Fireballs and Bolides: Why Some Meteors Are So Bright

What counts as a fireball

Astronomers measure brightness in magnitudes, and a lower or more negative number means a brighter object. The AMS defines a fireball as a very bright meteor, generally brighter than magnitude -4, which it equates roughly with Venus in the morning or evening sky. NASA's All Sky Fireball Network uses a plainer line: meteors brighter than Venus. NASA's Center for Near-Earth Object Studies (CNEOS) sets the cutoff at magnitude -3 or brighter when seen at the observer's zenith, the point straight overhead.

The word bolide is looser still. The AMS describes a bolide as a fireball that ends in a bright flash, often with visible breakup. CNEOS says fireballs that explode in the atmosphere are technically called bolides, though people often use the two words as if they meant the same thing. A 2022 NASA Jet Propulsion Laboratory (JPL) article describes bolides as very bright meteors that can even be seen in daylight.

The ordinary meaning of each word is covered in meteor, meteoroid and meteorite. The extra terms are shorthand for the bright end of the same phenomenon, and no single official line separates them.

Why some meteors get so bright

Many visible meteors come from particles between a sand grain and a small pebble, according to the AMS. A fireball needs more material than that, moving fast enough to turn it into hot, glowing gas. Entry speeds run from roughly 11 to 72 km/s (25,000 to 160,000 mph), and the energy of a piece grows with both its mass and the square of its speed.

Fireballs often break up, which adds flashes. JPL notes that a light curve records how the brightness changes during the several seconds of an object's breakup in the atmosphere, and that the pattern gives insight into its strength and composition. A fireball that fragments and brightens in steps is telling scientists something about the rock inside.

Only part of the energy comes out as light. CNEOS says the total radiated energy it lists is always less than the total impact energy, and it estimates the second number from the first with an empirical formula. For the largest events that gap matters, and the 2013 Chelyabinsk airburst shows how one very bright airburst was reconstructed from video and sensor data.

Sound and sonic booms

Most fireballs are silent to anyone on the ground. The AMS says sonic booms go with very bright fireballs, usually brighter than magnitude -8, that explode as a bolide below about 50 km (31 miles). Light reaches you at once, but sound is far slower, so the boom comes late. The AMS says it is generally 1.5 to 4 minutes after the visual explosion before any boom can be heard, and it asks observers to listen for a full 5 minutes after a fireball.

That delay carries information. Because sound is slow, a longer gap between flash and boom points to a more distant explosion, so noting both times is worth the effort.

How fireballs are detected

Two kinds of systems do most of the work. The first is camera networks. NASA's All Sky Fireball Network, run by the NASA Meteoroid Environment Office, consists of 17 cameras according to its own page: six in the Southeast, three in northern Ohio and Pennsylvania, five in New Mexico and Arizona, and three in Florida. The camera views overlap, so one fireball can be seen from several places. That lets scientists calculate its height and speed, and estimate its orbit, which hints at whether the object came from a comet or an asteroid.

The second is government sensors. CNEOS publishes a table of fireball events detected by U.S. Government sensors, and JPL says the database holds data back to 1988 for nearly one thousand events. JPL adds that bolides occur on the order of several dozen times per year. Each entry lists the date, time, location, altitude, velocity and energy when they were measured. The weather satellites called GOES also carry an instrument, the Geostationary Lightning Mapper, that can detect fireballs.

CNEOS is careful about what the table is. It says the data are not provided in real time, that not all fireballs are reported, and that CNEOS does not independently verify or reanalyze the events. The table is a record of the brightest events, not a complete census. For how astronomers track the larger objects that share Earth's neighborhood, see near-Earth asteroids and how they are tracked.

How to report a fireball in the United States

The AMS says the easiest way to report a fireball is its online form, and it keeps a log where you can check whether others saw the same event. It shares reports with the Meteoritical Society and with the International Meteor Organization's Fireball Data Center, and it alerts planetary scientists when a promising event happens in their area. Memory fades fast, so write things down before you talk it over with anyone.

• [ ] Note the date, the time and where you were standing.
• [ ] Record the start and end points, against background constellations or as a compass direction and height above the horizon.
• [ ] Estimate the brightness, the length of the path across the sky and how long it lasted.
• [ ] Note the color, and treat it with caution.
• [ ] Listen for a boom for a full 5 minutes, and note when it arrived.
• [ ] Submit the report on the AMS online form.

Fireball hunting needs no equipment. If you are ever tempted to use binoculars or a telescope to search a daytime sky, never point them at the Sun without a proper solar filter made for that purpose. A very bright fireball is usually visible without any help, and tips for watching a meteor shower cover the nights when more of them than usual are likely.

Common mistakes and myths

• "A bright fireball means a meteorite fell nearby." Not necessarily. The AMS says meteorite-producing fireballs must be about magnitude -8 to -10 or brighter, and the fireball must usually still be glowing below 20 km (12 miles) altitude. If you think a rock did fall, how to screen a suspicious rock is the place to start.
• "The boom arrives with the flash." It arrives 1.5 to 4 minutes later, according to the AMS. Sound is slow.
• "The color shows exactly what it is made of." The AMS says vivid colors are often reported but urges caution because of persistence-of-vision effects. Composition can tint a fireball, with sodium appearing yellow, nickel green and magnesium blue-white, but a quick memory of color is not a laboratory test.
• "It landed behind the next ridge." CNEOS notes that fireballs can be seen over a very wide area. One witness cannot tell how far away the end point was, which is why reports from many places are valuable.
• "Fireballs are rare, frightening events." JPL says bolides are detected several dozen times a year by U.S. sensors. They are a regular part of the sky.

Frequently asked questions

What is the difference between a fireball and a bolide?

A fireball is a very bright meteor. A bolide is a fireball that explodes or ends in a bright flash, often breaking apart. In everyday speech the two words are often used for the same thing.

Why do I hear a boom after the flash?

Sound travels much more slowly than light. The AMS says the boom generally arrives 1.5 to 4 minutes after the visual explosion, and only very bright fireballs that explode low enough produce one.

How often do fireballs happen?

JPL says bolides are detected on the order of several dozen times per year by U.S. Government sensors. Any one observer sees far fewer, since an event is noticed only if someone is looking the right way under a clear sky.

Can a fireball be seen in daytime?

Yes. JPL describes bolides as very bright meteors that can even be seen in daylight, and CNEOS says ground observers see these events at night and, more rarely, in daylight.