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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.
Uniquely among unmanned spacecraft, the iconic Hubble Space Telescope has had its hardware changed several times during its mission since its launch in 1990. While most of those upgrade missions concentrated on replacing instruments, two included processor upgrades.
The original controlling computer was the DF-224 built by Rockwell Autonetics, which has three 8-bit 1.25MHz custom CPUs and six 8K 24-bit plated-wire memory units. This was a general purpose, space-qualified unit.
By 1992, two of the six memory units on the DF-224 memory units had failed, and a minimum of three working units are needed for spacecraft operation. A co-processor was installed on the first servicing mission in 1993. It had two sets of 80386/80387 processor/numeric processor pairs, each with 1MB of RAM and 256kB EEPROM, plus 384kB of non-alterable permanent ROM.
The DF-224 was replaced altogether on the third servicing mission in 1999 by the Advanced Computer. This has three Intel 486 processors running at 25MHz, each with 2MB of SRAM and 1MB of EEPROM. This remains the main processor on the spacecraft, and with the retirement of the space shuttle, no further upgrades are possible.
The Hubble is expected to remain in use until at least 2014.