A comet, Siding Spring, is scheduled to give Mars a buzz cut on the 19th of October. Its moving at about 126,000 miles per hour, and will come within about 88000 miles of the Red Planet. That's pretty close by any account.
Perhaps John Carter will look up from the deserts, and seeing the comet, think to himself that he has forgotten his home world. But if so, it won't last long, for he'll think about his love and Queen Dejah Thoris and all thoughts of Earth will fade into the black.
All for a comet!