This Sunday the Oort Cloud comet Siding Spring (also called Comet C/2013 A1) will pass within 140,000 kilometers of Mars. This is about half the distance from the earth to the moon, and much, much closer than any known comet fly-by of earth. It will be observed by rovers on Mars and spacecraft in Mars orbit, as well as terrestrial land-based and space-baced telescopes. Currently orbiting Mars are the Mars Reconnaissance Orbiter (MRO), MAVEN, Mars Odyssey, the ESA Mars Express, and India's Mars Orbiter Mission (MOM). It is only by blind, dumb luck that this one-in-a-million flyby is happening at a time when we are actively surveying Mars for eventual human colonization. This should be a very interesting week.