By Gert van Dijk and Abbydon
If you like speculative biology, you cannot escape the Avatar films: they are spectacular. Regular readers will know that this blog likes its science 'well done' rather than 'medium' or 'rare'. But with television and film 'medium' is usually the best you can hope for. If the story is good enough, we are more willing to suspend disbelief. The avatar films are spectacular but have their share of problems: the illogical anatomy and gaits of Avatar's six-legged animals were something best ignored in the first film, and the skimwing's size and mode of swimming in the second film did not withstand close inspection either. This post is aimed at the third film ('Fire and Ash'); as that is not even out yet, isn't it too early to start dissecting its biology?
Based on the trailer, we thought we could have a first close look. 'We' here means Abbydon, who is a physicist, and me (Gert van Dijk/ Sigmund Nastrazzurro). Abbydon has his own blog and has written guest posts here before, on the subject of aerographene and foam as a way to make viable 'ballonts'. 'Ballonts', by the way, is a term one of us (Gert) came up with to describe life forms that move through the air using a lighter-than-air principle. At one point, I imagined a large array of floating lifeforms on Furaha, ranging from tiny aeroplankton to immense 'zeppeloons'. That bubble burst when I did the mathematics, so all those lifeforms underwent sudden mass extinction. If you wish to follow the mathematics (they are just Archimedes' principle), there is a list of posts at the end of this post.
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| Click to enlarge; source: Avatar 3 trailer |
The trailer for Avatar 3 is out, and it's got ballonts in it. Seeing that nature seems to conspire against ballonts, we looked at it critically. Let's start with a description.
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| Click to enlarge; from Avatar trailer |
There seem to be two ballont species: a large one, a 'barge', towed by a smaller one, the 'tug'. Apparently, these are known as 'medusa' and 'manta kite', respectively. A Na'avi-made ship is suspended from the barge animal so the Na'avi can use it for aerial transport. The medusa/barge animal largely consists of a large sac, elongated from front to back. It has two lateral vertical surfaces that we will call sails and tendrils hanging down, probably to feed with and to anchor itself. The tug is much smaller and has lateral undulating fins, rather like Earth's rays, cuttlefish and Furahan cloakfish. Those fins propel it.
What does this tell us?
Ballonts need to be very large to work (read the posts on ballonts to understand why). Gravity on the moon Pandora, where all of this takes place, is said to be low, which sounds good for balloons. But, and this may surprise you, low gravity doesn't make a balloon more practical! 'Practical', as far as a balloon goes, means a small bladder and a large liftable mass. On Earth, physical circumstances makes balloons impractical by dictating that they must have a very, very large bladder to lift even a small mass. Gravity does NOT influence the balance between the size of the bladder and the mass to be lifted and so does not help to make a balloon practical. Two things that do help are a high density of the atmosphere, which can be achieved by adding heavy gases to it, and a high pressure. Pandora is said to have a density that is 20% more than that of Earth, while the surface pressure is a bit lower at 0.9 atmosphere. Those changes are not impressive from a ballooning point of view.
The Pandoran barge looks very large, which it has to be; so far so good. But why does it have those two sails at its sides? To catch the wind for propulsion? We hope not, as that cannot work! Balloons are, by their nature, as light as the air around them, so they will, after a short while, move at exactly the same speed as the air around them. That leaves no wind to power anything! You can only harness the power of the wind if the air moves relative to you, for instance because you are held back by the ground or by water.
Or do the 'sails' serve some other purpose? Are they themselves a source of propulsion? They could perhaps function like oars, folded up when moving forwards and spread out when going backwards. Or do they undulate? As they are vertical, undulation would allow vertical but not horizontal mobility. But the sails are completely immobile and the barges do not seem to have any kind of propulsions by themselves (if they did, they probably wouldn't have to be towed). Or do the sail serve another purpose, such as heating? This is unlikely, as they are transparent and the sac should offer enough surface area anyway. Do they help to orientate the animal with help of the wind? If so, the purpose eludes us, and you would want them at one end of the animal, not the middle. In short, we cannot make sense of the barge's sails.
Is the tug, the manta kite, large enough to float? Without a better estimate of its size there is no way to check. The undulating fins can provide some propulsion force in air, but probably not much. If this were an animal swimming in water, fins of this relative size would work because they would displace a substantial volume of water, which is heavy. But swimming through air differs from swimming through water: there is a thousandfold difference in density that affects thrust and drag, as well as a fiftyfold difference in viscosity. No air animal use undulation to achieve true flight on Earth, making it difficult to predict how undulating flight would work out. We suspect that you would need very large or very fast-moving fins to effectively swim through air. Whether an animal like the manta kite would swim well in air is air is uncertain, but its proportions suggest to us that it might feel more at home under water than in the air.
The tug does not only have to move itself but also has to drag the barge along. And 'drag' is a key word here, in terms of resistance to movement. We can be certain about one thing: those immense barge sails will function as air brakes, making the tug's job that much harder...
Mind you, there are some interesting loose ends about balloons and their steering that may deserve another post. Meanwhile, we hope that the film will solve the riddles. Our biggest surprise was that the trailer seems to show sails to catch the wind on a free-floating balloon; but surely the designers wouldn't have done that?
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Posts on ballonts that help understand the physics
Ballonts III: basic physics
Ballonts IV: effects of density and pressure
Ballonts V: ballonts in gas giants
Ballonts VI: effects of the envelope
Ballonts VIII: foam
Ballonts IX: aerographene
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