Fireballs and meteorites

A fireball is a meteor much brighter than usual: brighter than about magnitude -4, roughly the brightness of Venus in the evening or morning sky.^[1][2] Several thousand meteors this bright enter Earth's atmosphere every day, but most happen over oceans or empty land, or in daylight, so few are seen.^[1]

What makes a fireball

A meteoroid enters the atmosphere at between 11 and 72 km per second and is slowed and heated by the air until its outer layers vaporize.^[1] A fireball that explodes in a bright final flash, often breaking into visible fragments, is called a bolide.^[1] Scientists estimate that about 44 tonnes of meteoritic material reach Earth every day, almost all of it vaporized in the atmosphere.^[3] US government sensors also detect large fireballs, and NASA's Center for Near-Earth Object Studies publishes their location, altitude, speed and energy.^[4]

The brighter the fireball, the rarer it is. Each step brighter in magnitude brings about a third as many events; an experienced observer sees a fireball of magnitude -4 about once in every 20 hours of watching, and one of magnitude -6 about once in every 200 hours.^[1]

Sounds

Sounds from fireballs are rare.^[1] A very bright one, usually brighter than magnitude -8, that reaches below about 50 km before exploding can produce sonic booms on the ground.^[1] Because sound travels slowly, the boom arrives 1.5 to 4 minutes after the flash.^[1]

Reporting a fireball

The American Meteor Society collects fireball reports from around the world, and when several people report the same event, it is sometimes possible to work out the object's actual path through the atmosphere.^[1] A useful report notes the time, the brightness, the color, how long it lasted, and where in the sky it began and ended, by constellation or by compass direction and height above the horizon.^[1] The International Meteor Organization also takes reports through its Fireball Data Center.^[1]

Before reporting, rule out an aircraft contrail lit by the setting Sun. A fireball usually lasts only a few seconds; a trail that stays in the sky for minutes, or that sits below clouds, is not a fireball.^[1]

When meteorites reach the ground

Most fireballs leave nothing to find. Less than 5% of an incoming object usually survives to the ground, and meteor showers, whose particles come from fragile comet material, are not known to have produced meteorites.^[3] A fireball generally has to be brighter than about magnitude -8 to -10 to drop a meteorite, and its source must be sturdy, asteroid material entering at a relatively slow speed.^[1]

Visible light stops at about 15 to 20 km altitude, when the remaining fragments have slowed too much to glow.^[1] From there they fall in darkness at 90 to 180 m per second, and they reach the ground at little more than air temperature, not glowing hot.^[1] When a meteoroid breaks up, its fragments land in a roughly elliptical area, larger pieces generally traveling farther.^[1]

Falls are more common than finds. About 10 to 50 meteorite-dropping events happen around the world each day, but two-thirds are over the ocean and many more over uninhabited land; on average, any one square kilometer receives a fall about once every 50,000 years.^[1]

What meteorites are

There are three main kinds: stony, iron and stony-iron. Most meteorites that fall are stony, but most found long after their fall are irons, which are heavier and easier to tell from Earth rocks.^[3] A fresh meteorite usually has a dark, sometimes shiny fusion crust, melted onto it during its passage through the atmosphere.^[3] About 99.8% of meteorites come from asteroids; the rest, about 0.2%, come from Mars and the Moon in roughly equal shares.^[3] Deserts and the ice of Antarctica are the best places to find them, because dark stones stand out there.^[3]

Meteorites rarely hurt anyone. The first documented injury in the United States came in November 1954, when a 3.6 kg stony meteorite crashed through a roof in Sylacauga, Alabama, and bruised Ann Hodges.^[3] Larger impacts are covered in How close, how big, how often.