Thread

Thread is the name of a deadly phenomenon that appears throughout Anne McCaffrey's series of science-fiction novels about the fictional planet Pern. Threads are thin silver filaments of a space-borne mycorrhizoid spore that devours all matter that it touches. Pern periodically experiences Threadfalls where this deadly organism rains down from the sky.

How it arrives
Thread is brought to Pern by another planet in its solar system, the inexactly named Red Star. The Red Star has a 250 Turn (Pernese year) elliptic orbit around its sun, Rukbat. When at aphelion, the Red Star travels as far as Rukbat's Oort Cloud, the source of Thread. As the Red Star moves through the Cloud, some of the Thread falls into the planet's gravity well, remaining there as the Red Star leaves the Cloud on its journey sunward. As the Red Star approaches perihelion, it gets close enough to Pern for that Thread to rain down on the planet.

Frequency
Threadfalls occur on Pern for about 50 Turns while the Red Star is near Pern's orbit. This is called a Pass, while the 200 Thread-Free Turns during which the Red Star is farther away are called an Interval. Occasionally the gravitational effect of Rubkat's other satellites prevent Thread from falling on Pern, even though the Red Star is near perihelion, leading to a Long Interval of 450 Turns. McCaffrey's first Pern novel, Dragonflight begins near the end of such a Long Interval, when most of the population of Pern believes that Thread is no more.

Life cycle
Thread remains dormant in space as a small ovoid, but is reactivated by passage through the atmosphere. Heat and atmospheric friction burn off the outer shell, releasing "thread-like" strings which float down to the surface in sheets, tangles and clumps. The size of a single strand is comparable to a long, silver strand of yarn yet while feeding, Thread grows visibly, being described as a "heavy hawser" rope or "wriggling sausages" about 10 cm by 3 meters in size. Structurally, Thread is composed of many thin, tighly wound fillaments within a very fine outer shell or film. Complex proteins allow Thread to consume any carbon-based substance through direct physical contact as well as providing wriggling mobility. Contact with Thread results in "Threadscore," similar to a chemical burn. A small clump of Thread can devour a fully-grown cow in moments. Feeding Thread grows rapidly, pulsing with sickly grey, pink and green colors as is consumes. Its exterior thickens the larger it gets. As if a Thread's need to eat interferes with its biological process, Thread dies from the inside out, hardening a thick, dead shell on the outside and unravelling/melting on the inside, leaving a foul stench. Water drowns Thread immediately, leaving a soggy, bubbling mess.

Thread hisses and thrashes about wildly when captured. The majority of Thread dies after landing, either due to starvation or simply "eating to death." However, what Thread does manage to survive can burrow into the ground and multiply extremely rapidly, devastating miles of land before perishing.

Close examination of Thread ovoids in a vaccume reveal a durable outer shell embedded with cometary matter (ice and dirt), requiring the use of diamond-edged tools. Dissection revealed a mass of tightly wound fibers, tubes, and bubbling yellow goo (boiling helium). Captured Thread ovoids that are exposed to warmth and oxygen explode into a writhing, devouring mass, melting into a dead puddle after about twenty minutes without food.