carry enough fuel to enter orbit. This split-up between the four spacecraft was done because the 1973 Mars launch window was inefficient, and the Proton rockets couldn't deliver the same amount of mass to Mars as they were able to do in 1971.
Mars 4 was launched on July 21, 1973, and arrived at Mars on February 10, 1974. Due to a flaw in a computer chip, resulting in degradation of the chip during the voyage to Mars, the retro-rockets designed to slow the craft into Mars orbit didn't fire, and Mars 4 flew by the planet at a range of 2200 km. It returned one swath of pictures and some radio occultation data, which constituted the first detection of the nightside ionosphere on Mars. It continued to return interplanetary data from solar orbit after the flyby.
Mars 5 was launched four days later, on July 25th, and arrived on February 12th. The spacecraft was able to achieve an elliptical orbit that took 24 hours, 53 minutes to complete, with an inclination of 35.3°. Nearly synchronized with the rotation of the planet, its two photo-television cameras could take up to 12 pictures during each close approach. The "Vega" camera used a wide area 52mm lens with color filters; the "Zulfar" camera used a telescopic 350mm lens and a long-pass orange filter. Images were transmitted in a rapid 220-line mode, and then selected pictures were retransmitted at 880 or 1760 line resolution. Mars 5 collected data for 22 orbits until a loss of pressurization in the transmitter housing ended the mission. About 60 images were returned over a nine day period showing swaths of the area south of Valles Marineris, from 5° North, 330° West to 20° South, 130° West.
Mars 6 was launched on August 5th, and arrived at Mars on March 12th. The descent module separated from the bus at a distance of 48,000 km from Mars. The bus continued on into a heliocentric orbit after passing within 1600 km of Mars. The descent module entered the atmosphere at 09:05:53 UT at a speed of 5.6 km/s. The parachute opened at 09:08:32 UT after the module had slowed its speed to 600 m/s by aerobraking. During this time the craft was collecting data and transmitting it directly to the bus for immediate relay to Earth. Contact with the descent module was lost at 09:11:05 UT in "direct proximity to the surface," probably either when the retrorockets fired or when it hit the surface at an estimated 61 m/s. Mars 6 landed in the Margaritifer Terra region of Mars (23.90° South, 19.42° West). The descent module transmitted 224 seconds of data before transmissions ceased, the first data returned from the atmosphere of Mars. Unfortunately, much of the data were unreadable due to a flaw in a computer chip, leading to a degradation of the system during its journey to Mars.
The final spacecraft, Mars 7, was launched four days after Mars 6, on August 9th, and arrived three days prior, on March 9th. Due to a problem in the operation of one of the on-board systems (attitude control or retro-rockets) the landing probe separated prematurely (4 hours before encounter) and missed the planet by 1300 km. The early separation was probably due to a computer chip error which resulted from degradation of the systems during the trip to Mars. The intended landing site was 50° South, 28° West, in the Noachis Terra region, east of Argyre Planitia. The Mars 7 lander and bus continued on into heliocentric orbits.
The above image is a composite of three photos taken by the Mars 5 "Vega" camera on February 23, 1974. Frame 9 was taken with a green filter, frame 10 was taken with a red filter, and frame 11 was taken with a blue filter. Unfortunately, I do not know the location of these three pictures. However, the image does give a decent indication of the quality of the Soviet camera technology at the time.