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The Viking project to land robot laboratories on Mars saw many developments that influenced spacecraft design throughout the '70s and '80s.
The landers pioneered full redundancy and local processing, and were capable of autonomously guiding themselves to the surface. The processors themselves were dual Honeywell HDC 402 processors, 24-bit discrete logic designs running at around 300KHz and each with 18 Kwords of plated-wire memory.
The system software was developed by aerospace contractor Martin Marietta and included virtualisation and multitasking features.
The original plan was to use a more capable computer that could have its instructions changed remotely, but weight problems caused by other aspects of the spacecraft design forced the selection of the lighter — but only third-choice — Honeywell systems.
However, Martin Marietta had adopted a software-first design policy, in contrast to previous projects where the hardware came first, imposing strict limits to changes. With software-first, the developers ran their software in an emulator — indeed, the final hardware was only delivered a couple of months before launch — and were thus able to adapt more quickly to specification changes and discoveries.
More details on the Viking and related projects can be found here.
The two Voyager probes launched in 1977 to study Jupiter, Saturn, Uranus and Pluto, and remain in contact with Earth as they move into interstellar space.
The probes have identical designs, with six computers apiece in three roles. Each has a duplicate Computer Command System (CCS) — 18-bit word, interrupt-type processors with 4,096 words each of plated wire, non-volatile memory; a duplicate Flight Data System (FDS) — a 16-bit word machine with modular memories and 8,198 words each; and a duplicate Attitude and Articulation Control System (AACS) — 18-bit word machines with 4,096 words each. That's a total of around 88 Kbytes of memory.
Each computer is a custom design by the Jet Propulsion Laboratory (JPL), which runs the project, and is built by General Electric. The CCS runs the spacecraft and reports problems back to base; the FDS manages the scientific instruments and prepares data for transmission back to Earth; while the AACS performs thruster and instrument platform manoeuvres.
Both Voyager probes have suffered a variety of hardware failures during their lives. Most of their experiments are now turned off, following the last planetary encounters, but with reprogramming and about eight years of life left in the nuclear generators, both have some distance left to run.
At seven metres high and four wide, with a launch weight of 5,600kg, the Cassini mission to Saturn is the largest planetary mission to date. Blasting off in 1997, it arrived at Saturn in 2004 and continues to observe the planet, its rings and moons, with mission-end planned for 2017.
Its 12 instrumentation packages contain many processors, but the core guidance and control is provided by dual Harris (now Intersil) RTX 2010RH processors running at six million instructions a second, 192kB of radiation-hardened static RAM (SRAM), 8 or 4kB of ROM, and 64kB radiation-hardened electrically erasable ROM (EEPROM, a precursor to flash). There's more memory on a support board — 832kB SRAM and 64kB EEPROM.
These systems are most notable for being hardware-implemented Forth systems — Forth being an obscure programming language initially designed to control radio telescopes with a small but fanatical following among technical programmers.