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The same technology that has revolutionised our lives has changed space science forever, building ever-cleverer robots exploring ever farther into space.
As the Voyager probes celebrate their 35th birthday and prepare to leave the Solar System, and as the Curiosity rover starts on the most exciting Mars mission ever, we look back at the IT under the hood that has made science fiction into everyday fact.
Launched in 1972, Pioneer 10 (above) was the first imaging mission to fly past Jupiter, Although it is widely reported that it used an Intel 4004 processor for navigation, mission sequencing and communication, it in fact used a custom processor system built from a variety of standard logic chips, with individual commands handled by dedicated circuits.
Storage was 49,152 bits of magnetic core memory, which was too heavy to be expanded further. But alternatives were pricey — a proposed upgrade to 1 megabit (256kB) of chip memory for a later mission was priced at $630,000 in 1974.
Pioneer 10 and its sister probe Pioneer 11 were the first missions to have a recognisably modern computer system capable of multiple commands and a measure of independent operation, due in part to the availability of high-density logic chips and the need to manage short, high-intensity science-gathering flypasts at a distance from Earth that precluded direct commands in real time.
However, they were still recognisably derived from earlier probe thinking, where pre-programmed sequences unaffected by local information drove manoeuvres.
An in-depth NASA design document discussing the Pioneer architecture and future developments into what would become the Voyager probes can be found here (PDF).
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.