The Grand Comets are six large icy bodies orbiting Thalassa's solar system with a very regular periodicity (they are hinted here when describing the cometary nebula - entry 13b) ). When such comets come near to Thalassa's sun (Naryan) they emit impressive tails, far brighter than those of the average "common" comet randomly appearing in Thalassa's sky: a Grand Comet tail is a persistent shining belt, clearly visible throughout the entire solar system. Every decade or two a Grand Comet is seen crossing Thalassa's sky. Superposition of two or three comets appearing in Thalassa's sky are relatively common (once every 50-100 years), while the passage of all six comets during the same year is a very rare event happening once every 4200 years.
The six comets are actually six millennia-old intelligent bacterial communities living on large icy asteroids from another plane of existence which were moved in the space around Thalassa by the extraplanar gates created from a planar alignment on the Edge about 266000 years ago.
The original colony needed sunlight to gather enough energy to eat and reproduce, but also new icy bodies both to replenish the material lost during close encounters with the sun and also to establish new settlements. The bacterial communities living on the icy body adjusted the orbit of their home asteroid in a elliptic orbit with a period of 200 years. As the icy body was quite big (about 100km in diameter) it produced an impressive tail each time it got closer to Thalassa's sun, thus becoming the first Grand Comet - much larger than an average random comet approaching Naryan now and then - visible in the whole solar system. On every approach the tail emission was slightly modified by the intelligent bacteria to counter the gravitational influence of other planets and adjust the orbit to a very regular period. The tail was also used to diffuse and disperse bacterial spores in space which are then able to colonize other icy bodies passing nearby.
The first colony on another icy body required a very patient and slow process of gravitational influence on the bodies of the external solar system, requiring more than 120000 years before both comets showed themselves together close to the sun, but since then the colonization process accelerated.
In the following millennia the bacterial community managed to move away from their former orbits in the outer rims of Thalassa's system four other icy bodies. Now six Grand Comets with orbital periods varying between 60 years and 1050 years travel across Thalassa's solar system, passing close to the sun in the same period every 4200 years (such a configuration is ongoing since the last 37700 years). These meetings are used by bacterial communities to exchange knowledge and population through the tails of the icy bodies. The colonizing effort of other icy bodies is still ongoing and accelerating, and by the next 135000 years a total of eleven Grand Comets will be orbiting in Thalassa's solar system, appearing at the same time around the sun once every 63000 years. By then, the original settlement of the intelligent bacteria will be more than 400000 years old, but it will not show any sing of aging.
At present time, there is on average a Grand Comet transit every 20 years, with multiple transits happening on average once every 50 years.
While the comets and their meetings do not directly affect life on Thalassa or on other planets of the Mundane Plane, their appearance are often interpreted as omens of good and bad fortune to local civilizations.
Many dwarves, in particular, came to believe that their golden ages would start and end with the appearance of all six comets in Thalassa's sky; a former golden age expired long ago, and another is expected to happen 80 years in the future.
Some sages believe that being exposed to the light or to the tail of a Grand Comet might enhance or diminish personal magical powers, due to the influence of the magical field of Thalassa's sun reflected by the cometary body, or due to the adsorption of some stranded bacterial spores which would establish a symbiotic relationship in the host's body. Such claims are quite difficult to demonstrate or verify anyway, as the exact procedure needed to gather and concentrate such extra powers on a recipient is a well-guarded secret in the hands of few.
The following table lists all the passages of a Grand Comet in the past 500 years and in the next 150 years.
We left our old worlds, each of us. This is it. We can't go any farther. This is The Edge.
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