Unfrozen assets

The dark side of the moon, the deepest lunar craters and Jupiter’s satellite, Europa, are so cold that even robots may refuse to work there.

This presents a significant hurdle because it’s in these places where valuable resources may lie, in the form of ice. With the temperature as low as 43K (-230ºC), only rovers whose circuits are made of special materials that allow electrons to flow will keep moving.

So integrated circuits capable of withstanding extreme cold have to be designed if robotic rovers are to pave the way for the expected resumption of manned exploration.

The problem is that standard silicon circuitry effectively freezes when the temperature plunges and the electrons stop flowing. Silicon doped with germanium is a potential answer because laboratory experiments have shown it can still function right down to 5K.

But that’s in the lab — NASA would not risk wrecking a multi-million dollar high-profile mission by assuming the same material will behave identically in space. The fall-out from such a failure would make Beagle 2 seem successful by comparison.

So universities and industrial partners are collaborating to perfect circuits and components made of silicon-germanium (SiGe) on behalf of NASA.

The work is led by Prof John Cressler of Georgia Institute of Technology, with five other US universities and NASA’s own Jet Propulsion Laboratory.

Boeing is a major partner, along with the US division of BAE Systems, IBM and Lynguent.

The SiGe alloy is common and cheap enough — it is used in high- speed communications on Earth, operating at speeds of several gigahertz. In extreme space, though, it is more likely to operate at less than 5MHz, assuming that the researchers can make it work.

The project is well under way and Boeing’s team leader Dr Leora Peltz explained their achievements so far.

‘We completed phase one in April, to mitigate the technical risks of the project by fabricating and demonstrating prototypes of a “circuit primitive”, a digital-to-analogue converter,’ said Peltz. They are building on that success by working towards developing mixed-signal circuits for processing analogue and digital signals. These are vital for the electronics that operate, control, monitor and reconfigure many space systems.

‘First, we have to design new tools that can be used to design the SiGe circuits,’ said Peltz. ‘The current CAD tools only know the alloy’s physics to -55ºC, which is the lowest temperature required by the military. We’ve got to get that down to 43K and calibrate the model against experimental data.’

The next stage is to use the tool to design and fabricate model circuits.

These have to be integrated and packaged, and the types of application for which they are suitable have to be specified. The reliability and radiation tolerance of the materials, circuits and packages has to be assessed. Then the performance of the devices in extreme cold and radiation has to be demonstrated.

And it all has to be done within three years if SiGe is to help the next lunar robot mission, currently planned for 2011.

If all goes to plan, SiGe electronics will eliminate the need for housing the electronics in warm boxes which in turn will conserve energy, reduce launch weight and improve reliability.

The technology will allow extended mission range and duration, could be used in lunar landers, hoppers, rovers or data-gathering stations, and could also benefit space transportation systems for returning astronauts to the moon, Mars and beyond. And robots will never freeze.