Although Earth and the moon inhabit the same cosmic neighborhood, our planet has far fewer scars from extraterrestrial impacts because incoming objects burn up in its atmosphere. A new computer model suggests that Earth's thin layer of air is an even better shield than previously thought.
Scientists have identified fewer than 200 impact craters on Earth (SN: 6/15/02, p. 378: Presto, Change-o!), and only for a few do they suspect the type of object that gouged the hole. Of those, most were blasted out by asteroids, which come in rocky and iron-rich varieties, says Philip Bland, a planetary scientist at Imperial College in London. From observations of asteroids in space, and analysis of the composition of their meteorite remnants on Earth, scientists believe that about 5 percent of the asteroids that enter the upper atmosphere are of the iron-rich type.