Space is filled with countless small objects called meteoroids, which are chunks of rock or metal. When a meteoroid enters the Earth’s atmosphere, intense friction causes it to heat up and glow, creating the bright streak of light we call a meteor. The troposphere is the lowest layer of the atmosphere, extending from the ground up to about \(12 \text{ kilometers}\). Meteors are rarely seen as bright streaks of light within the troposphere because the vast majority of incoming space debris is destroyed or significantly slowed down at much higher altitudes.
The Extreme Velocity of Atmospheric Entry
The root cause of meteor destruction is the tremendous speed at which meteoroids plunge into the atmosphere. Objects approach Earth at velocities ranging from \(11 \text{ kilometers per second}\) up to \(72 \text{ kilometers per second}\). This extreme velocity means the meteoroid carries a massive amount of kinetic energy.
Upon encountering the thin air molecules of the upper atmosphere, this kinetic energy is rapidly converted into heat and light. The sheer speed compresses the air directly in front of the object, generating a powerful shockwave. This violent conversion of energy begins the destruction process long before the object reaches the denser air closer to the ground.
Atmospheric Density and the Ablation Mechanism
The Earth’s atmosphere becomes progressively denser closer to the surface. The main destructive phase, where the bright meteor is visible, typically occurs in the mesosphere and thermosphere, at altitudes between \(100 \text{ and } 80 \text{ kilometers}\). Here, the air is dense enough to create resistance but thin enough for the object to maintain its high velocity.
The primary mechanism of mass loss is called ablation, which is the superheating and vaporization of the meteoroid’s outer layers. As the surface material is heated, it turns into gas and plasma, forming the luminous trail we observe. Aerodynamic stress, caused by increasing atmospheric pressure, also contributes by causing less robust meteoroids to fragment and break apart. This rapid destruction at high altitude ensures the light show is finished far above the troposphere.
Survival Criteria and the Transition to Meteorite
Only objects that are large and structurally strong can survive the ablation process and continue toward the troposphere. To endure the intense heat and pressure, a meteoroid must have an initial mass greater than \(100 \text{ grams}\) and be composed of dense, heat-resistant materials like iron or stony-iron. The entry angle is also a factor, as a shallower angle allows the destructive energy to be dissipated over a longer period.
When a surviving fragment enters the troposphere, typically below \(20 \text{ kilometers}\), it has lost nearly all of its cosmic velocity. The intense heating and light production cease because the object is moving too slowly to cause ablation. The object enters a non-luminous phase known as “dark flight,” simply falling to Earth under gravity while rapidly cooling. A space rock that successfully completes this journey and lands on the ground is then classified as a meteorite.