101e331cb369a528620f6a7067009617The 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. NASA’s five robotic explorers at Mars — three orbiters and two rovers — are being repurposed to witness a comet named Siding Spring make its first known visit to the inner solar system. So are a European and an Indian spacecraft circling the red planet. The orbiting craft will attempt to observe the incoming iceball, then hide behind Mars for protection from potentially dangerous dusty debris in the comet tail. Shielded by the Martian atmosphere, the Opportunity and Curiosity rovers may well have the best seats in the house, although a dust storm on Mars could obscure the view. More