1. Asteroid Collision
The first likely threat is a collision with an extraterrestrial object like a meteor, an asteroid or a comet. Most paleontologists say it was a 9-mile wide asteroid that crashed into what is now Mexico's Yucatan Peninsula that was responsible for the extinction of dinosaurs about 65 million years ago. According to NASA's Near Earth Object Program, there is some possibility that a 0.6 miles-wide asteroid collides with earth about every 100,000 years or so. A smaller asteroid with a diameter of 55 yards or so, could statistically hit earth every 100 years. Such an interstellar traffic accident might provoke acid rain that would impact crops worldwide and generate enough dust and debris to shield earth from sunlight.