Spacecraft and Subsystems
The Mariner 4 spacecraft consisted of an octagonal magnesium frame, 127 cm across a diagonal and 45.7 cm high. Four solar panels were attached to the top of the frame with an end-to-end span of 6.88 meters, including solar pressure vanes which extended from the ends. A 116.8 cm diameter high-gain parabolic antenna was mounted at the top of the frame as well. An omnidirectional low-gain antenna was mounted on a seven foot, four inch (223.5 cm) tall mast next to the high-gain antenna. The overall height of the spacecraft was 2.89 meters. The octagonal frame housed the electronic equipment, cabling, midcourse propulsion system, and attitude control gas supplies and regulators.
Scientific instruments included:
- A helium magnetometer, mounted on the waveguide leading to the omnidirectional antenna, to measure the magnitude and other characteristics of the interplanetary and planetary magnetic fields.
- An ionization chamber/Geiger counter, mounted on the waveguide leading to the omnidirectional antenna nearer the body of the spacecraft, to measure the charged-particle intensity and distribution in interplanetary space and in the vicinity of Mars.
- A trapped radiation detector, mounted on the body with counter-axes pointing 70° and 135° from the solar direction, to measure the intensity and direction of low-energy particles.
- A cosmic ray telescope, mounted inside the body pointing in anti-solar direction, to measure the direction and energy spectrum of protons and alpha particles.
- A solar plasma probe, mounted on the body pointing 10° from the solar direction, to measure the very low energy charged particle flux from the Sun.
- A cosmic dust detector, mounted on the body with microphone plate approximately perpendicular to the plane of orbit, to measure the momentum, distribution, density, and direction of cosmic dust.
- A television camera, mounted on a scan platform at the bottom center of the spacecraft, to obtain closeup pictures of the surface of Mars. This subsystem consisted of 4 parts, a Cassegrain telescope with a 1.05° by 1.05° field of view, a shutter and red/green filter assembly with 0.08s and 0.20s exposure times, a slow scan vidicon tube which translated the optical image into an electrical video signal, and the electronic systems required to convert the analogue signal into a digital bitstream for transmission.
The electrical power for the instruments and the radio transmitter of Mariner 4 was supplied by 28,224 solar cells contained in the four 176 x 90 cm solar panels, which could provide 310 watts at the distance of Mars. A rechargeable 1200 W·h silver-zinc battery was also used for maneuvers and backup. Monopropellant hydrazine was used for propulsion, via a four-jet vane vector control motor, with 222-newton (50 lbf) thrust, installed on one of the sides of the octagonal structure. The space probe's attitude control was provided by 12 cold nitrogen gas jets mounted on the ends of the solar panels and three gyros. Solar pressure vanes, each with an area of 0.65 square meter (seven ft²), were attached to the tips of the solar panels. Positional information was provided by four Sun sensors, and a sensor for either the Earth, Mars, or the star Canopus, depending on the time in its spaceflight. Mariner 4 was the first space probe that needed a star for a navigational reference object, since earlier missions, which remained near either the Earth, the Moon, or the planet Venus, had sighted onto either the bright face of the home planet or the brightly lit target. During this flight, both the Earth and Mars would be too dim to lock onto. Another bright source at a wide angle away from the Sun was needed and Canopus filled this requirement. Subsequently, Canopus was used as a reference point in many following missions.
The telecommunications equipment on Mariner 4 consisted of dual S-band transmitters (with either a seven-watt triode cavity amplifier or a ten watt traveling-wave tube amplifier) and a single radio receiver which together could send and receive data via the low- and high-gain antennas at 8⅓ or 33⅓ bits per second. Data could also be stored onto a magnetic tape recorder with a capacity of 5.24 million bits for later transmission. All electronic operations were controlled by a command subsystem which could process any of 29 direct command words or three quantitative word commands for mid-course maneuvers. The central computer and sequencer operated stored time-sequence commands using a 38.4 kHz synchronization frequency as a time reference. Temperature control was achieved through the use of adjustable louvers mounted on six of the electronics assemblies, plus multilayer insulating blankets, polished aluminum shields, and surface treatments. Other measurements that could be made included:
- Radio signal occultation
- Celestial mechanics based on precision tracking
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