Why are witnessed falls more useful to science than finds?
Falls are fresh, unweathered and unbiased by searching, often with known orbits, so they give the clearest view of what reaches Earth and of delicate space chemistry.
Meteorites are divided into falls, seen falling and recovered soon afterwards, and finds, discovered later without a witnessed fall. Both are valuable, but falls have special scientific advantages.
Freshness. Falls are usually collected within hours to weeks, before Earth's weathering has had much effect. Their metal is unrusted, their minerals unaltered, and their delicate components, such as water-bearing clays, salts and organic molecules, are better preserved. For carbonaceous chondrites, this freshness is especially important.
Short-lived isotopes. Cosmic rays create radioactive isotopes in meteoroids, some with half-lives of days to weeks, such as sodium-22 and cobalt-58. These can only be measured in very fresh falls. They reveal information about the cosmic-ray environment and the solar cycle.
Unbiased statistics. Finds are biased: iron meteorites are more likely to be noticed and survive, while fragile meteorites weather away. Falls give the truest picture of what kinds of meteorites reach Earth today.
Known orbits. If a fall is captured by cameras, scientists can calculate its orbit before it reached Earth, linking it to source regions in the asteroid belt. Each fall with a known orbit adds to the map of where meteorites come from.
Context. Eyewitness reports, sounds, videos and strewn fields give information about how meteoroids break up in the atmosphere.
Finds remain essential, especially from Antarctica and deserts, which supply many rare types. But each new well-documented fall, like Winchcombe in 2021, is a scientific prize.