Why the mega comet is so fascinating — and not a threat to Earth

A colossal comet, maybe the largest ever discovered, is hurtling through our solar system. The newly found icy object, thankfully, is on a path that won't threaten us, or any planet. Yet at some 60 miles across, or perhaps even larger, the comet — a mass of ices, dust, and rocks — is a fascinating and atypical visitor to our...