Ziggy Stardust, one of David Bowie's guises, had a band called the Spiders from Mars. Bowie may have had an inkling of the future, since insect occupation looks more likely than human colonisation, despite Nasa's announcement this week of what could be evidence of a frozen ocean.
Space researchers are now planning to explore Mars using not spiders, but their common prey, flies. Man-made dragonflies, in fact. Science and technology often learn things from nature. Dragonflies flit around with wingspans of up to six or seven inches, but in the Carboniferous era, around 280m years ago, they had cousins almost five times that size.
One of the largest early dragonfly fossils, 24in from wingtip to wingtip, was found deep below ground in the north-east Somerset coalfields, a steamy jungle in those days. Why don't such behemoths inhabit today's swamps and village ponds?
The favoured answer is oxygen. The oxygen content of the atmosphere a few hun dred million years ago was around 35%, compared with today's 21%. Larger insects could thrive. The reason stems from the way in which insects (and other invertebrates such as arachnids) breathe.
Vertebrate land animals such as ourselves repeatedly suck large volumes of air into our lungs, whereas insects depend to some extent upon the rate of diffusion of oxygen within the air.
Flies and the like have spiracles, openings on their bodies through which air molecules can enter, but the rate at which they are replenished (as oxygen is extracted and carbon dioxide expelled) is limited, resulting in a maximum size to which insects can grow. This and other mechanical reasons, such as the weight an exoskeleton can bear without having a prohibitively large bulk, imposes a size limit, meaning that sci-fi movies with giant mutated ants and spiders are just that: works of fiction.
Scientists often claimed it was impossible for bumblebees to fly, but five years ago, its flight was at last explained. A team under Charlie Ellington, professor of animal mechanics at Cambridge University, worked out how these leviathan bees buzz around. Now they're looking at applying this understanding in other areas: in improving the performance of helicopter rotors, for example.
But there are applications that are literally off this planet. Another limitation on aerial insects is imposed by the viscosity of the atmosphere. To butterflies, the air is like treacle, enabling them to fly but limiting how large they can grow. At some finite wingspan, the muscle power needed to beat those wings results in too bulky a body, and so gravity wins.
This would not be the case on Mars, though. First, Martian surface gravity is only 37% that on Earth, representing an advantage for a flying machine. Second, and more significant, the atmosphere of Mars is much thinner, making a robotic dragonfly a feasible tool for the near-surface exploration of that planet.
The essential parameter is what is called the Reynolds number. This dimensionless number indicates the conditions under which there will be a change from laminar (or streamlined) to turbulent flow of a fluid around a solid object, with a concomitant change in the drag.
On Mars, the atmosphere is so thin that the Reynolds number is equivalent to that of an altitude of around 30 kilometres on Earth. This means we should be able to make artificial dragonflies, or their kin, with wings that will beat fast enough to hover just over the Martian surface. This would be a huge boost for Mars exploration. Mapping from orbit is wonderful for wide area coverage, but the detail is limited by the distance of the satellite above the ground. At the other extreme, a surface rover is able to examine the soil and rocks grain by grain, but only a verysmall region is covered.
To date, the two favoured options, suggested for Nasa missions later this decade, are aeroplanes and balloons. The aircraft idea suffers severe drawbacks: a speed of at least 250mph is needed to get enough lift, which necessitates a huge wingspan, and there's nowhere to land: one strike and you're out. The balloon-borne probe would be easier to implement, but it has no navigational capabilities. The Martian surface relief is also vast, with volcanoes far taller than Everest and a chasm vaster than the Grand Canyon. For safety, the balloon would need to maintain a fair altitude, limiting the detail of the data collected.
Proponents of the artificial dragonfly point to its ability to overcome these problems. Professor Robert Michelson, of the Georgia Tech Research Institute, argues strongly for such a mechanical beast, which he calls an entomopter, after entomology, the study of insects. As Michelson points out, "because of its ability to fly slowly, land, and refuel, the entomopter may be the longest lived and therefore potentially the most useful unmanned machine to fly on Mars." It could stop to study specific features, perhaps collecting samples to return to a home base where it would be recharged for its next foray.
Making imitation wings strong and light enough is relatively straightforward. Michelson's team has this in hand. Rather more complicated is driving those wings. Conventional electric motors and batteries are far too heavy, a problem exacerbated by the gears and cams needed to facilitate the continually changing tilt and drive of the flapping wings.
Again, nature provides the answer. The entomopter uses what is known as a reciprocating chemical muscle. In short, these are man-made muscles, and they use chemical energy to generate movement. Entomopter development has taken off. An entomopter could easily be tested in a large vacuum chamber at 1% atmospheric pressure, the same as on the red planet. Planetary exploration is only one of the possible implementations. If a model can be built that will fly in our atmosphere, then the surveillance of hostile regions will be revolutionised. The concept of being buzzed will obtain a whole new meaning.
· Duncan Steel is reader in space technology at the University of Salford
