Mars 6

Description

Mars 4, 5, 6, and 7 comprised an associated group of Soviet spacecraft launched towards Mars in July and August of 1973. The Mars 6 interplanetary station consisted of a flyby bus and an attached descent module. The descent module separated from the bus on reaching Mars and was designed to enter the martian atmosphere and make in-situ studies of the Mars atmosphere and surface.

Mission Profile

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. After one course correction burn on 13 August 1973, it reached Mars on 12 March 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 Sinus 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.

Scientific Instrumentation

The Mars 6 Descent Module carried a panoramic telephotometer to image the martian surface around the lander, atmospheric temperature, pressure, density, and wind sensors, an accelerometer to measure atmospheric density during the descent, a mass spectrometer to estimate atmospheric composition, a radio altimeter, an activation analysis experiment to study soil composition, and mechanical properties soil sensors. The flyby module contained a telephotometer to image Mars, a Lyman alpha sensor to search for hydrogen in the upper atmosphere, a magnetometer, an ion trap and narrow angle electrostatic plasma sensor to study the solar wind and its interaction with Mars, solar cosmic ray sensors, micrometeorite sensors, and a French-supplied solar radiometer to measure solar long-wavelength radio emissions. It was also equipped to perform a radio occultation experiment to profile the atmosphere and ionosphere.

Scientific Results

Data returned by the Mars 6 descent module allowed a profile of tropospheric structure from the base of the stratosphere at 25 km altitude at 150 K to the surface at 230 K and atmospheric density from 82 km to 12 km. A surface pressure of 6 mb and temperature of (230 K) -43 C were measured. Instruments also indicated "several times" more atmospheric water vapor than previously reported. The mass spectrometer data were stored on-board during the descent and scheduled to be transmitted after landing and were therefore lost. The current to the vacuum pump was transmitted as an engineering parameter, however, and a steep increase in current was found. It was hypothesized to indicate an inert gas which could not be removed by the pump, leading to an estimate of argon abundance in the atmosphere of 25% to 45%. (The actual value is now known to be about 1.6%.) The Mars 6 flyby bus performed a radio occultation experiment and the results, in concert with results from Mars 4 and 5 occultation measurements, showed the existence of a nightside ionosphere with a maximum electron density of 4600 per cubic cm at an altitude of 110 km and a near surface atmospheric pressure of 6.7 mbar.