A tiny strip of metal could pave the way for a generation of spacecraft made of "smart" materials.

When the European Space Agency's Rosetta spacecraft lands on comet Wirtanen sometime around 2012, it will be performing something of an engineering experiment on top of its other duties. A super-simple yet sophisticated one-piece mechanism made of a shape memory alloy (SMA) will be crucial to understanding the comet's nucleus.

SMAs are so special because they behave rather like rubber bands or muscles, yet they're made of metal. Tony Anson, SMA expert and Director of Anson Medical Ltd. in Oxford, England, explains.

"Smart materials might be described as materials that can adapt in some way to a change in their physical environement - automatically," says Anson.

"Shape memory alloys can change their shape by heating. But, they have two generic properties, thermally induced shape recovery and super elasticity." he said. "This last term means that the metal alloy can be dramatically deformed, when the deformation load is removed, the metal springs back to its original shape simply by elsatic response; just like an elastic band."

At the ESA's Rosetta mission, engineer Martin Whalley faced a tiny but puzzling challenge -- keeping his helium clean.

Whalley is the Project Engineer for the Rosetta Lander's Ptolemy instrument; a device designed to "smell" comet Wirtanen for different substances.

"Ptolemy will ride on board the Lander down onto the surface of the comet's nucleus. Once there, the Lander will drill down below the surface and deliver tiny samples into an oven," Whalley explains.

"The oven converts the solid samples into gases, which then pass along a pipe into Ptolemy. Ptolemy can then determine which chemicals are present in the comet sample, and hence help to build up a detailed picture of what the comet is made from."

Which seems fairly straight forward, until one realizes that there's no way to "suck" up gas in the vacuum of space. On Earth, taking a sniff of something is an easy matter; the movement of a larger body of air coaxes particles of the sample into your nose or the intake of a sensing device. But to blow things around in space, you first have to supply the "air." And ideally, you don't want that air to carry its own aroma that will confuse your results. The Ptolemy team's choice? Pristine helium.

"Since the spacecraft takes over 8 yrs to reach the comet, even a tiny leak from the storage tanks would lead to a significant loss of gas. So the tanks are filled and then sealed completely before being integrated into the instrument," says Whalley.

But the big problem was how to "pop the cork" without any stray atoms getting mixed in after nearly a decade of knocking around the Solar System.

"The actuators used to break the seals are often explosive devices ("Pyrotechnic actuators") which are larger, heavier, consume more power and could contaminate the helium supply," Whalley recalls. That would be like opening a bottle of a valuable wine using a firecracker.

"We've been aware of SMAs for a while, but had never found a suitable application for them before," says Whalley.

"Here their light weight and simplicity are ideal. The shape of the SMA came about from thinking about the pushing action required to snap the seal - we tried several geometries before coming up with one we used; which straightens out in a manner similar to an extending finger (e.g. to press a button, doorbell etc.)."

So Whalley and his team brought in SMA experts like Anson to help design a tiny "finger" that could sit patiently poised over a button for eight years.

"The SMA we've used is an alloy of nickel and titanium, in the form of a strip (20mm long x 14mm wide x 0.5mm thick)," Whalley explains.

"This strip can be bent out of shape when cold, but when heated above a particular temperature, it recovers to a predetermined "memorized" shape, and produces force while doing so. We use this recovery force to snap open the seal in the tank and thus allow gas into the instrument."

Still, the makeup of the final mechanism presented a few more challenges.

"One difficulty was in getting the heat into the SMA to make it recover its memorized shape," Whalley remembers. "We used a small electrical film heater (like the heated rear window in a car) stuck to directly onto the SMA strip."

But then they ran afoul of the bane of every model kit builder: fumes from the glue. It took some shopping around, but they finally found an adhesive that wouldn't smell, crack, or melt when heated. But with that problem solved, Ptolemy is nearly ready for its January, 2003 launch aboard its mothership, Rosetta.

And Whalley points out that while the SMA on board Ptolemy may seem tiny, the uses for elastic alloys themselves are growing.

"SMAs are already at work in the world around us; directing the airflow in heating/air conditioning units, in resetting the switches on kettles and coffee makers as the water boils, and in various medical applications," he says. "Anywhere where a change in temperature can be used to trigger motion."