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THE hair-like projections that sweep debris out of your windpipe have
inspired a new docking mechanism for miniature spacecraft. Engineers in the US
say a coating of cilia-like protuberances on the skin of a spacecraft could
guide miniature satellites as they dock with their mother ship.

Such cilia could be key to future space missions that will rely on the help
of tiny 鈥減icosatellites鈥� weighing less than a kilogram and small enough to fit
in the palm of your hand. These diminutive rovers will act as mobile eyes that
can inspect a larger spacecraft for damage from micrometeoroids, say, or check
that sensors are deployed successfully. Precision docking is critical for
refuelling these rovers and downloading their video and sensor data.

Arrays of microcilia will make the docking quick and efficient, say its
inventors. An incoming picosatellite manoeuvres towards a cilia-coated area of
the spacecraft and uses magnets to cling to the bristly surface. The undulating
movement of the thousands of cilia will then sweep the picosatellite along the
surface of the craft towards its docking bay, in the same way windpipe cilia
pass particle-laden mucus towards the throat.

鈥淭he space cilia are difficult to damage, which is an advantage in that harsh
environment,鈥� says Karl B枚hringer at the University of Washington in
Seattle, who has worked with Mason Terry and Joel Reiter to adapt the idea for
spacecraft docking. The cilia were originally developed by Gregory Kovacs and
John Suh of Stanford University, now at Xerox in California.

Although the cilia are only 0.5 millimetres tall, they are remarkably
complex. Suh made them by depositing layers of various materials on a flat
silicon plate, and then carving them out in rosettes of four, each rosette
looking like a four-leaf clover
(see Graphic). Inside each cilium is a
titanium-tungsten heating element. The cilium is normally curved up and away
from the plate, but when the bimetallic strip heats up, it forces the cilium to
flatten down towards the plate. Cilia rhythmically straightening and then
relaxing in waves propels the picosatellite to its docking station.FIG-mg23214901.JPG

To move a picosatellite to the right, for example, B枚hringer applies
heat only to the right-hand cilia of each four-leaf rosette. As these cilia
flatten they drag the satellite slightly to the right. Then B枚hringer
allows the left-hand cilia to cool and curl up to the right, pushing the
satellite slightly further along. By continually repeating the cycle, he can
waft the picosatellite to the right at about one centimetre a minute.

In tests, the microcilia arrays successfully shifted 40-gram blocks of
aluminium. He calculates that a circular patch of cilia just 50 centimetres
across should be sufficient to manoeuvre a 40-kilogram satellite.

Picosatellites carry little fuel, so they must be able to dock quickly and
efficiently, says Robert Twiggs, who designs and builds picosatellites at
Stanford University. 鈥淢icrocilia sound like a fabulous way to accomplish this.鈥�