Bright streaks of light crossing the night sky, often called “shooting stars,” are a common astronomical event. These fleeting displays are the visible consequence of small space rocks entering Earth’s atmosphere at tremendous speeds. Traveling at high velocities, these objects encounter the gaseous envelope, transforming their kinetic energy into light and heat.
Defining Meteoroids, Meteors, and Meteorites
The identity of a space rock changes depending on its location relative to Earth’s atmosphere. While the object is still traveling through the vacuum of space, it is properly termed a meteoroid. Meteoroids are small pieces of debris, ranging in size from grains of sand up to small boulders, that originate from comets, asteroids, or even planets.
Once this space rock enters the Earth’s atmosphere and begins to vaporize, the resulting streak of light is known as a meteor. This visible event is transient, lasting only as long as the object is heated to incandescence by its passage through the air. Should any portion of the original meteoroid survive the fiery descent and land on the ground, the remnant is then classified as a meteorite.
The Atmospheric Layer Where Ablation Occurs
The visible streak of a meteor most frequently ignites and breaks apart in the atmospheric layer known as the Mesosphere. This region is the middle layer, situated directly above the Stratosphere and below the Thermosphere. The Mesosphere extends roughly from an altitude of 50 kilometers up to about 85 kilometers above the Earth’s surface.
At these heights, the atmospheric density is sufficient to create resistance, which causes the space rock to heat up rapidly and begin the process of ablation. Most meteors, particularly the small, sand-grain-sized particles, completely vaporize between 70 and 100 kilometers.
The Physics of Why Meteors Incinerate
The intense heat that destroys the meteoroid is not caused by burning in the presence of oxygen, but rather by an effect called compression heating. As the space rock hurtles through the atmosphere at hypervelocity speeds, it violently compresses the air directly in front of it. This rapid compression of gas molecules generates extreme temperatures, sometimes reaching thousands of degrees Celsius.
This superheated air transfers thermal energy to the meteoroid’s surface, causing its outer layers to vaporize and melt in a process known as ablation. The vaporized material and the surrounding superheated air become ionized, creating the glowing plasma trail perceived as the bright streak of the meteor. The speed of the incoming object, often exceeding 20 kilometers per second, is the main factor driving this mechanism.
What Happens When Space Rocks Survive Entry
A small fraction of space rocks possess the size and structural integrity to survive the thermal and mechanical stresses of the Mesosphere. Larger meteoroids or those composed of dense materials like iron-nickel alloys have a higher chance of penetrating the upper atmosphere. These survivors, having shed much of their mass and slowed significantly, cease to glow brightly at altitudes around 15 to 20 kilometers.
The remaining fragment then enters a phase known as “dark flight,” falling through the lower atmosphere under the influence of gravity and air resistance. By the time the object reaches the Troposphere, it has decelerated dramatically and typically falls at its terminal velocity. This means the object strikes the ground as a cool rock, not a fiery projectile, resulting in stony or iron meteorites.