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 known as a meteorite. While most meteorites are natural in origin, on occasion manmade space debris can reenter the atmosphere and also become a meteor or even a meteorite!
This astronaut photograph, taken from the International Space Station while over China (approximately 400 kilometers to the northwest of Beijing), provides the unusual perspective of looking down on a meteor as it passes through the atmosphere. The image was taken on August 13, 2011, during the Perseid Meteor Shower that occurs every August. The Perseid meteors are particles that originate from Comet Swift-Tuttle; the comet’s orbit is close enough for these particles to be swept up by the Earth’s gravitational field every year—leading to one of the most dependable meteor shower displays.
Green and yellow airglow appears in thin layers above the limb of the Earth, extending from image left to the upper right. Atoms and molecules above 50 kilometers in the atmosphere are excited by sunlight during the day, and then release this energy at night, producing primarily green light that is observable from orbit. Part of a space station solar panel is visible at image upper right; behind the panel, a bright region indicates the Sun low on the horizon.