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Mariner 6; 1969 // USA
Topic Started: Friday 31-07-2009, 00:25 (625 Views)
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Mars > Missions > Mariner 6 > Index
 
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  1. Index
  2. [go2=missionoverview]Mission overview[/go2]
  3. [go2=spacecraftandinstruments]Spacecraft and instruments[/go2]
  4. [go2=results]Results[/go2]
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Mars > Missions > Mariner 6 > Mission overview
 
Objectives
  1. perform a flyby of Mars and study its surface and atmosphere
  2. establish the basis for future investigations of the planet
  3. demonstrate and develop technology required for future missions to other planets
  4. provide experience and data to help Mariner 7 in its Mars encounter




Summary

Launch date: 24 february 1969

Mars flyby & mission end: 31 july 1969

Mariners 6 and 7 were identical probes sent to Mars in early 1969. Since they were unable to orbit or land on the planet, they were designed to perform a single flyby and take photographs during it.

Both of them completed their mission successfully and returned many more pictures than their predecessor, Mariner 4.
Sources: Wikipedia / Mariner 6 and 7 | Absolute Astronomy / Mariner 6 and 7 | James H Wilson, Two over Mars
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Mars > Missions > Mariner 6 > Spacecraft and instruments
 
Mariner 6, like Mariner 4, consisted of an octagonal magnesium frame base. The base was 138.4 centimeters diagonally and 45.7 centimeters tall. On top of the frame were a high-gain parabolic antenna one meter in diameter and four 215-by-90-centimeter solar panels. Next to the high-gain antenna stood a 2.23-meter-tall mast with a low-gain antenna at the top. Under the frame was a scan platform with the scientific equipment.

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Bottom image © NASA
The spacecraft propelled itself by means of a monopropellant hydrazine rocket motor within the frame. The motor nozzle protruded from one of the sides of the frame.

Attitude was controlled by a set of three gyroscopes, nitrogen jets at the ends of the solar panels, a Canopus tracker and six Sun sensors.

Power was provided by the 17,472 cells in the solar panels (4,368 cells per panel), which provided 800 watts at Earth and 449 watts at Mars. Power requirement reached 380 watts at Mars encounter but stayed below that throughout the mission, so power shortages were never a problem for Mariner 6. In any case, backup power was available via a 1,200-watt-hour rechargeable silver-zinc battery.

Temperature was controlled by adjustable louvers on the sides of the frame.

The science instruments were the camera used to image Mars, an infrared spectrometer, an ultraviolet spectrometer, a two-channel infrared radiometer for measuring Mars's surface temperature, a thermal control flux monitor, a celestial mechanics experiment and a general relativity experiment.

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Both images © NASA
Three telemetry channels were available for communications: one for carrying engineering data at 8 1/3 or 33 1/3 bits per second, one for carrying scientific data at 66 2/3 or 270 bits per second, and one for carrying scientific data at 16,200 bits per second. The antennas achieved communication via dual S-band travelling-wave tube amplifiers and a receiver.

Up to 195,000,000 bits of imaging data could be stored in an analog tape recorder for later transmission, and scientific data was stored in a digital recorder.

The command system consisted of a computer and a sequencer. This executed specific commands at specific times, making the probe automated, but ground instructions overriding the preset mission and backup mission could be sent at any time.

Mariner 6's total height was 3.35 meters. With its solar panels deployed, the spacecraft's span was 5.79 meters.
Source: Wikipedia / Mariner 6 and 7
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Mars > Missions > Mariner 6 > Results
 
All stages of liftoff were successful, and some ten minutes after countdown Mariner 6 was under way.

Shortly after launch, Mariner 6's radio tried to lock on itself instead of on the ground signal transmitted to it. The spacecraft would then not have been able to communicate with Earth for navigational purposes. The problem was not solved; rather, it solved itself: a few weeks later the radio turned out not to be locked on itself anymore.

Afterwards, when the scientific platform was released from its protective restraints, particles were released. Shining in the uninterrupted sunlight of interplanetary space, they looked brighter than Canopus, so the star-tracking sensors on Mariner 6 locked on them instead. This problem was solved by using the gyroscopes for orientation instead of the star-tracking system for a while.

Canopus is not exactly 90 degrees from the Sun, so, as Mariner 6 moved around the Sun, Canopus appeared to change position, moving in a circle. This means that the spacecraft's tracker must move constantly to keep locked on the star. On 20 april, the Canopus sensor moved in the wrong direction, losing the star completely. Ground control was able to move the sensor back in the right direction, but only the same distance it had moved originally. The ground team tried to use the Large Magellanic Cloud instead, but the sensor would lock off periodically, since it is not a bright enough source. After a while, the sensor finally responded to the initial ground commands and moved another step in the right direction, locking onto Canopus. Weeks later there was a similar problem, but this time the sensor responded immediately to the commands sent from Earth.

The rest of the cruise phase went with no other threats to the mission.

Every instrument, with the exception of one of the two detectors of the infrared spectrometer, functioned as planned. Their data was collected and transmitted to Earth for analysis.





Pictures of Mars

On 28 july, Mariner 6 was some 1,255,000 kilometers from Mars. It was then that its scientific instruments were turned on and it began to gather data. It also took 33 pictures at 37-minute intervals. A second series of 17 pictures was begun as soon as the first series had been downloaded to Earth and the craft's memory had been erased. It was as these 17 pictures were taken that the craft reached its final approach to Mars and the climax of its mission. It then took a final set of twenty-odd close-up pictures.

The craft crossed and imaged the planet's equatorial regions primarily. Combined, the images taken by Mariners 6 and 7 cover about 20% of the planet's surface.

The closest approach to the planet was 3,431 kilometers from the surface.

Although they did show the craters which were expected, Mariner 6's pictures of Mars showed that the planet was not as similar to the Moon as had been thought when Mariner 4 sent its own pictures. Rather, the red planet had its own personality and characteristics. It was shown to be a vast, cold, dry desert with several different terrain types and a thin carbon dioxide atmosphere with clouds and frost.

The photographs also showed what is now known as chaos: chaotic terrain scarred by a labyrinth of small ridges and featuring no craters at all. It is believed that these regions formed when underground ice or water escaped from below, although the exact reason for their existence is still unknown.

Far-encounter pictures taken by Mariner 6

Near-encounter pictures taken by Mariner 6

Mosaics created with the pictures taken by Mariner 6

Mariner 6's trajectory





The martian atmosphere

Readings of Mars's atmosphere at the tropics returned warmer temperatures than predicted by theory. In other places, however, the temperature readings were consistent with theory. Data indicated that carbon dioxide clouds could form at any altitude near the north pole. Atmospherical composition turned out to be dominated by carbon dioxide (CO2) and its chemical derivatives, especially carbon monoxide (CO). It was surprising at the time that no nitrogen or ozone was detected.

Clouds on Mars are formed by CO2 ice, water ice and dust particles. Although it was determined to be impossible for rain to fall on Mars, frost was detected and snow was predicted (the snow was confirmed almost four decades later by Phoenix).

Temperature readings indicated highs of about 16°C near the equator during summer and lows of about -151°C near the south pole.
Sources: James H Wilson, Two over Mars | Stewart A Collins, The Mariner 6 and 7 pictures of Mars | Wikipedia / Mariner 6 and 7
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