Once in a million years

CAPE CANAVERAL, Fla. -- The heavens are hosting an event this weekend that occurs once in a million years or so. A comet as hefty as a small mountain will pass mind-bogglingly close to Mars on Sunday, approaching within 87,000 miles at a speed of 126,000 mph. ...For comparison, the flyby distance of 87,000 miles is about one-third of the way from here to the moon. Siding Spring's tail could extend from Earth all the way to our moon. Its gaseous coma, the fuzzy head surrounding the nucleus, might stretch halfway to the moon.

Boom.