Atmospheric entry: from orbit to dark flight

A meteoroid can travel around the Sun for millions of years, then cross the whole of Earth's atmosphere in well under a minute. In that time it is squeezed, melted, broken and slowed from many kilometres per second to the speed of a falling stone. Here is the journey, stage by stage, with the heights and speeds involved.

1. Step 1 of 7

   On a collision course

   A meteoroid is a piece of rock or metal in space, travelling around the Sun on its own orbit. Most that drop meteorites have come, over millions of years, from the asteroid belt, nudged by collisions, by the pull of Jupiter and by the faint push of sunlight on a spinning rock.

   Sooner or later some cross Earth's path at the moment Earth is there. Earth's gravity alone guarantees an arrival speed of at least 11.2 kilometres per second. The highest possible is about 72 km/s, for a body meeting Earth head-on from a retrograde orbit. Stones that survive to the ground usually arrive at the slow end, around 12 to 20 km/s.

2. Step 2 of 7

   First contact, about 100 kilometres up

   At about 100 kilometres the air is a million times thinner than at sea level, but the meteoroid is moving so fast that every air molecule it meets strikes it with great energy. Air piles up in front of it, is compressed and heated, and passes that heat into the surface.

   This is where a meteor usually becomes visible, between about 80 and 120 kilometres up. The faster the meteoroid, the higher it starts to glow: fast shower meteors light up higher than slower stones from the asteroid belt.

3. Step 3 of 7

   Ablation: the glowing trail

   Heated from the front, the surface melts and vaporises and the melt is swept away almost as fast as it forms. This loss of material is called ablation. Most of a meteoroid's mass can be lost this way, so a fireball that drops a stone of a few kilograms may have entered as a body of hundreds of kilograms or more.

   The light of a meteor comes mainly from the cloud of hot vapour and air around and behind the body, whose atoms glow at particular colours as they are excited. Because the heat is stripped away with the melt, it never has time to soak far into the rock: just a few millimetres below the surface, the interior stays at the cold temperature of space.

4. Step 4 of 7

   Breaking up

   As the meteoroid plunges into thicker air, the pressure on its front face rises steeply, because it grows with the density of the air and with the square of the speed. When that pressure exceeds the strength of the rock, it shatters. Real meteoroids are full of cracks, so they usually break at much lower pressures than a solid lab sample would.

   Break-ups often happen between about 20 and 50 kilometres up and are seen as bright flares. Each fragment then has more surface for its mass, so it heats and slows faster. A large, weak body can break up almost all at once and release its energy as an airburst, as over Chelyabinsk in 2013.

5. Step 5 of 7

   The light goes out

   High up, the air is too thin to slow a meteoroid much, so its speed hardly changes. Lower down, drag takes over and the speed drops fast. Small fragments are braked first, large ones last.

   Once a fragment has slowed to roughly 2 to 4 kilometres per second, the air can no longer heat it enough to melt the surface. Ablation stops, the glow fades, and the visible fireball ends, typically somewhere between about 15 and 30 kilometres up for meteorite-dropping falls. The last film of melt on the surface freezes into a thin, dark fusion crust.

6. Step 6 of 7

   Dark flight

   From here the stones fall unseen, a stage called dark flight. They are now ordinary falling objects: gravity pulls them down, the air resists, and they soon settle at their terminal speed, a few hundred kilometres per hour for stones of a few kilograms or less. Their paths curve from a slant to almost vertical.

   Dark flight lasts a few minutes, so winds at different heights can push the stones sideways by a kilometre or more, more for small pieces than large ones. During this time people on the ground often hear sonic booms, and sometimes a whirring or buzzing as stones pass overhead.

7. Step 7 of 7

   Landing in a strewn field

   The stones land in an elongated area called a strewn field, often several kilometres long, lying along the direction of travel. Small pieces lose speed fastest and drop out first; the largest keep their momentum longest and land at the far end. Searchers use this pattern to know where to look for big and small pieces.

   Meteorites arrive at the speed of a falling stone, so they make at most a small pit. They are not hot: the interior is still cold from space, and a freshly fallen stone is usually cool or just warm to the touch. Only bodies large enough to keep much of their cosmic speed, usually tens of metres across or massive irons, reach the ground fast enough to blast out a crater.

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