Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


11.3.3  Avian Propulsion

The air is such a useful niche that it will probably be occupied on any world with atmosphere. The ability to fly has been evolved by each of the three major animal phyla of Earth. Among the molluscs there is a cephalopod of the genus Omnastrephes, often called the "flying squid," having broad lateral flippers or fins which allow it to leap far out of the water. Most prominent among the arthropod fliers are the insects, whose aerial acrobats are well known.

The chordates too have many avian species. Flying fishes (e.g., the "flying gurnards") can leap from the water and glide hundreds of meters before touching down. The South American Gastropelecus has been observed to progress through the air by rapid beats of the enormous pectoral muscles of its fins. With its keel hardly cutting the water’s surface, this fish eventually emerges into true (but not prolonged) powered flight. The Catalina Flying Fish is also reputed to be a strong flier.224

Reptilian gliders are widely known, including the extinct giant pterodactyl (9-12 meter wingspan) and archaeopteryx, the flying lizards (Draco), the so-called Flying Snake of Borneo, and so forth. Birds are an entire class of chordates subscribing to an avian existence, and most members of Aves can fly. There are also the gliding mammals, such as the flying lemur and the many species of flying squirrels, and flying foxes and bats beat the air with their wings much like birds. There are at least three species of aerial marsupials.450

Powered flight presents a unique challenge for lifeforms on other planets. The central problem is the need for a high power/weight ratio -- that is, to generate enough power to get above the stall speed for the design and into the air. (The stall speed is the slowest an object can fly and still remain aloft.)

Typical power utilization curves for extraterrestrial avians are shown in Figure 11.4. (The exact details of the curves are governed by something called the Reynolds number of the wing, which relates power consumption to the actual shape of the airfoil.) As we see, for any given wing shape and given atmospheric conditions, there is some optimum velocity at which minimum power is required. Flying faster or slower is less efficient and costs more energy.


Figure 11.4 Avian Power Utilization Curves2426

Small Avians
Large Avians
Power required for flight is high during hovering (far left of each curve) and greatest at high velocities (far right of each curve) than at the intermediate "optimum velocity" in the trough.

Small avians such as pigeons tend not to have serious power/weight ratio problems. Continuously available muscular energy is sufficient to propel the animal comfortably through a wide range of speeds (shaded area in trough). Short duration "sprint power" permits an even wider velocity range, and is often enough to allow jump-takeoffs from a standing start ("grasshopper effect") or brief periods of hovering.

Large avians such as vultures and albatrosses are not so fortunate. Their power/weight ratio is too low to allow protracted flight even at the optimum velocity for their wings. Hence, the large aerial animal must find a long run way, or dive from a high perch, to reach stall speed (the minimum for flight), and then use short duration sprint power to remain airborne long enough to attain optimum velocity and begin relatively exertionless soaring flight.


If the creature flies too slowly it cannot remain aloft, and will stall out. The speed at which this stall occurs is inversely proportional to the square root of the density of the atmosphere.224

Given the same aerodynamic design, avians on a world with air as thick as the atmosphere of Venus could remain airborne at speeds ten times slower than on Earth. Conversely, on planets with Martian-thin air the stall speed would be ten times faster than on Earth.

Stall speed is also inversely proportional to the square root of the surface area of the wing.

An ET with huge wings can fly much slower -- and not stall out -- than an avian with the same shape but with tiny wings. An alien with 100 m2 of wing should be able to cruise as much as ten times slower than a creature of similar design with only 1 m2 of airfoil -- assuming identical atmospheric conditions.

Exactly how do we go about designing an extraterrestrial avian? How big can they be?

On Earth, the albatross is pretty close to the maximum. This 10 kg bird has been known to achieve wingspans of up to four meters. (The most massive aerial animal that has ever lived probably weighed less than 20 kg, although there are reports of an enormously fat cock bird shot down over the Transvaal in 1892 which measured 24½ kg.360) The albatross requires a lengthy "runway" for takeoff. When it lands, it must use wing flaps like a commercial jetliner to lower the stall speed sufficiently to land safely at about 20 kph.

The primary determinant of avian size turns out to be atmospheric pressure, not gravity as many erroneously believe. It should also be pointed out that the two are unrelated. High gravity does not imply high surface pressure, as is clearly demonstrated by the members of our own solar system. For instance Venus, our sister planet, has 9000% more pressure but 12% less gravity than Earth.

A good empirical relation that seems to work well for most aerodynamic lifeforms is: S = 0.24P-1(MG)0.82, where S is total wing surface area (m2), P is air pressure (atm), M is total body mass (kg), and G is planetary surface gravity (Earth-gees).1749

So on high pressure worlds, alien avians can make do with vastly smaller wings. If larger wings are retained, massive bodies can be maintained aloft. An extraterrestrial with the mass of a man, standing on the surface of a one-gee, 5-atm world, could fly with the wings of an albatross. An albatross, on the other hand, could make do with less than half the original wingspan. On a 100-atm world, the 10 kg bird could be supported by stubby finlike airfoils a mere 30 cm in breadth.

Low-pressure worlds are not amenable to large avian lifeforms (Figure 11.5). Lift falls off rapidly, and nothing more massive than perhaps a small pigeon would be able to take to the air. The force of gravity plays a secondary role in fixing the size and flight characteristics of extraterrestrial bird life. On a heavy 2.2-gee planet, the wing area would have to be about 90% larger than an Earthly avian of comparable design. On a bantamweight 0.16-gee world, wing area could be reduced by as much as 80% without losing the ability to fly.


Figure 11.5 Power Requirements for Active Flight vs. Walking Compared, for Earth-Dwellers

Minimum cost of transport is given both for surface and for aerial niches. Open circles are the insect data, squares are for birds, and filled circles are land mammals. The values represent the coat to an animal of transporting one kilogram of its bulk one kilometer of distance. Note that it is generally much cheaper to fly than to walk or run, except for very large organisms.

The best-fit curves for the above data are as follows:
    AIR: Cost : 1.25(MG)-0.227
    LAND: Cost 10e-1, e = 1.67(MG)-0.126
where M is total mass of the animal (kg), G is planetary surface gravity (goes), and "Cost’ is in Kcal/kg-km.

Kcal may be converted to joules by multiplying by the factor 4190 joules/Kcal.

Full line at lower right indicates the power/weight ratio needed to sustain steady flight. Animals in the hatched area generate insufficient power to fly.

 Values are calculated on the basis of aerodynamically optimal wing design, at the given mass on a 1-gee planet. This "minimum power" line varies for different worlds, and in fact: P ~ 1/SQRT(r) and P ~ g where P is the minimum power for flight, r is the atmospheric pressure (or density), and g is surface gravity.

Horsepower may be converted to watts by multiplying by the factor 745.7 watts/horsepower.


Gravity also affects the stall speed. In fact, the minimum velocity at takeoff ~g1/2. This has several interesting consequences.

On a 2.2-gee planet the stall speed would be about 50% higher. An extraterrestrial heavy-world albatross would require a correspondingly lengthened runway to get off the ground.

The problem of takeoff is greatly simplified on lighter planets. At 0.16 gees, liftoff occurs at a speed 60% below the nominal Earthly value for the same animal. Avian ETs could perhaps take advantage of what has been called the "grasshopper effect": An animal the size of a large pigeon could easily hop into the minimum airspeed from a standing start on the ground.86

In light of all the evidence, we are most likely to encounter large, intelligent winged avians on relatively small worlds with high atmospheric surface pressures.

How many wings are best?

The first winged insects on ancient Earth probably had no more than three pairs. A few modern insects retain vestigial traces of the third pair, notably the Stenodictya of the Order Strepsiptera.1212 By and large, however, aerial arthropods have cut down on the number of wings, Locusts, ant lions, fishflies, termites and aphids each have two pairs apiece, but a single pair is far more common in the animal world,

There are good reasons to reduce the wing count. The usual arrangement is a single pair, which serves more or less in the capacity of a helicopter rotor -- that is, to generate active lift. Another less common design is to use one pair of wings to generate passive lift (like the wings of an airplane) and a second pair taking the more active role. (Beetles come close to doing just this.) But no useful purpose is served by adding more wings, which would only interfere with the smooth airflow and ruin the aerodynamics of the design.1212

Using the principle of economy, extraterrestrial avians will have one pair, or at most two pairs, of wings.

Or they may have no wings at all. Thus far we have discussed only the most common techniques of flight known on Earth. But there are many other -- wingless -- ways to get into the air. On this planet, the principles of the rocket, the kite, and the balloon have not been widely exploited.

Imagine a world with a thick atmosphere rich in oxygen and abundant seas. Evolution might favor large but slow-moving insects as discussed earlier in connection with respiration. We might expect a kind of "rocket fish" to arise near the coasts, feeding where such insect life swarmed thickly in the air.

Like the small plastic toy projectiles that shoot high into the air when fully charged with water and compressed gas, the rocket fish suddenly would bolt from the sea skyward and mouth their aerial prey at high speeds. To be able to eat on the run, this jet-propelled alien predator has to evolve a sturdy posterior pressure canister which can be discharged rapidly through a rigid nozzle. The rocket fish might have a recycling time on the order of minutes, and might charge their canisters using osmotic pressure.

Another class of alien creatures might take to the air on other worlds. A low gravity planet with fast rotation and a thick atmosphere could be ideal for the evolution of "parachute beasts." These ETs could travel virtually anywhere for free, simply by extending their retractable ‘chutes. With strong winds, they could tack across the planetary surface. With a good stiff breeze, even fairly massive parachute beasts could kite hundreds of kilometers each day without effort.

There is some precedent among Earthly fauna. In the majority of spider species, aerial dispersal of the young takes place. A spiderling crawls to the end of a blade of grass or other protuberance, raises its tiny abdomen, and lets fly a thin thread of silk into the wind. As the gossamer strand is caught by a breeze, the spiderling leaps from its perch and climbs to the middle of this floating "magic carpet." Air currents carry the animal to considerable distance: Spider threads have been sighted at least as high as 8 km and as far seaward as 500 km from shore, Couldn’t extraterrestrial "hang gliders" do just as well?

The idea of the balloon principle in relation to living organisms is a common one, both in science and science fiction.20,442,2427 It appears, for instance, in a nineteenth-century novel by the French writer Charles Ischir Defontenay -- Psi Cassiopeia (1854):

Here is what the Starian naturalists say of this animal, which they call the psargino: Its skin, which has great extensibility, is only attached at the eyes, the mouth, the other natural openings of the body and the soles of the feet. Over the rest of its expanse, it is only juxtaposed to another membrane or internal skin having the property of secreting, at the animal’s will, {hydrogen} gas. The psargino, thus surrounded by gas, becomes a sort of balloon lighter than the atmosphere, and it makes use of this property to rise into the air and escape its enemies. A kind of aperture furnished with valvules on its abdomen relieves it of part or all of the gas burdening it and serves for descent to earth when the predator has lost its track.564

Bonnie Dalzell has designed an "airship beast" for a planet with cold winters, heavy gravity and a thick atmosphere.736 Twice a year this herbivorous 100 kg animal inflates its lifting bags with metabolically generated hydrogen gas* and drifts to the opposite hemisphere to avoid the cold. Strong winds are an advantage, but predators are widespread. And many of these living balloons are lost during their semiannual migration when lightning from an electrical storm strikes and ignites their bodies.

There are a few indirect precedents for such a creature among the lifeforms of our own world.2574 For example, the Portuguese man-of-war (Physalia) is a shapeless, baglike marine organism whose large air sac serves both as a float and a sail in the water, Its numerous tentacles perform many functions, including stinging its prey into senselessness,

Another creature even more analogous to the airship beast is the chambered nautilus. Some 3000 species of this animal once flourished in the primitive, shallow seas of our planet. They are found as deep as 700 meters, and although they measure a mere 0.2 meters today their ancestral forebears left fossil remains up to three meters in diameter.584

The nautilus is a miniature submarine, consisting of a series of as many as forty individual chambers partially filled with air. All are connected by a thin tube called the siphuncle, which is thought to control the buoyancy of the animal as it dives and ascends in water.

As the man-of-war and the chambered nautilus utilize the principle of buoyant lift in the seas of Earth, why could not giant living extraterrestrial gasbags ply the skies of alien worlds?


* Many bacterial photosynthetic autotrophs generate hydrogen metabolically. Examples include Clostridium, Chromatium, Athiorhodaceae, and green algae under certain special circumstances.


Last updated on 6 December 2008