How fireball cameras triangulate a fall

Most meteorites are found by chance. A growing number, though, are tracked from the sky to the ground by networks of automatic cameras that watch the whole sky every night. From a few seconds of video seen from different places, scientists can work out the fireball's path, speed and orbit, and predict where to look for the stones. Here is how it is done.

1. Step 1 of 7

   A network watching the whole sky

   A fireball network is a set of automatic cameras spread across a region, typically tens to around a hundred kilometres apart. Each one looks straight up through a fish-eye lens that sees the whole sky from horizon to horizon, and records every clear night.

   Networks of this kind have run since 1959, when a camera network in Czechoslovakia recorded the Příbram fall, and now operate across Europe, Australia, North America, the UK and elsewhere. The cameras' views overlap, so a bright fireball is usually caught by at least two and often many stations.

2. Step 2 of 7

   Measuring positions against the stars

   Every star in the image has a precisely known position in the sky, so the stars act as a grid. By measuring where the fireball lies among them, scientists convert each point along the trail into an exact direction from that camera: so many degrees round from north and so many degrees above the horizon.

   Many cameras also chop the trail into dashes, using a rotating shutter or a liquid-crystal shutter that opens and closes at a fixed rate, or simply by recording video frames at known times. Each dash marks where the fireball was at a known instant, which is what later gives its speed.

3. Step 3 of 7

   Two views fix the path in space

   From one camera, the fireball's trail only gives a set of directions: a flat fan of sight-lines from that camera, called a plane. The fireball lay somewhere in that plane, but one view cannot say how far away it was.

   A second camera in a different place gives a second plane. The two planes meet along a single line, and that line is the fireball's real path through the atmosphere: its position, heights and direction of travel. This is triangulation. With more cameras the path is over-determined, so scientists can fit the best solution and estimate how accurate it is.

4. Step 4 of 7

   Speed and slowing down

   With the path known in kilometres, the dashes tell scientists how far the fireball travelled in each fixed time step, which gives its speed at every point. Early on the speed is almost the entry speed, often 13 to 20 km/s for meteorite-dropping fireballs; towards the end, the dashes crowd together as the air brakes it.

   How quickly a body slows depends on its mass and shape compared with the air it pushes aside. A light object slows fast; a heavy one ploughs on. So the deceleration gives an estimate of the mass still present at the end of the bright flight, which tells searchers whether there is anything worth looking for.

5. Step 5 of 7

   Running the orbit backwards

   The fireball's speed and direction at the top of the atmosphere describe how it was moving relative to Earth. Scientists correct for Earth's own motion around the Sun, its spin and the extra speed its gravity added on the way in. What remains is the meteoroid's own path through the Solar System: its orbit.

   Příbram in 1959 was the first meteorite with a measured orbit. Several dozen more have followed, and almost all trace back to the asteroid belt, the main evidence that most meteorites are pieces of asteroids. Orbits can even point to particular regions of the belt, linking meteorite types with families of asteroids.

6. Step 6 of 7

   Modelling the dark flight

   Once the light goes out, cameras see nothing more, but the stones are still falling for a few minutes. Scientists model that dark flight by computer, starting from the last measured position and speed and including the drag of the air and the winds at every height, taken from weather balloons or weather models.

   Because the masses are uncertain, the model is run for a range of sizes, from grams to kilograms. Small stones are blown further by the wind and land closer along the track; large ones carry on further. The result is a fall line or search ellipse on the map, often only a few hundred metres wide, with each part labelled by the mass expected to land there.

7. Step 7 of 7

   Searching and recovery

   With a predicted fall area, teams of volunteers and scientists walk slowly in lines across it, always with the landowner's permission, scanning the ground. Drones with cameras and computer image-searching now help too, and weather radar sometimes shows the falling stones directly.

   Fresh meteorites are recovered as quickly and cleanly as possible, ideally before rain, and handled with gloves or foil, because their chemistry is precious. The UK's Winchcombe meteorite, which fell on 28 February 2021, was tracked by several camera networks; the first stones turned up on a driveway in Winchcombe the next morning, and the trajectory pointed searchers to the field where another piece was found days later.

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