Jörmungandr

The second child of Loki and Angrboða, Jörmungandr was hurled into the ocean by Odin, who foresaw its destiny. There it grew until it could encircle the entire world and bite its own tail. Thor fished for it once and nearly hauled it from the sea before the giant Hymir cut the line in terror. At Ragnarök the Serpent will release its tail, wade to land, and spray venom across sky and earth — Thor will kill it but walk only nine steps before collapsing from its poison.

Jörmungandr is described as an unfathomably large and monstrous sea-serpent or 'worm' that dwells in the world sea and encircles the Earth (Midgard), biting his own tail—an image explicitly cited as an example of an ouroboros. Astronomically, the modern namesake 471926 Jormungandr is an Apollo-group asteroid; the supplied astronomical sources do not provide a physical descriptive portrayal in mythic terms.

Mythic accounts present Jörmungandr as dwelling in the world sea with a body that encircles Midgard, effectively forming a cosmological boundary; the act of releasing his tail is described in sources as one of the signs of the beginning of Ragnarök, giving the creature a portentous, eschatological role. In modern usage the name refers to asteroid 471926 Jormungandr (2013 KN6), an Apollo-class near-Earth object whose orbital behavior crosses Earth's path (sources note membership in the Apollo group and list an orbital period of about 1 year, 283 days).