At over 600掳C, a jet engine鈥檚 exhaust stands out like a beacon when seen in infrared, making it an obvious target for heat-seeking missiles 鈥� but a US Navy device might be able to thwart such missiles with ghost images projected in mid-air.
The traditional method of defence has been to eject hot flares to draw missiles away from the aircraft. This new approach would use lasers rather than pyrotechnics.
When focused to a point, a laser can produce a spot or filament of ionised gas in the air, known as a laser-induced plasma (LIP). The US military has long shown interest in LIP to create artificial lightning to defuse bombs or produce 鈥渘on-lethal鈥� effects.
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Now, Alexandru Hening at the Naval Information Warfare Center Pacific in California is using lasers to create an array of plasma columns in the air. These are rastered like the beam in an old cathode-ray screen to create two or three-dimensional images in mid-air of ghost aircraft that can distract incoming missiles.
The patent describing the work was published in February, although it was filed in 2018. The exact status of the work and how close it is to operational deployment are unknown. The US Navy didn鈥檛 respond to requests for comment.
In principle, by tuning the plasma used in such a device, any desired wavelength could be produced from radio to gamma rays, so it should be possible to adapt the device to fool any future sensor. The challenge is likely to be producing a stable plasma with a powerful enough infrared output to be effective.
Gianluca Sarri at Queen鈥檚 University Belfast in the UK believes the plasma decoy is plausible in theory. He says the mechanism is well known and widely used in laser-plasma applications.
However, existing high-power lasers generally work at shorter wavelengths, which would be unsuitable. If a powerful mid-infrared laser were available, it would certainly be able to create a suitable plasma channel, he says.
While more complex and expensive than flares, a laser system would have significant advantages. A flare only works for a few seconds, and aircraft can only carry a limited supply, whereas a laser can continue producing decoys for as long as it has power.
Flares also take some time to deploy and quickly fall away from the launch aircraft, whereas a laser can generate a ghost image in less than a microsecond and place it up to a few kilometres away from the plane carrying the device.
One intriguing possibility is that plasma ghosts from this type of defensive system could account for some recent sightings of UFOs by US Navy pilots, which were recorded by infrared sensors but which skipped around faster than physical objects.
Sarri says there is little information on the UFO sightings, making them difficult to assess. If anything, they look too steady to be plasma.
鈥淭hey look a bit too stable over such a long distance,鈥� says Sarri. 鈥淭his stable propagation might be possible at surface level, but I find it hard to believe that it could be achieved at such an altitude.鈥�
