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. The comet called Siding Spring will approach within 87,000 miles of Mars at a speed of 126,000 mph.

NASA's five robotic explorers at Mars are being repurposed to witness the comet's first known visit to the inner solar system. So are a European and an Indian spacecraft circling the red planet.

The orbiting craft will observe the incoming iceball, then hide behind Mars for protection from potentially dangerous debris in the comet tail. NASA's Opportunity and Curiosity rovers will be shielded by the Martian atmosphere. They should have the best seats in the house.