Once a thing is set in motion, it is inertia that continues to propel it along the same trajectory. If no force acts against it, it will continue at the same rate for eternity. This is how meteors move through the vacuum of space, hurtling at thousands of kilometers per second, forever, unless they meet an opposing force: they collide with another body, or their trajectory is warped by a gravitational pull. When a meteor enters the Earth’s atmosphere, the friction caused by our dense air burns the surface, it is set aflame, giving them tails of light, and for those large enough to not be burnt completely before impacting, this friction leaves thumb-sized pockmarks across their surface.