In the early hours of May 5, a camera in Waycross, Georgia, captured footage of a strange object moving through the skies. The bright and fast subject, followed by a glowing oblong aura, was described to look like a “space jellyfish” on Twitter, with many being left baffled by what they were looking at. However, it was not some kind of glowing alien squid, but rather a SpaceX Falcon 9 rocket that launched from Florida’s Kennedy Space Center, approximately 400 kilometres from the camera.
Professor Chris Combs, from the University of Texas at San Antonio, explained why the rocket looked like a jellyfish, saying: “Why do we see this?
“It’s a combination of compressible flow physics and perfect timing where the gas is illuminated by light just over the horizon.”
He wrote that it was actually the exhaust leaving the SpaceX rocket’s engine nozzle that created the long blobby “body” of the jellyfish.
He noted that the exhaust took this strange shape the pressure difference inside and outside the nozzle.
He wrote: Under-expanded means nozzle exit pressure is greater than surroundings, so gas needs to expand more (drop pressure) to match.
“Over-expanded means nozzle exit P is less than surroundings—expanded too much.
“In under-expanded exhaust, you get expansion fans at the nozzle exit to decrease pressure and match background: jellyfish, at high altitude
“In over-expanded exhaust, you get shock waves to increase pressure back up to background: shock diamonds, lower altitude.”