It happens once in a million years or so. A comet “as hefty as a small mountain” will buzz pass Mars on Sunday, at the speed of 126,000 mph. The comet, aka Siding Spring, is making its first known visit to the inner solar system. And NASA’s five robotic explorers on Mars hope to catch a glimpse of it–or at least the people who program the robots hope. (Robots don’t yet hope.) It’ll be difficult to see with binoculars or telescope from Earth. Those in the Southern Hemisphere (South Africa and Australia) will have the best shot. (Siding Spring refers to the Australian observatory used to detect the incoming iceball in January 2013.)
With a nucleus estimated to be at least a half-mile in diameter, the comet hails from the Oort Cloud on the extreme fringe of the solar system. According to NASA: “It formed during the first million or two years of the solar system’s birth 4.6 billion years ago and, until now, ventured no closer to the sun than perhaps the orbits of Jupiter, Saturn, Uranus or Neptune.”