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To use a comet to send a message into the future - or indeed already sent from the past.
The orbit of a comet can be calculated to predict its return into the inner solar system, this was proven by Edmund Halley when he predicted the return of 'his' comet following an elliptical orbit around the sun, at a future date after his death. This indeed showed that the path of a comet is both predictable and calculable.
So in understanding this particular science, it would have certainly been scientifically feasible during the time of Atlantis, that a physical body in the form of a large rock of known mass could be 'seeded' with light encoded information and launched into space at a given speed trajectory, having calculated its path and taking into account the position of other gravitational masses of large bodies and other known influences such as planets, dwarf planets and other known comets.
Its trajectory orbit, speed would allow it to pass rapidly into the vast reaches of space only to return again based on the gravitational pull of the sun which could be calculated to return at a particular position and time in space, to release its conscious / physical payload into the path of the Earth's orbit for absorption into the atmosphere (through sublimation) in the same manner as the annual meteor showers that we already pass through on an annual basis such as the Lyrids in April, the Perseids in July, the Leonids in November etc)
So for a message to arrive now from Atlantis, the orbit would have been calculated at approximately 12,500 years ago.