Meteor

A meteor, often referred to colloquially as a shooting star, is a streak of light produced when a small extraterrestrial body, usually a meteoroid, enters Earth’s atmosphere and becomes incandescent due to rapid heating. This luminous phenomenon occurs as the object collides with air molecules in the upper atmosphere, creating a bright trail of glowing gases and ablated material. Although meteors may appear to be close to the ground, they typically occur high in the mesosphere, where entry speeds and atmospheric density combine to produce the characteristic flare of light.

Characteristics and Formation

Meteors are produced when meteoroids—solid particles ranging from microscopic dust grains to pebble-sized fragments—encounter Earth’s atmosphere at high velocities. Millions of such meteoroids enter the atmosphere each day, but most are only millimetres in size and therefore vaporise completely. The meteoroid’s velocity is influenced by its own orbit, Earth’s orbital motion, and the gravitational acceleration experienced during entry. Visibility generally begins at high altitudes and terminates as the meteoroid disintegrates through heating and fragmentation.
Meteor showers occur when Earth passes through streams of debris associated with comets, producing clusters of meteors that appear to radiate from a specific point in the night sky. Sporadic meteors not associated with any particular debris stream can occur at any time. Occasionally, the trajectory of a meteor is recorded in sufficient detail to reconstruct the orbit of the original meteoroid.
Heating during atmospheric entry results not from friction but from extreme ram pressure. The compression of atmospheric gases in front of the meteoroid raises temperatures enough to melt or vaporise the material and to ionise surrounding gases. This ionised region contributes substantially to the glow and often lasts only for a second.

Historical Understanding

Before the nineteenth century, meteors were generally interpreted as atmospheric phenomena, comparable to lightning. Their extraterrestrial origin was not widely accepted until 1807, when Benjamin Silliman investigated a meteorite fall in Connecticut and suggested a cosmic cause. Broad attention, however, came after the dramatic Leonid meteor storm of 1833, during which thousands of meteors appeared to radiate from the constellation Leo. The consistency of the radiant point led to the conclusion that meteors had a celestial origin.
Subsequent research, notably that of Denison Olmsted and Hubert Anson Newton, established the periodic nature of meteor storms and connected them with the debris streams of comets such as Tempel–Tuttle. By the mid-nineteenth century, the astronomical nature of meteors was firmly recognised.

Fireballs, Bolides, and Special Phenomena

A fireball is an exceptionally bright meteor. The International Astronomical Union defines it as a meteor brighter than apparent magnitude –4, whereas other organisations such as the International Meteor Organization adjust the definition to account for viewing angle. Fireballs that reach magnitude –14 or brighter are commonly termed bolides, especially if they explode in the atmosphere, producing a meteor airburst. Some bolides create audible effects, such as detonations or sustained rumbling.
An extremely rare subset, superbolides, reach magnitude –17 or brighter and may produce spectacular light and sound displays. Occasionally, meteoroids skim the atmosphere and exit back into space, creating Earth-grazing fireballs. Another unusual occurrence is a meteor procession, in which a meteoroid breaks apart into fragments that travel along nearly parallel trajectories.
Although hundreds of thousands of fireballs occur annually, only a fraction are noticed due to their occurrence over oceans or during daylight. Networks such as the European Fireball Network and NASA’s All-sky Fireball Network track many of these events.

Effects on the Atmosphere

Meteor entry produces three prominent atmospheric effects: ionisation, dust deposition, and acoustic signals. Ionisation trails form as air molecules are energised along the meteoroid’s path; these trails may persist for several minutes and provide reflective surfaces for radio waves in meteor burst communications. Meteor radars use these ionised trails to measure atmospheric winds and density by observing how the trail dissipates.
Most meteoroids burn up entirely, leaving behind meteoric dust—tiny particles that can remain aloft for months. Such particles influence atmospheric chemistry and radiative balance by scattering sunlight and acting as catalysts in chemical reactions.
Fragments that survive to low altitudes slow to terminal velocity and enter dark flight, a stage characterised by absence of visible light. If they reach the surface, they are classified as meteorites.

Colours and Visual Appearance

The colour of a meteor’s light depends on both the composition of the meteoroid and the plasma generated during entry. Different chemical elements emit characteristic colours as they ionise:

• orangeyellow from sodium
• yellow from iron
• bluegreen from magnesium
• violet from calcium
• red from atmospheric nitrogen and oxygen

Layering within a meteoroid can also cause the colour to shift as successive mineral layers ablate.

Acoustic Effects and Electrophonic Sounds

Sound produced by meteors usually reaches observers long after the visual display, as acoustic waves travel far more slowly than light. Sonic booms and rumbling effects accompany larger bolides. A more puzzling phenomenon is the occasional report of electrophonic sounds—hissing, crackling, or swishing noises that coincide instantaneously with the meteor’s appearance.
These synchronous sounds cannot be conventional acoustic waves and are thought to arise from electromagnetic interactions. One hypothesis is that ionised meteor trails generate strong radio-frequency pulses that induce vibrations in nearby objects, producing audible sounds. Similar reports occur during intense auroral displays, suggesting a related mechanism involving geomagnetic disturbances.

Scientific Significance

Meteors serve as natural probes of the upper atmosphere. The study of meteor trajectories, light curves, and ionisation trails provides data on atmospheric winds, density profiles, and chemical processes. Meteors also offer insights into the composition and history of the Solar System, as meteoroids originate from comets, asteroids, and interplanetary dust.
As an atmospheric and astronomical phenomenon, meteors illustrate the dynamic interactions between Earth and the wider Solar System, with their brief flashes of light marking the continual influx of cosmic material onto the planet.