1. Mission Objectives

Mars 7 was the final spacecraft of the 1973 Soviet armada, a twin to Mars 6. Its mission was the most complex of the launch window:

Engineering and Operations:

• Transport a lander module to the vicinity of Mars.
• Execute the separation of the module and perform a direct atmospheric entry for a soft landing.
• The flyby module (Bus) was to continue its trajectory to serve as a communications link with Earth.

Scientific:

• Study the composition of the Martian atmosphere and surface in situ.
• Measure physical parameters during descent (pressure, temperature, density).
• Analyze the plasma and radiation environment in interplanetary space and near Mars.

2. Spacecraft Specifications (3MP Platform)

The spacecraft utilized the 3MP configuration, optimized to carry a descent vehicle instead of fuel for orbital insertion.

• Launch Mass: 3,260 kg (Some sources cite higher figures including upper stages, but the standard 3MP probe mass is ~3.2t).
• Dimensions: Cylindrical bus ~2.8 m in height with two extendable solar panels.
• Lander Module: Spherical capsule of ~635 kg (similar to Mars 6) equipped with an aerodynamic heat shield and multi-stage parachute system.
• Propulsion: KTDU-425A main engine for mid-course corrections and nitrogen gas thrusters for attitude control.

3. Scientific Instrumentation

The payload was distributed between the cruise vehicle and the lander:

Flyby Bus (Coast Stage):

• Magnetometer: To study the planetary magnetic field.
• Plasma Traps and Cosmic Ray Detectors: To monitor solar wind and radiation.
• Micrometeoroid Detector: To measure interplanetary dust flux.
• Lyman-Alpha Photometer: To detect hydrogen in the upper atmosphere (cited in general series configurations).

Lander Module:

• Mass Spectrometer: For chemical analysis of the atmosphere.
• Environmental Sensors: Thermometer and barometer for surface conditions.
• Radio Altimeter and Accelerometer: Critical instruments for descent control.

4. Failure Analysis

The Mars 7 mission suffered a disappointing fate due to systemic failures in Soviet electronics of the era.

1. Arrival at Mars: The spacecraft reached the vicinity of Mars on March 9, 1974, three days before its twin Mars 6 (launched earlier but on a different trajectory).
2. The Separation Failure: Due to a malfunction in the onboard computer (likely corrosion in the chips or an error in the attitude control/retrorocket system), the lander separated prematurely or failed to execute the final correction maneuver necessary to intercept the atmosphere.
3. Trajectory Result: Instead of entering the atmosphere, the lander flew past the planet at a minimum distance of 1,300 km from the surface.
4. Root Cause: Subsequent investigations identified the use of transistors with aluminum components instead of gold, which corroded rapidly, as the primary cause of control system failure across the entire Mars M-73 series.

5. Technical Conclusion

Mars 7 was a total failure regarding its surface objectives. The inability to correct its trajectory or properly time the lander separation resulted in both the bus and the lander entering a permanent heliocentric orbit. This failure, combined with the problems of Mars 4, 5, and 6, marked the end of the Mars M-73 probe series and forced a complete re-evaluation of space electronics manufacturing standards in the Soviet Union.