The story
space + jellyfish. From having a jellyfish-like appearance, and being in the sky.
Etymology from Wiktionary, CC BY-SA 4.0.
space jellyfish
Today it means: An atmospheric phenomenon of the early morning or late evening, where the ground is in nighttime, but sunlight shines through the upper atmosphere, whereby rocket exhaust, the rocket plume, expanding in the upper atmosphere reflects light, creating a visual appearance similar to that of a jellyfish. It can result in reports of UFO sightings. Full definition →