Mariner 4
Mariner 4 was the fourth in a series of spacecraft intended for planetary exploration in a flyby mode. It was designed to conduct closeup scientific observations of Mars and to transmit these observations to Earth. Launched on November 28, 1964, Mariner 4 performed the first successful flyby of the planet Mars, returning the first close-up pictures of the Martian surface. It captured the first images of another planet ever returned from deep space; their depiction of a cratered, dead planet largely changed the scientific community's view of life on Mars. Other mission objectives were to perform field and particle measurements in interplanetary space in the vicinity of Mars and to provide experience in and knowledge of the engineering capabilities for interplanetary flights of long duration. Initially expected to remain in space for eight months, Mariner 4's mission lasted about three years in solar orbit. On December 21, 1967, communications with Mariner 4 were terminated.
Spacecraft and subsystems
The Mariner 4 spacecraft consisted of an octagonal magnesium frame, across a diagonal and high. Four solar panels were attached to the top of the frame with an end-to-end span of, including solar pressure vanes which extended from the ends. A elliptical high-gain parabolic antenna was mounted at the top of the frame as well. An omnidirectional low-gain antenna was mounted on a tall mast next to the high-gain antenna. The overall height of the spacecraft was. The octagonal frame housed the electronic equipment, cabling, midcourse propulsion system, and attitude control gas supplies and regulators.The 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 four parts: a Cassegrain telescope with a 1.05° by 1.05° field of view, a shutter and red/green filter assembly with 0.08 and 0.20 second 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 telecommunications equipment on Mariner 4 consisted of dual S-band transmitters 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 occultation
- Celestial mechanics based on precision tracking
Mission profile
Launch
Mariner 3 had been a total loss due to failure of the payload shroud to jettison. JPL engineers suggested there had been a malfunction during separation of the metal fairing exterior from the Fiberglas inner lining. Pressure differential between the inner and outer part of the shroud could have caused the spring-loaded separation mechanism to become tangled and thus fail to detach properly.Testing at JPL confirmed this failure mode and an effort was made to develop a new, all-metal fairing. The downside of this was that the new fairing would be significantly heavier and reduce the Atlas-Agena's lift capacity. Convair and Lockheed-Martin had to make several performance enhancements to the booster to wring more power out of it. Despite fears that the work could not be completed before the 1964 Mars window closed, the new shroud was ready by November.
After launch from Cape Canaveral Air Force Station Launch Complex 12, the protective shroud covering Mariner 4 was jettisoned and the Agena-D/Mariner 4 combination separated from the Atlas-D booster at 14:27:23 UTC on November 28, 1964. The Agena's first burn took place from 14:28:14 to 14:30:38. The initial burn put the spacecraft into an Earth parking orbit and the second burn from 15:02:53 to 15:04:28 injected the craft into a Mars transfer orbit. Mariner 4 separated from the Agena at 15:07:09 and began cruise mode operations. The solar panels deployed and the scan platform was unlatched at 15:15:00. Sun acquisition occurred 16 minutes later.
Lock on Canopus
After Sun acquisition, the Canopus star tracker went searching for Canopus. The star tracker was set to respond to any object more than one-eighth as, and less than eight times as bright as Canopus. Including Canopus, there were seven such objects visible to the sensor. It took more than a day of "star-hopping" to find Canopus, as the sensor locked on to other stars instead: a stray light pattern from the near Earth, Alderamin, Regulus, Naos, and Gamma Velorum were acquired before Canopus.A consistent problem that plagued the spacecraft during the early portion of its mission was that roll error signal transients would occur frequently and on occasion would cause loss of the Canopus star lock. The first attempt at a midcourse maneuver was aborted by a loss of lock shortly after the gyros began spinup. Canopus lock was lost six times within a period of less than three weeks after launch and each time a sequence of radio commands would be required to reacquire the star. After a study of the problem, the investigators concluded that the behavior was due to small dust particles that were being released from the spacecraft by some means and were drifting through the star sensor field-of-view. Sunlight scattered from the particles then appeared as illumination equivalent to that from a bright star. This would cause a roll error transient as the object passed through the field-of-view while the sensor was locked onto Canopus. When the object was bright enough that it exceeded the high gate limits at eight times the Canopus intensity, the spacecraft would automatically disacquire Canopus and initiate a roll search for a new star. Finally, a radio command was sent on December 17, 1964, that removed the high gate limit. There was no further loss of Canopus lock, although roll transients occurred 38 more times before encounter with Mars.