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Star Cores: Computers in space

Posted on 3 Sep 2008 at 14:26

The Phoenix Mars Lander mission will be facing the sun for the duration of its three-month mission, so can rely purely on solar power to conduct its work. But solar panels produce modest power, and the RAD6000 computer is given an allocation of only 20W. "Typical power budgets for deep-space missions are of the order of a few tens of watts for the entire computer system," said Some. "This impacts not only the hardware that may be flown, but also the software that can be accommodated - both of which must be extremely efficient to ensure correct and timely operation within the power constraints."

While solar power is the most viable source of power for missions such as the Phoenix Mars Lander, once you get further away from the sun, it becomes less viable. In these cases - such as for the twin Voyager craft - radioisotope thermoelectric generators (RTGs) are used instead. The energy generated by the isotopic decay process of plutonium oxide is converted into heat and, in turn, to power by a thermoelectric converter. Each of the Voyager craft has three such generators on board that provide power to all its systems, including the on-board computers. Over time, there is a gradual decrease in the amount of energy they create, which means non-essential systems are slowly being switched off, but the RTGs are expected to provide enough power for the crafts to function until at least 2020.

The sheer variation in temperature is another problem that computer systems are exposed to. "Space can be extremely hot - surfaces facing the sun, or entering the atmospheres of Venus or Jupiter, for example - or extremely cold - surfaces facing deep space and shielded from the sun or when operating on the surface of Mars," Some told us. "There are two basic approaches used for dealing with these extreme environments: placing the computer equipment in a thermally-controlled environment, or designing the computer to operate in extreme temperature environments in the first place.

"The former, involving the use of 'cold plates' and 'warm boxes', is the most commonly used strategy. This allows the computer to experience relatively benign temperature ranges at the cost of electric heaters, thermal radiators, heat pipes and other thermal-management hardware. The latter - based on the use of custom materials and circuits that can operate through extremely hot or cold environments - is non-standard, but is the only approach available in certain cases, and is becoming more prevalent as available power and mass margins continue to decrease, and the environments being visited become more challenging."

Hardly software

Of course, the strongest, most durable hardware in the world means nothing if the software that controls it isn't designed to the same standard. You won't find Windows loaded on board any space shuttle - the conditions and the need for far higher levels of stability than any commercial package can offer means the software found on spacecraft is as bespoke as the hardware.

NASA's Some outlines the challenges: "Space missions require custom, one-off software that is correct and catastrophic-bug-free. It must operate in extreme environments, and in circumstances which are often unknown. In addition, the design of software systems needs to be tolerant to hardware faults as well as software faults.

"It must be possible for these software systems to be easily and reliably modified (patched, augmented or modified to accommodate new and unanticipated requirements, for example) after launch, from the ground, over low-bandwidth communication links and with minimal ability to remotely observe or control system operation during software mod installation, checkout and operation."