Protective Blanket: Atmosphere blocks many small stony asteroids

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.