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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.