Sometimes a fiery streak crosses the night sky. It's a piece of rock from space burning up in the air, and the color of the flash can tell you what elements it contained. Astronomers pointed sensitive cameras at such flashes and, among 180 images, found two rocks that stand out from the majority of shooting stars.
One meteor flashed with bright lines of magnesium and silicon, but almost no iron. The other showed calcium, aluminum, and titanium, like rock melted from the depths of a shattered planet. The first likely broke off an asteroid similar to white stones without iron; the second came from a body that once had a crust, like Earth's. Both rocks traveled on short orbits from the inner part of the asteroid belt, and their densities matched those of such rare rocks.
The most interesting part happened mid-flight: for a fraction of a second, the first rock flashed with calcium, manganese, and titanium. This occurs when a rock contains inclusions of a different composition that evaporate faster than the main material. So, brief flashes can reveal the interior of a flying meteor, showing that it's not uniform but assembled from different pieces.
The density of one of the found meteors turned out to be 3.16 grams per cubic centimeter, like a piece of basalt used to pave streets.
In Peter Watts's novel Blindsight, characters study unusual objects in the Kuiper Belt, and their conclusions depend on how they read spectra — much like in this work, where light lines reveal a rock's composition.