The two planets are racing toward each other at a rate of about 30 kilometers every five seconds, until they are as close as they ever can be on August 27.
The slightly elliptical curve of their orbits causes this rare meeting, which occurs only once every 60,000 years -- meaning Neanderthals were the last people to have such an up-close look at Mars.
More distant, but still close encounters are slightly more common. The most recent of those were in the months of August in 1924, 1845 and 1766.