In cryptozoology, a skyfish, or “rod”, is a supposed atmospheric entity that travels too fast to be seen by the unaided eye. A relatively new addition to the cryptozoological laundry list, rods were ‘discovered’ in the early 1990s but debunked by 2003 at the latest. It turns out that they are just videographic artifacts, produced by the motion blur of a conventional insect being filmed at 60 fps.
How fast does something need to travel to move too fast to be seen? Of course this depends on its size and distance. According to this analysis of human vision, Air Force pilots were able to identify an image of a plane flashed in front of them for only 1/250th of a second. This is around the limits of human vision. If the flash were only for 1/500th of a second, it is nearly certain that they wouldn’t even notice it.
Imagine a rod that moves 500 times its own length in a second. Even using a super-expensive 500 fps video camera, the skyfish wouldn’t show up in the same place in more than a frame for any longer than 1/500th of a second, making it thoroughly invisible to the naked eye. For a rod 10 cm in length, that would be 50 m/s, or about 110 mph. For a larger rod 50 cm in length, it would need to travel at around 250 m/s, or more like 550 mph. Indeed, the Peregrine falcon, which moves at speeds of up to about 240 mph during steep dives, almost travels too fast to be seen without high fps photography.
Clearly, if a flying creature were discovered that traveled at 550 mph, it would be a boon to modern science. It is hypothesized that these creatures are made further harder to observe by their partial or total transparency. It has even been asserted (by UFO nuts) that they may be composed of an entirely undiscovered phase of matter, or possess the ability to pass in and out of an alternative dimension.
Unfortunately for insufficiently skeptical cryptozoologists, the skyfish has been thoroughly debunked. But might it one day be possible to engineer a small unmanned aerial vehicle (UAV) that moves too fast to be seen by the naked eye?
The surveillance potential of an artificial skyfish is undeniable. Imagine tens of thousands of these beasts flitting about in the mountains of Afghanistan, taking snapshots of people and facilities with greater resolution and at more angles than spy satellites or conventional UAVs could possibly muster. Traditional spying techniques would become all but obsolete.
The supposed method of locomotion for the fabled skyfish are undulating fins, similar to those used by the cuttlefish. According to this paper on cuttlefish locomotion, it appears that around 30% of the mass/volume of a cuttlefish is used for locomotion, images of the skyfish suggest that more like 50% of its mass/volume consists of its undulating ‘fins’, a fin-to-body ratio of 1:1, an improvement on the apparent 1:2 ratio found in cuttlefish. But a factor-of-2 improvement would hardly be enough to make up for 1:800 density difference between air and water. Cuttlefish also use jetting, like squids, to propel themselves along, whereas an artificial skyfish would probably not have that convenience.
A man-made rod would qualify as a µAV (micro air-vehicle). µAVs today are about 15 cm long and fly at 10 to 20 m/s (22 to 45 mph). Here is a diagram comparing MAVs to other flight vehicles:
Because the Reynolds number for tiny flyers is so low, they cannot rely on lift to move along, but must instead flap, undulate, or use propellers operating at 10 Hz+ to stay aloft. Power systems with high energy densities are essential. For this reason, leaders in the field of µAV research, the Entomopter Project in particular, use chemical energy sources, which offer superior energy density to electrical power storage. The amount of energy you can store in a drop of gasoline is much greater than what could be stored in a battery of similar size. A milliliter of gasoline contains 32 kilojoules, which is about 9 watt-hours condensed into an amount of fuel that weighs less than one gram.
The Entomopter Project uses something called Reciprocating Chemical Muscle (RCM), “motivated chiefly by the basic necessity for very high rate of energy release from compact energy sources”. Here is their graph for necessary power versus forward velocity for various masses, using their particular µAV design:
A key observation made in the paper is that with each doubling of mass, nearly eight times as much power is required to propel the craft to a given speed. The upshot is that with each halving of mass, almost eight times as little power would be necessary to achieve the same velocity. In the entomopter testbed experiments, 1 ml of fuel offered 13 watts for a 100 g craft, allowing it to fly for about 30 seconds. The paper argues that if the mass of the µAV were halved, it would have been able to fly for 3 minutes using the same amount of fuel, underscoring the importance of weight to effective micro-flight.
The entomopter’s actuator system was milled from steel. In the not-too-distant future, we will have actuators built from superior materials, such as carbon nanotubes, which achieve 60 times the strength of steel with 1/5th the density. More sophisticated synthesis techniques are dropping the costs of carbon nanotubes to dollars per gram. Already, highly purified nanotubes are available for $500/gram.
If a µAV today weighs about 100 g and flies at 10 to 20 m/s using 13W, surely we can imagine a µAV in 2025, built primarily from carbon nanotubes, that weighs approximately 10 g and travels 100 to 200 m/s (220 - 440 mph), too fast to be seen with the unaided eye. These artificial skyfish could be deployed by aircraft by the hundreds, or launched out of shoulder-mounted launchers and controlled using laptops. To keep the skyfish from being captured by the enemy, they would need to be programmed to automatically return to base at appropriate intervals for refueling or recovery. Using simple robotic joints and locks, skyfish that run out of fuel could be instructed to break themselves into hundreds of tiny pieces, too small to identify on the ground without sophisticated equipment. Metamaterials could be used to actually make the skyfish totally transparent.
The list of potential applications for artificial skyfish is quite large, if you use your imagination. They could be designed to kamikaze into enemy planes or missiles, rendering them impotent. They could deposit tiny sensors into enemy buildings, recording the conversations inside and transmitting them to the skyfish, which forwards the recordings to intelligence. Working in cooperation, skyfish could even carry kg-sized payloads to their destinations, or embed themselves in enemy vehicles, awaiting activation. More advanced versions of the future could weigh even less, traveling longer distances without refueling. We may not see an artificial skyfish project proposed in the next ten years, but when you see the headline from DARPA announcing cutting-edge research into high-velocity, stealth µAVs, remember that you first heard of the concept from some guy named Anissimov.
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