Mars 1
The Mars 2 and Mars 3 missions consisted of identical spacecraft, each with an orbiter and an attached lander; they were the first human artifacts to touch down on Mars. The orbiters' primary scientific objectives were to image the Martian surface and clouds, determine the temperature on Mars, study the topography, composition and physical properties of the surface, measure properties of the atmosphere, monitor the solar wind and the interplanetary and Martian magnetic fields, and act as communications relays to send signals from the landers to Earth. Mars 2 and 3 were launched by Tyazheliy Sputniks.

Mars 2 and 3

Main article: Mars 2 Mars 2

Main article: Mars 3 Mars 3
Mars 4, 5, 6, and 7 comprised an associated group of Soviet spacecraft launched towards Mars in July and August of 1973. The Mars 4 and 5 automatic stations 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 to the Mars 6 and 7 landers. They were launched from Earth by Proton SL-12/D-1-e boosters.
The 1973 Mars launch window was inefficient and thus the Proton rocket could not deliver the mass to Mars, as had been possible in 1971. Thus, the mission was split somewhat extravagantly into four separate, but lighter vehicles: two relay orbiters without landers and two orbiter type buses with landers but without fuel to enter orbit. The political purpose of these missions was to beat the American Viking probes (scheduled for 1975 launches) to be the first spacecraft to soft land on Mars, at which they did not succeed.

Mars 4, 5, 6 and 7
The Mars 4 orbiter reached Mars on 10 February 1974. Due to a flaw in the computer chip which resulted in degradation of the chip during the voyage to Mars, the retro-rockets designed to slow the craft into Mars orbit did not 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.

Launch Date/Time:

• Mars 4: July 21, 1973 at 19:30:59 UTC
  On-orbit mass:
  
  
  
  • Dry: 2270 kg
    Fully-fuelled: 3440 kg Mars 4
    Mars 5 reached Mars on February 12, 1974 at 15:45 UT and was inserted into an elliptical 1755 by 32,555 km, 24 h 53 min orbit with an inclination of 35.3 degrees. Nearly synchronized with the rotation of the planet, its two phototelevision cameras could be commanded to take 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 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° N, 330° W to 20° S, 130° w.
    
    Launch Date/Time:
    
    
    
    • Mars 5: July 25, 1973 at 18:55:48 UTC
      On-orbit mass:
      
      
      
      • Dry: 2270 kg
        Fully-fuelled: 3440 kg Mars 5
        Mars 6 successfully lifted off into an intermediate Earth orbit on a Proton SL-12/D-1-e booster and then launched into a Mars transfer trajectory. Total fueled launch mass of the lander and bus was 3260 kg. It reached Mars on March 12, 1974. 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 at 23.90° S, 19.42° W in the Margaritifer Terra region of Mars. The landed mass was 635 kg. 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 which led to degradation of the system during its journey to Mars.
        
        Mars 6
        Mars 7 successfully lifted off into an intermediate Earth orbit on a Proton SL-12/D-1-e booster and then launched into a Mars transfer trajectory. Total fueled launch mass of the lander and bus was 3260 kg. It reached Mars on March 9, 1974. Due to a problem in the operation of one of the onboard 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° S, 28° W. The lander and bus continued on into heliocentric orbits.
        
        Mars 7
        
        Later missions
        
        
        Main article: Phobos program Mars 96