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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.
The most recent robotic space mission to start work is the Curiosity Mars Science Laboratory rover, which has just embarked on its workload following a month of post-landing checks.
Its main computers are a pair of identical Rover Compute Element boards, based on RAD750 processors running at 200MHz. This is a radiation-hardened version of the PowerPC 750 architecture, most notably used in Apple products, including the original iMacs. The Curiosity systems have 2GB of flash memory, 256MB of DRAM and 256kB of EEPROM.
The two systems are entirely interchangeable in use, with the main/backup designation being arbitrary.
The boards run the real-time VxWorks OS from Wind River Systems, which is now owned by Intel. VxWorks has been used or is planned for use on 11 spacecraft, including the two previous Mars Rovers, Opportunity and Spirit, as well as in BMW cars, Apache attack helicopters and Drobo network storage devices.