A Celestial event would be one that was caused from an event that originated from outside our atmosphere.
Such events include solar flares, comets, asteroids or other objects that threaten the earth. The section on solar flares gives more information on an event that happened in current historical times and the impact. There have been many near misses in the last few years of asteroids that weren’t detected until a few days before they passed by the earth. Not the most recent but an asteroid about 200 feet in diameter barely missed the earth in 2009.
Your Emergency Preparedness Plan may not need to address this type of event singularly but the effects and magnitude of the emergency of such an event would be the same as other events.
An event caused by such an object could include tsunamis, earthquakes, large explosions, mini ice ages or knocking out satellites. Results from search an event could include food shortages, civil unrest, loss of communications and loss of the power grid.