A photo from the Leonid Kulik expedition to the Tunguska region of Russia in 1929. A meteorite or comet knocked down millions of trees in one of the largest space-object-meets-Earth events in recorded history.

The Leonid Kulik Expedition, St. Petersburg Museum.

Near a small village in Russia, Marina Ivanova stepped into cross-country skis and kicked toward a hole in the snow. The meteorite specialist with the Smithsonian Museum of Natural History and Vernadsky Institute in Moscow was hunting for fragments of the great Chelyabinsk Meteorite that exploded three days earlier.