The Perseid meteor shower, from the top down: an astronaut snapshot from the International Space Station.
Many people have spent time outdoors under a dark sky, watching for “shooting stars” to streak across the firmament. In some cultures, this event is an occasion to make a wish; in others it is viewed as a herald of important events, such as the birth of a future ruler. While not actual stars, “shooting stars” do come from outer space, in the form of meteoroids entering the Earth’s atmosphere. Meteoroids are small objects moving through the solar system that are attracted to the Earth by its gravitational pull. These small objects—typically fragments of asteroids or comets, though they can also originate from the Moon or Mars—begin to heat and burn up as they collide with air molecules in Earth’s atmosphere, creating a bright vapor trail or streak. At this point, the object is known as a meteor. If any remnant of the object survives to impact the Earth’s surface, it becomes...