Meteor

Full definition

When you see a 'shooting star', you are not watching a star fall. You are watching a tiny rock — typically between a fraction of a millimeter and a few centimeters across — slam into Earth's atmosphere at high speed and burn up in less than a second. That rock is called a meteoroid before entering the atmosphere, and the glowing trail it produces is called a meteor.

At speeds between 11 and 72 km/s, it is not friction with air molecules that heats the meteoroid — it's adiabatic compression of the gas in front of it (similar to what melts the heat shield of an Apollo capsule). Temperatures reach 1,600-3,000 °C, which rapidly vaporizes the object's surface and ionizes the atoms in the air column. It is this plasma column, not the rock itself, that briefly glows — a bit like the gas column of a fluorescent tube.

Most meteors disintegrate between 75 and 120 km altitude, well above commercial aircraft. Larger fireballs descend deeper and can end in a spectacular burst called a terminal explosion — the case of the Chelyabinsk event on February 15, 2013, a ~20 m bolide that exploded at 30 km altitude over the Urals with an energy of about 500 kilotons.

Each day, Earth receives 40 to 100 tonnes of extraterrestrial matter — almost all of it in the form of microscopic dust producing these fleeting flashes. The vast majority of meteors are never seen: they fall in daylight, over oceans, or beneath clouded skies.

Numbers, structure and distribution

Under a dark sky, in good conditions, an observer typically sees 5 to 10 sporadic meteors per hour — the steady 'sporadic background' produced by diffuse interplanetary dust. During a major shower, this rate can exceed 100 per hour at peak (the famous ZHR — Zenithal Hourly Rate, corrected for a zenith-altitude radiant and a perfect sky).

Entry speed depends on direction: minimum 11 km/s (Earth's escape velocity) for a body caught from behind, up to ~72 km/s for a body meeting Earth's orbit head-on. The Leonids, at 71 km/s, are among the fastest; the Geminids (36 km/s) are among the slowest and brightest.

The meteoroids that feed meteors come mainly from two populations. Grains released by comets on their perihelion passes are strung along the comet's orbit and form the streams that cause meteor showers. Fragments chipped off asteroids in collisions produce the random falls and most of the meteorites recovered on the ground.

Meteor brightness is measured in magnitude (inverse scale: lower number = brighter). Beyond magnitude -4 (Venus's brilliance), we speak of a fireball. Beyond magnitude -17 (brighter than the full Moon), we speak of a superbolide (a very energetic fireball that can produce a ground-fall meteorite).

The different types

Meteors are classified by origin, brightness and behavior.

Sporadic meteors. They arrive at any time, from any direction, unrelated to any identified stream. They make up about two-thirds of the total annual flux.

Meteor showers. When Earth crosses a cometary or asteroidal debris stream, all meteors appear to radiate from a single point in the sky — the radiant, which gives the shower its name (Perseids = radiant in Perseus, Geminids = in Gemini, Leonids = in Leo). The seven reliable annual showers:

• Quadrantids — peak around January 3-4 (source: asteroid 2003 EH1) • Lyrids — peak around April 22 (source: comet C/1861 G1 Thatcher) • Eta Aquariids — peak around May 5-6 (source: 1P/Halley) • Perseids — peak around August 12-13 (source: 109P/Swift-Tuttle), the most famous northern-hemisphere shower, summery and comfortable • Orionids — peak around October 21-22 (source: 1P/Halley, second shower from Halley) • Leonids — peak around November 17-18 (source: 55P/Tempel-Tuttle), famous for its storms every 33 years • Geminids — peak around December 13-14 (source: asteroid 3200 Phaethon), now the most abundant of the year

Fireballs and superbolides. A meteor brighter than Venus (mag -4). They can cast shadows, change color, leave a persistent train, even produce audible booms. Superbolides are rare but well documented by modern surveillance cameras (Chelyabinsk 2013, Botswana 2018, Bering Sea 2018).

How do we observe them?

Watching meteors is the simplest astronomical activity there is: no instrument required. Quite the opposite — binoculars and telescopes narrow the field of view too much.

Ideal conditions. A dark sky, far from city lights. A reclining chair or blanket to contemplate a wide patch of sky, ideally oriented toward the radiant but without staring at it: the longest, most spectacular trails are seen far from the radiant. Give your eyes 20-30 minutes to adapt to the dark (avoid screens). The best hours are generally between midnight and dawn, when the observer is on the 'forward' side of Earth plowing into the stream.

Amateur science. Many observers contribute to scientific networks: the IMO (International Meteor Organization) centralizes visual counts, while camera networks like FRIPON (France) or the Global Meteor Network (GMN) automatically triangulate trajectories to compute the parent body's orbit and, where possible, locate a meteorite fall.

Missions and satellites. Space surveillance of fireballs is coordinated by NASA (Meteoroid Environment Office, in Huntsville) and by military sensors (US Government Sensors) that detect energetic explosions in the atmosphere. The GOES-R and Meteosat Third Generation weather satellites carry imagers able to spot the brightest fireballs.

And concretely, to plan an observing night? Our astronomy calendar lists the peaks of the major showers and the associated Moon phases (a gibbous Moon can ruin a Perseid night), and What's up tonight? suggests the best time window from your location.

Not to be confused with

Three similar-sounding words must be clearly distinguished.

Meteoroid. The rock before entering the atmosphere. Still in space. The IAU sets its size between 30 micrometers and 1 meter; larger objects are asteroids, smaller ones are interplanetary dust. A meteoroid is not visible to the naked eye — too small and too far.

Meteor. The luminous phenomenon when the meteoroid crosses the atmosphere. That's what you see in the sky. 'Shooting star' is a popular synonym. A meteor is an event, not an object.

Meteorite. The solid fragment that survives descent and reaches Earth's surface. Very few meteors yield a meteorite — it requires a sufficiently large, dense parent body (asteroidal rather than cometary).

Re-entering artificial satellite. A piece of space debris re-entering the atmosphere produces a light phenomenon similar to a fireball, but slower (only 7-8 km/s vs. 11+ km/s for a meteoroid) and more fragmented (trail of multiple bright points). Controlled re-entries of ISS cargo and recent rocket fallbacks are increasingly common.