I believe that a comet consisting of extremely dense materials will pass by the earth and the moon fairly soon.

Phase 1: The gravitational pull exhibited by this Near Earth Object will cause greater tides, resulting in coastal flooding, and unsettle soft rock deep within the earth, resulting in catastrophic volcanic and seismic events.

Phase 2: The gravitational pull exhibited by the earth on this Near Earth Object will alter this objects transitional orbit, essentially pulling the Near Earth Object back towards earth. I believe it is a comet, and I believe this comet's tail will impact with earth, and not the comet itself. The comet itself will pass back by earth.

I believe this has taken place many times throughout earth's history.