stylistic:style_transfer, information:classification, structural:expansion
Fireball

A fireball is a brighter-than-usual meteor that also becomes visible when about 100 km from sea level. The International Astronomical Union (IAU) defines a fireball as "a meteor brighter than any of the planets" (apparent magnitude −4 or greater). The International Meteor Organization (an amateur organization that studies meteors) has a more rigid definition. It defines a fireball as a meteor that would have a magnitude of −3 or brighter if seen at zenith. This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude −1 at 5 degrees above the horizon would be classified as a fireball because, if the observer had been directly below the meteor, it would have appeared as magnitude −6.

Fireballs reaching apparent magnitude −14 or brighter are called bolides. The IAU has no official definition of "bolide", and generally considers the term synonymous with "fireball". Astronomers often use "bolide" to identify an exceptionally bright fireball, particularly one that explodes in a meteor air burst. They are sometimes called detonating fireballs. It may also be used to mean a fireball which creates audible sounds. In the late twentieth century, bolide has also come to mean any object that hits Earth and explodes, with no regard to its composition (asteroid or comet). The word bolide comes from the Greek βολίς (bolis) which can mean a missile or to flash. If the magnitude of a bolide reaches −17 or brighter it is known as a superbolide. A relatively small percentage of fireballs hit Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs. Such an event happened in broad daylight over North America in 1972. Another rare phenomenon is a meteor procession, where the meteor breaks up into several fireballs traveling nearly parallel to the surface of Earth.

A steadily growing number of fireballs are recorded at the American Meteor Society every year. There are probably more than 500,000 fireballs a year, but most go unnoticed because most occur over the ocean and half occur during daytime. A European Fireball Network and a NASA All-sky Fireball Network detect and track many fireballs.