Mariner 4 was designed around an octagonal magnesium frame 127 centimeters across a diagonal and 45.7 centimeters high; this frame was called the bus, and each of its eight sides was called a bay. The bus housed the electronic equipment, cabling, midcourse propulsion system, and
attitude control gas supplies and regulators. Most of the science experiments were mounted outside the bus. Four solar panels were attached to the top of the frame for an end-to-end span of 688 centimeters including solar pressure vanes which extended from the ends.
A 116.8-centimeter-diameter
high-gain parabolic antenna was mounted at the top of the frame as well. An omnidirectional
low-gain antenna was mounted on a 223.5-centimeter-tall mast next to the high-gain antenna. The low-gain antenna mast held a
magnetometer. At the bottom-center of the spacecraft the camera was mounted on a scan platform.
Posted ImageScience instruments, in addition to the camera and the magnetometer, were a dust detector, a
cosmic ray telescope, a trapped radiation detector, a
solar plasma probe, and an ionization chamber /
Geiger counter.
There was to be an ultraviolet photometer as well, but it wasn't completed in time for the mission. It was replaced by a nonfunctioning instrument called the "mechanical and electrical subassembly simulator subsystem" and by a scan-platform inertial and thermal simulator which prevented a mechanical or thermal imbalance.
The subsystems were located as follows:
| Bay I | - power subsystem (distributed electricity to other components)
|
| Bay II | - propulsion, including a small rocket motor for midcourse correction
|
| Bay III | - some scientific instruments
- data automation system (organized data)
|
| Bay IV | - data encoder (organized data)
- command (interpreted ground commands)
|
| Bay V | - radio frequency communications
- videotape recorder
|
| Bay VI | - radio transmitter and receiver
|
| Bay VII | - attitude control
- gas supply
|
| Bay VIII | |
Power was supplied by 28,224 solar cells contained in the four 176-by-90-centimeter solar panels, which could provide 310 watts at Mars. A rechargeable
silver-
zinc battery with an energy of 1,200
watt-hours was also used for maneuvers and backup.
Monopropellant hydrazine was used for propulsion via a motor installed on one of the sides of the bus (the motor is seen in the picture above).
The telecommunications equipment consisted of a dual
S-band 7-watt
triode cavity amplifier / 10-watt
travelling-wave tube amplifier transmitter and a single receiver which could send and receive data via the low-gain and high-gain antennas at 8
1/
3 or 33
1/
3 bits per second. Data could also be stored on a tape recorder with a capacity of 5,240,000 bits for later transmission.
Temperature control was achieved through the use of adjustable louvers mounted on six of the electronics assemblies, multilayer insulating blankets, polished
aluminium shields, and surface treatments.
Attitude control was provided by 12 cold-
nitrogen-gas jets mounted on the ends of the solar panels and three
gyroscopes. Solar pressure vanes, each with an area of 0.65 square meters, were attached to the tips of the solar panels. Positional information was provided by four
Sun sensors, an
Earth sensor, a Mars sensor and a
Canopus sensor. Attitude control was achieved via the spacecraft's nozzles and vanes. Keeping the solar panels oriented so they would receive maximum sunlight was of primary importance. If the Sun and Canopus sensors indicated any drift away from the acceptable position, small puffs of gas were ejected from nozzles located at the ends of the solar panels. Any unequal
pressure from sunlight on various parts of the spacecraft which might affect its orbit was corrected by the adjustable vanes at the ends of the solar panels. In the event a midcourse correction was required, the rocket motor nozzle, which was fixed, had to be aimed by changing the spacecraft attitude.
Since it was not known exactly where the spacecraft would be as it passed Mars, the camera was mounted on the scan platform. A few hours before it reached the planet, power was applied and the platform bobbed up and down. When the planet came in view of the wide-angle (40-degree-field-of-view) sensor, the platform-driving motor switched from scanning to tracking mode, which kept the scan sensor, and hence the camera, pointed directly at Mars. When the planet came in view of the camera with a narrow field of view (one degree), it started taking a series of
pictures as the spacecraft swept across the planetary
surface.
The imaging subsystem consisted of a
Cassegrain narrow-angle reflecting telescope with a 30.5-centimeter effective focal length and a 1.05°-by-1.05° field of view; a shutter and filter assembly that had 0.08-second and 0.2-second exposure times and used red and green filters; a slow-scan
vidicon tube with a 0.22-inch-by-0.22-inch square target, which translated the optical image into an electrical video signal; and related electronics, including a television data encoder. When the picture-recording sequence commenced, vidicon output would undergo analog-to-digital conversion and data would be stored at 240,000 bits per picture on a
1/
4-inch, 300-foot-long magnetic tape loop on the spacecraft. Two of every three pictures taken were recorded on the tape, resulting in a chain of pairs of overlapping, alternately-filtered pictures extending across the disk of Mars. Data was transmitted after occultation of the spacecraft by Mars via the radio subsystem from 15 july 1965 to 24 july 1965. Conversion from electrical signals to an optical image was performed by a video-to-film recorder using 64 shades. Computer processing programs yielded photographs with greater contrast than the raw image data..
A vector low-field
helium magnetometer was used to measure the interplanetary magnetic field.
The cosmic dust detector consisted of a 22-centimeter-by-22-centimeter aluminium impact plate which was coated on both sides with a nonconducting material. The detector also had a crystal acoustical transducer bonded to one side. It was to be used to continually monitor dust-particle flux and mass distribution from Earth proximity through the Mars encounter. The impact plate and dielectric aluminium film combination constituted a penetration detector in the form of a capacitor. The experiment was also intended to observe the degree of dust-particle concentration near Earth and near Mars, the rate of change of the dust-particle flux density with respect to distance from Earth, and the perturbation effects of large planetary bodies on the dynamic behavior of the dust particles. The instrument was mounted above the main spacecraft bus, with the case just inside the thermal shield to protect it from the Sun. The sensor protruded through an opening in the thermal shield. The instrument memory consisted of two 8-bit binary data-analysis registers and a microphone accumulator which would have recorded the number of microphone events observed by the instrument.
A set of three
silicon surface barrier detectors was used in the form of a telescope to determine the flux of protons in the energy intervals 15-70
megaelectronvolts and 70-170 megaelectronvolts,
alpha particles in the energy ranges 15-70 megaelectronvolts per
nucleon and 70-∞ megaelectronvolts per nucleon, and protons and alpha particles in the energy interval 1.2-15 megaelectronvolts per nucleon. The detector was mounted on the spacecraft so as to point always in the antisolar direction. A 128-channel
pulse-height analyzer was used to sample the energy loss in the top detector element of the telescope. It was possible to pulse-height analyze protons and alpha particles in the 15-70-megaelectronvolt-per-nucleon range, protons in the 70-170-megaelectronvolt range, and alpha particles with energies above 70 megaelectronvolts per nucleon.
A detector composed of four
Geiger-Müller tubes and a solid state device was used to detect low-energy cosmic rays --protons and electrons-- while the spacecraft journeyed to and passed Mars. The Geiger-Müller tube was mounted away from the main body of the spacecraft so that at the tube the spacecraft subtended 14% of 4π
steradians (not including the thin solar panels).
Two levels of expertise existed for each scientific instrument: scientists, who proposed experiments and the parameters to be measured, and engineers, who designed them to their specifications. Each bit of data was displayed in a line; these lines were transmitted to Earth and decoded by computers. To indicate the start of each frame, each one was preceded by an identifier: 1111111 for engineering, 000011101100101 for science. The computer, seeing either of these sequences, would decode the data accordingly. Occasionally there would be a gap in the received data -- the spacecraft signal might be temporarily lost or a glitch occurred at the tracking station or in the connection between there and the Jet Propulsion Laboratory. The computer would search for the next occurrence of either of the synchronization signals. It was quite possible that 1111111 could occur anywhere in the data. If the computer locked on this, its output would be gibberish until it picked up sync once again. In this event, it was necessary to visually inspect the raw data for the true occurrence of a line start. The scientists were required to continuously monitor the data. In another case, the receiving station might lock onto a sideband instead of the carrier and the data became inverted: 1s turned into 0s and 0s turned into 1s. They developed methods for coping with this condition as well.
All operations were controlled by a command subsystem which could process any of 29 direct command words or 3 quantitative word commands for midcourse maneuvers. The central computer and sequencer operated stored time-sequence commands using a 38.4-kilohertz synchronization frequency as a time reference. Thus, the spacecraft was completely autonomous. However, the stored commands could be modified from Earth. Its memory consisted of ferromagnetic doughnuts strung on wires.
The overall height of the spacecraft was 289 centimeters, and its mass was 260.68 kilograms.