Home Table of Contents What's New Image Index Copyright ScienceViews Search

On Mars: Exploration of the Red Planet. 1958-1978
 
 
MARINER MARS 71
 
 
 
[159] The battle over NASA's budget during the summer of 1968 had caused the agency's leadership to postpone beginning work on a Mariner Mars 71 project. NASA had begun the year by asking for $4.37 billion for fiscal 1969, or $218 million less than appropriated the preceding year. After the budget cycle was completed, President Lyndon B. Johnson signed an appropriation bill for $3.995 billion on 4 October 1968, the lowest since 1963. This figure, more than half a billion dollars less than the fiscal 1968 budget, sent NASA planners groaning back to their drawing boards. 8
 
Despite the tight budget, $69 million was earmarked for the planetary program, to support Mariner Mars 69's flight and preliminary study of Mariner Mars 71 and Viking 73. Two and a half months after the project [160] approval document for the 1971 mission was signed, NASA Headquarters announced on 14 November 1968 that Jet Propulsion Laboratory had been authorized to begin work on the project. Dan Schneiderman was appointed project manager at JPL, and Earl W. Glahn was named program manager at NASA Headquarters. 9
 
Mariner Mars 71 was described as part of a continuing program of planetary exploration. Unlike the previous Mariner flights, however, the 1971 mission was designed to orbit the planet with two spacecraft for a minimum of 90 days each. At a December 1968 meeting of the American Institute of Aeronautics and Astronautics, Oran W. Nicks, deputy associate administrator for space science and applications at NASA Headquarters, spoke of the value of orbiter flights and future orbiter-lander missions for the examination of Mars. He noted that Mariner 4 , 6 , and 7 had given "snapshot views of the planet." The two 1971 orbiters would "provide powerful new tools for our survey of dynamic Mars." They were scheduled to "arrive at a time in the Mars cycle when the most striking seasonal changes are evident in the southern hemisphere." A combination of different orbits for the two 1971 craft would provide a complete survey of the entire planet. "The life-times expected from these orbiters will allow observations of the dynamic changes in clouds and surface features over a period of several months." 10 In addition to the improved observations the two orbiters would meet several other scientific objectives.
 

Mariner F and G spacecraft (below)-to be christened Mariner 6 and 7 on launch-are tested in preparation for their five-moth journeys to Mars to investigate the planet's atmosphere and surface. Solar arrays are not yet installed. At left, an Atlas-centaur launch vehicle thrusts Mariner 7 toward space from Cape Kennedy, Florida, on 27 March 1969, following the Mariner 6 launch in February.

 
[161] Scientists had four general objectives for the 1971 missions, including the search for "exo-biological activity, or the presence of an environment that could support exo-biological activity." They hoped to gather information that might help answer nagging questions about the origin and evolution of the solar system. A third goal was to collect "basic science data related to the general study of planetary physics, geology, planetology, and cosmology." The specialists were also interested in information that would assist in planning and designing a Viking lander mission on Mars, especially data that would affect landing site selection.
 
Five specific investigations also demanded the attention of the planetary scientists. The orbiter cameras would provide imagery that could update topographic maps of the planet's surface. The television team, led by Harold Masursky of the U.S. Geological Survey, anticipated photographs of a much higher quality (better resolution) than those taken by the 1964 and 1969 spacecraft. These images, and other orbiter sensors, would also allow the scientists to examine time-variable surface features. Some specialists thought the most obvious of these features-the "Wave of Darkening"-was seasonal. Were the variations the results of moisture, vegetation, or the movement of air-borne dust? 11 The long stay in orbit also would permit study of the composition and distribution of the Martian atmosphere, to gain clues about the planet's weather. A fourth area of study included temperature, composition, and thermal properties of the planet's surface; scientists would be looking for warm spots where life forms might have had a chance to survive. And the Mariner investigators wanted a closer look at the seasonal waxing and waning of the polar caps.12 Besides studying these five areas, scientists would also be getting information on the internal activity, mass distribution, and shape of the planet.
 
To meet the objectives, the Mariner Mars 71 mission plan called for two spacecraft to perform separate but complementary missions. Mission A was designed primarily as a 90-day reconnaissance. The orbital path would give the spacecraft instruments a look at a large portion of the planet's surface. Orbiting the planet every 12 hours, the flight path would permit communication with the Goldstone tracking station during a lengthy portion of every alternate orbit. Mission B would study more closely the time-variable features of the Martian atmosphere and surface for at least 90 days, moving in a wide, looping orbit around the planet once every 32.8 hours.13 Nicks believed that the Mariner 71 orbit missions and the 1973 Viking orbiter lander flights would be powerful study tools, permitting man to gain at least partial answers to several important questions: "Is there life elsewhere? Has life existed on nearby planets and disappeared for any reason? Can nearby planets be made suitable for life?" 14 But before they could begin to look for answers, the NASA-contractor team had to build the hardware.
 
Engineers at JPL had a basic philosophy about incorporating changes into each new generation of spacecraft: modifications would be included to
 
(l) adapt the previous design to unique requirements for the new mission,
(2) [162] overcome difficulties demonstrated in the previous mission, and
(3) incorporate new technology when a major improvement would provide a significant benefit in cost, weight, or reliability. 15
 
The Mariner 71 spacecraft designers wanted to carry over as much of the design of the early Mariner spacecraft and ground equipment as possible. As they were quick to point out, the repeated use of experienced personnel, procedures, documentation, and facilities was a benefit to the project during tests, launch, and flight operations. The Mariner 71 spacecraft grew in size, weight, and complexity, however.
 
 
Table 27
Mariner 69 and 71 Spacecraft Comparisons

Spacecraft Feature

Mariner 69

Mariner 71

Shape

Octagonal magnesium frame

Octagonal magnesium frame

Size

127 cm diagonal; 45.7 cm depth

127 cm diagonal; 45.7 cm depth

Solar panels

112 cm x 90 cm (4); 4.0 sq m

112 cm x 90 cm (4); 7.7 sq m

Launch weight

412.8 kg

997.9kg

 
 
Besides growing much larger than its predecessors, Mariner 71 was also taking on a new major task, orbiting the planet Mars, not just passing by. As a consequence, the propulsion subsystem had to be completely redesigned to provide the necessary propulsion capability-a 1600-meter-per-second velocity change-to inject the spacecraft into Mars orbit. The 1971 design incorporated a 1335-newton (300-pound-thrust) engine, instead of the 225-newton (51-pound thrust) engine on Mariner 69. Nearly all the components needed for the 1971 propulsion subsystem (valves, regulators, and the like) had been used on previous spacecraft, but they had not been used in this particular combination. Although the propulsion subsystem was a new design, some inheritance from earlier Mariner systems was realized at the parts level by using flight-proven components.
 
Mariner 71's data storage subsystem was a completely new design, too. This all-digital, reel-to-reel tape-recording unit was, however, derived from earlier development activities at JPL. It incorporated selectable playback speeds of 16,8,4,2, and 1 kilobits* per second, with an eight-track capability [163] using two tracks at a time. High-packing density for this electronic information provided a total storage capability of 180 million bits on a 168-meter tape. Data could be recorded at 132 kilobits per second. In this subsystem, there was little or no design-hardware carry-over from previous programs.
 
Design of the central computer and sequencer was altered to increase this onboard system's memory from 128 words to 512 words.** The modification provided the operational flexibility required for orbital operations, permitting repetitive sequences to be carried out. Other changes in the central computer and sequencer led to improved operations between the computer and the sequencer, better checks on stored information, and generally improved control over the spacecraft.
 
Of the four Mariner 71 onboard science instruments-television, infrared radiometer, ultraviolet spectrometer, and infrared interferometer spectrometer-only one was new to the Mariner series. The infrared interferometer spectrometer (IRIS) had been flown on the Nimbus weather satellites. It would provide information on the composition of the Martian atmosphere-measuring water vapor, temperatures at the surface, and the temperature profile of the atmosphere-and would examine the polar caps. Although the instrument was an adaptation of a previous design, many changes had to be made in it so that it worked on Mariner. To Mariner systems engineers, IRIS was a new instrument that they had to incorporate into their spacecraft design.
 
Television was another subsystem that was extensively modified. Installing two cameras on Mariner 71, the engineers could use circuitry, optics, and vidicon components from other systems. But there were difficulties. The Mariner 69 television equipment had developed background noise problems; a considerable amount of processing had had to be done to both analog and digital signals to convert them into usable video images. And the 1969 system had less dynamic range and was not as adaptable as the scientists needed for the orbiter mission. The Mariner 71 team developed an all-digital television system with eight selectable filters in the wide-angle camera, automatic and commandable shutter speeds, and picture sequencing. Another improvement reduced the effects on the optics of long exposure to the harsh space environment. Relying on existing technology minimized development costs and risks and provided the Mariner 71 scientific team a high-performance television system.
 
Major changes were made in the attitude control subsystem to adapt it to the requirements of orbital flight. To accommodate a new autopilot and computer logic changes, the Mariner 71 engineers designed new attitude control electronics and redesigned the inertial reference unit (a device that. . . .
 
 
[164] Mariner Mars 1964
[165] Mariner Mars 1969
 
[166] Mariner Mars 1971
 

 
 
[167]....gives continuous indication of position by integration of accelerations from a starting point). They included an acceleration sensor (accelerometer) that would control the firing duration of the propulsion-subsystem rocket engine. To maintain spacecraft attitude stability, gyroscopes were modified from Mariner 69 hardware. Sensors, both solar and star, which help determine the spacecraft's location in space, were considerably altered for the orbital flight. Mariner 71's attitude-control gas-jet system was similar to the 1969 subsystem with only minor modifications.
 
The data automation subsystem was designed to contain a new logic function to accommodate the requirements of the scientific instruments and orbital flight. Integrated circuitry and packaging techniques were directly borrowed from Mariner Venus 67 and the 1969 Mars craft. The structural subsystem, or the basic chassis of the spacecraft, was a successful adaptation of the 1969 octagonal frame. Electrical energy requirements were provided by an adapted power subsystem , which used new nickel-cadmium batteries and enlarged solar panels like those used in 1969. The radio subsystem, which borrowed technology from the Apollo program was altered to eliminate earlier problems. Other systems requiring only minor changes included command, telemetry, antennas, scan platform control, infrared radiometer, and ultraviolet spectrometer. The Mariner 71 final project report notes, "The design changes which were incorporated underwent considerable review and debate prior to approval so that the maximum inheritance could be realized," keeping the total number of changes the engineers had to make in the Mariner hardware to a minimum.16
 
* Bit , is the abbreviation for binary digit and stands for the smallest unit of computer-coded information carried by a single digit of binary notation. This form of notation is a system of expressing figures for use in computers that use only to two digits, one and zero. A kilobit equals 1000 bits.
 
** A word in a computer memory is a binary number containing a specific number of bits and is used as the unit of meaning.