The journey through the atmosphere

Every meteorite survived one of the most violent journeys in nature. Here we follow a meteoroid from the edge of space to the ground, step by step.

Arriving at cosmic speed

A meteoroid enters Earth's atmosphere at between about 11 and 72 kilometres per second. The lower limit is set by Earth's own gravity: anything falling in from far away is speeded up to at least about 11 km/s. The upper limit comes from adding Earth's speed around the Sun, about 30 km/s, to the fastest speed an object can have while still orbiting the Sun near Earth, about 42 km/s. A meteoroid meeting Earth head-on arrives fastest; one catching up from behind arrives slowest.

Slower meteoroids have a much better chance of producing meteorites. They are heated less fiercely and lose less of their mass. Most meteorites come from meteoroids that entered at less than about 25 km/s, on orbits that brought them in from the asteroid belt.

Ablation: the burning skin

From about 100 kilometres up, the air becomes thick enough to matter. The meteoroid smashes into air molecules so fast that a layer of compressed, glowing gas forms in front of it. This is what we see as the meteor. The rock's surface reaches well over 1,500 °C, melts and is swept away in tiny droplets. This process of melting and removal is called ablation.

Ablation is surprisingly efficient at protecting the inside. The melted layer is blown off almost as soon as it forms, carrying the heat away with it. The interior stays cold. That is why a meteorite's fusion crust is so thin, normally less than a millimetre, and why the minerals just a few millimetres below it are usually unaltered.

Most meteoroids lose the majority of their mass during ablation and break-up.

Break-up and dark flight

As the meteoroid plunges deeper, the air pressure on its front face rises rapidly. Many stony meteoroids are cracked from ancient collisions, and when the pressure exceeds their strength they fragment, often several times. Each break-up exposes fresh surfaces, causing bright flares in the fireball.

Somewhere around 15 to 30 kilometres altitude, the surviving fragments have slowed to a few kilometres per second or less. Ablation stops and the glow fades. The fragments now fall freely in what is called dark flight, reaching a steady terminal speed of roughly 100 to 400 km/h depending on their size and shape.

During dark flight, winds at different heights push the fragments sideways. Larger, heavier pieces carry further along the track than small ones, so the strewn field is sorted by size, with the biggest pieces usually at the far end.

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