|From National Geographic|
At this distance there is a chance, albeit a decreasing chance, that Siding Spring will be caught in Mars' gravity pull and impact the planet. If that would happen the event would be epic on a dinosaur-killing scale. As National Geographic writes
If Siding Spring would hit, the force of impact may truly be monumental. Based on observations to date the comet nucleus could be a real monster – as big as 9 miles (15 km) to 31 miles (50 km) wide. With it’s velocity clocked at 35 miles (56 km) per second, the energy force of the collision could be measured in the billions of megatons, resulting in a crater hundreds of miles wide. This could be an impact that rivals the one that wiped out the dinosaurs 65 million years ago on our planet and would be bright enough to be even seen with the naked eye from Earth.The Earth is surrounded by space rocks and ice both known and unknown. If this were headed to/very near the Earth what would humanity do? We need to have plans or a completely unknown event will have devastating impact. The Aztec thought they knew the cosmos but then the world expanded.