Mariner 4 images
Detailed Information on the Spacecraft, Experiments, and Data Sets
Mariner 4 was the first spacecraft to obtain and transmit close range images of Mars. After its launch on November 28, 1964 and a journey of hundreds of millions of kilometers, Mariner 4 passed within 9844 kilometers of Mars on July 14, 1965. Beginning at a range of 16,900 km, the probe acquired a surface image composed of 200 lines, consisting of 200 pixels each (average resolution was about 3 km/pixel) every 48 seconds until, at a range of 11,900 km, 22 television pictures of the Martian surface had been stored on the spacecraft's 4-track tape recorder (the last 3 of these pictures were beyond the terminator). It took 4 days to transmit the image information to Earth and the spacecraft returned useful data until October 1965, when the distance from Earth and its antenna orientation temporarily halted the signal acquisition. Data acquisition resumed in late 1967 and continued until December 20, 1967.
Mariner 4 was designed with 3 main objectives in mind. First, the probe was to conduct field and particle experiments in interplanetary space, including measurements of the Martian magnetic field, cosmic dust and cosmic rays, and the solar wind. Secondly, Mariner 4 was to provide close range images of Mars in order to provide us with the most detailed surface pictures ever obtained of the planet, hopefully revealing geologic and atmospheric processes. Finally, as interplanetary probes were still a new technology, the mission would provide experience in operational and engineering techniques required for long term interplanetary missions. An interesting spin-off of the last objective was the development of the radio-occultation experiment which obtained approval only after the spacecraft was already launched. The experiment took advantage of radio waves from the spacecraft propagating through the Martian atmosphere as the spacecraft passed behind the planet. Variations in the amplitude, frequency, and phase of the returned signal allowed calculations of atmospheric density, temperature, and scale height as functions of altitude.
Other than some degradation of the plasma probe and a failed Geiger counter, the Mariner 4 mission was a success. The relayed surface images revealed a cratered and moon-like surface, contrary to even conservative estimates of the Martian topography (it was later learned that the images were of a geologically older region of the planet). The images covered about 1% of the surface within a discontinuous band stretching from 40oS, 96oW to 40oN, 187oW. It was not conclusively determined whether or not Mars had an intrinsic magnetic field, but to date it is generally accepted that Mars is essentially without an intrinsic magnetic field of any significance. One of the great achievements of the Mariner 4 mission was the modification of mission plans while the spacecraft was in transit in order to accomodate the radio-occultation experiment. Not only did the experiment indicate surface pressures of 4.1 to 7.0 mbar (for comparison Earth has a surface pressure of 1013 mbar), but the experiment revealed the benefits of maintaining some flexibility in mission plans even after a spacecraft is launched.
Mariner 6 & 7
Detailed information on Mariner 6 Spacecraft, Experiments and Data Sets
Detailed information on Mariner 7 Spacecraft, Experiments and Data Sets
Mariner 6 and Mariner 7 were identical spacecraft launched on February 24, 1969 and March 27, 1969 respectively, and their missions were entirely devoted to the flyby study of Mars. The probes passed closest to Mars on July 30 and August 4 of the same year (Mariner 7 took a more direct path than Mariner 6 and "caught up" to Mariner 6, arriving only 5 days behind). The spacecraft each acquired a series of far encounter images, composed of 704 lines consisting of 945 pixels each, as they approached the planet and a series of near encounter images (same numbers of lines and pixels/line) upon arrival. The far encounter photos had resolutions ranging from 4 to 43 km per pixel, while the near encounter images had resolutions as good as 300 m per pixel. In total, 143 far encounter images and 58 near encounter images were transmitted. Due to advances in technology, Mariner 6 and 7 carried computers that could be reprogrammed from Earth during the mission and the probes were able to transmit data at a rate of almost 2000 times that of Mariner 4. These spacecraft were heavier than any planetary probes previously launched by NASA and they were carried into space by the Atlas-Centaur rocket.
The probes were designed to concentrate entirely on Mars. Measurements of atmospheric composition, pressure, density, and temperature were made and the radio-occultation experiment designed during the Mariner 4 mission was once again implemented. Surface images of greater quality and quantity were hoped to provide a more complete picture of the Martian surface and, when combined with the atmospheric data, to allow planning for future missions in search of life on Mars. A celestial mechanics experiment was also included in the mission, which, by analysis of the extremely accurate tracking data of the paths of the spacecraft, provided a precise measurement of the mass of Mars. Finally, as long range interplanetary mission technology was still in its infancy, the missions provided valuable experience in the engineering and mission operations required for long-term flight away from the sun.
In transit to Mars, likely due to a battery rupture, contact was temporarily lost with Mariner 7 on July 30. After a 7-hour silence, contact was restored, but it soon became evident that the instrument responsible for reporting the orientation of the television cameras had been damaged and was no longer functioning. Without this information the Mariner 7 cameras could not be pointed properly and, with the Mars encounter close at hand, a solution was needed quickly. On August 1, manual calibration by ground crews brought Mars into the view of Mariner 7 cameras and, on August 2, Mariner 7 began to relay far encounter images of Mars. The restoration of the Mariner 7 imaging system was a prime example of the expertise being developed by mission operators during these early interplanetary missions and the event stood as a testament to the importance of having a reprogammable computer on the spacecraft.
The mission was a complete success. In total, 201 images of Mars were acquired; the near encounter images covering about 10% of the Martian surface. In the near encounter sequences, the two spacecraft viewed different areas of the planet with Mariner 6 imaging a band from the equator to 10oS, spanning 60oW to 320oW, and Mariner 7 imaging the edge of the polar cap at 60oS, 0oW as well as a band from 10oN to 30oS, spanning 20oW to 105oW. The images essentially deflated any of the theories proposing the existence of artificial canals on the surface and the images also showed a lack of any correlation between geographical features and the light and dark areas seen by far encounter images and Earth based images. The images still did not show the widely varied terrain that was hoped for, but later missions revealed the truth about the exciting surface of Mars. Atmospheric experiments indicated the presence of dust suspended in the atmosphere, carbon dioxide ice and water ice clouds, carbon monoxide, ionized hydrogen, and ionized oxygen. Among the species not detected were ozone and nitrogen; both of which would have been beneficial to the existence of life similar to that on Earth. Surface temperatures as warm as 280-290K (290K is about 17o Celsius or 63o Fahrenheit) were detected near the equator, and it was found that, as one would expect, the darker areas of lower reflectance were generally warmer than the lighter colored deserts of higher reflectance. The radio-occultation experiment reinforced the findings of Mariner 4 with atmospheric pressure falling between 3.8 to 7.0 millibars and this experiment also detected an ionosphere on only the sunlit side of the planet.
Detailed Information on the Spacecraft, Experiments, and Data Sets
Originally, Mariner 9 was to have an identical companion, Mariner 8, but on May 8, 1971, 365 seconds after launch, Mariner 8's Centaur main engine shut down and the upper stage of the rocket, along with Mariner 8, fell into the Atlantic about 560 km north of Puerto Rico. Mariner 9 was the first of NASA's Mars orbiters. The spacecraft, which weighed more than Mariner 6 and 7 combined, was launched on May 30, 1971 on top of an Atlas-Centaur launch vehicle. After a journey of more than 600 million kilometers, Mariner 9 arrived in Mars orbit on November 14, 1971 and began the most ambitious Mars exploration mission yet attempted. Mariner 9 carried sophisticated instrumentation similar to that of Mariner 6 and 7, but a larger propulsion system was required for controlling the spacecraft during its time in Mars orbit. By the end of 349 days in Mars orbit, 7329 images (including images of Mars' two moons, Phobos and Deimos) had been relayed back to Earth and a total of 54 billion bits of data had been transmitted. Mariner 9 completely revolutionized our view of Mars and the information from this mission would later serve as the foundation for the Viking program.
Mariner 9 was designed to provide the most complete view of Mars ever obtained with mission experiments similar to those of Mariner 6 and 7. Atmospheric structure, composition, density, and pressure were to be analyzed again using techniques similar to those used on previous Mariner Mars missions. Mission goals were to map over 70% of the Martain surface with resolutions ranging from 1 km per pixel to as good as 100 m per pixel during successive Mars orbits from an altitude of about 1,500 km. The spacecraft would also look for signs of volcanic activity denoted by heat anomalies on the surface using infrared radiometry. Mars' two moons, Phobos and Deimos, were also to be analyzed by Mariner 9. Mariner 9 was essentially designed to refine the findings of previous missions and to take away the veil of uncertainty which had so far concealed the Martain surface.
Mariner 9 far exceeded its expectations in every way. Upon its arrival in November, the spacecraft was presented with a Martian atmosphere full of dust which obscured the view of the surface. The opaqueness, explained as dust storms, of the Martian atmosphere had been previously observed from Earth during Martian southern hemisphere summers and now with Mariner 9 in orbit, the existence of these dust stroms was confirmed. With a reprogrammable computer on board, Mariner 9 was kept in orbit until the dust began to settle out of the atmosphere and the systematic imaging of the planet's surface began in January of 1972. The 7329 images, covering about 80% of the planet, revealed the surface and atmosphere of Mars to be as varied as planetary scientists had hoped. Some of the observed features included ancient river beds, craters, massive extinct volcanoes, canyons, layered polar deposits, evidence of wind-driven deposition and erosion of sediments, weather fronts, ice clouds, localized dust storms, morning fogs and more. With evidence of flow features, and therefore the possibilty of a time when water was in liquid form on the surface of Mars, the question of the existence of life on Mars was intensified. It was clear that Mars had brought about many more questions which a lander would be best suited to answer.
In addition to the vast array of images obtained, many other observations were made. These observations included: a lack of evidence for volcanic activity, irregularities in Mars' gravity field, properties of the daytime ionosphere, atmospheric pressures ranging from 2.8 to 10.3 mbar, measurements of atmospheric water vapor content, altitude measurements, detection of seasonal upper atmosphere ozone, and an ultraviolet spectrum of Phobos. In total, 54 billion bits of data were relayed back to Earth by Mariner 9. For comparison, this was 27 times the amount of data retrieved by all previous Mariner Mars missions combined.
The results of the Mariner 9 mission paved the way for the Viking program. Confirmation of atmospheric pressure allowed engineers to design the Viking landers for a safe descent, and the fantastic images and data retrieved fueled the fire for further Mars exploration. The possibility of life on Mars was an irresistable mystery and, on August 20, 1975, the Viking program was launched with a variety of experiments... including experiments designed to search for signs of Martian life.
A Listing of Mariner 4,6,7,9 References
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