The organism known as Thread is far more complex than either the settlers or the inhabitants of Pern knew, until steps were taken to finally capture and dissect the organism after the discovery of AIVAS. However, this can be disrupted by planetary alignments involving the other planets in the Rukbat system. Then for forty years, attacks occur fourteen hours apart, marching across the planet. During the first few years, there are several days between attacks, with the period shortening over time. The remaining 200 +/- years of the Red Star's orbit, when it was not close enough to Pern to drop Thread, was known as an Interval.ĭuring a Pass, Thread usually falls in a predictable pattern, with Threadfalls lasting approximately six hours. The fifty year period was known as a Pass. It took roughly fifty years for Pern to escape from the trail. Intense heat and friction of entry into Pern's atmosphere burned off the outer casing of the thread, leaving attenuated filaments of the voracious life form to fall to the planet's surface. About eight to ten years after passing out of the Oort cloud, the thread ovoids would be close enough to drop down to the planet. As it passed through the cloud, the planet would drag material with it, including the ovoids that would later become the organism known as Thread.

Roughly every 250 years the Red Star, a rogue planet in the Rukbat system with an extremely eccentric orbit, would pass through the Oort Cloud located on the edge of the system.

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