Earth is constantly being pelted with debris from outer space. Thousands of tons of material rain down on us annually, mostly in the form of dust; many of the bigger chunks break up in the atmosphere, but meteorites measuring at least a few feet across reach our planet about once a year. Stony meteorites, or chondrites—such as this fragment of the Axtell meteorite, found in Texas in 1943—are among the most common larger pieces. They are probably fragments of rocky asteroids that crashed into each other. Iron meteorites, derived from the shattered cores of larger asteroids, reach Earth only rarely. Chunks of the moon and Mars (blasted free by other impacts on those bodies) and comets can also make their way here. By analyzing the compositions of these space rocks, researchers are piecing together the 4.57-billion-year history of our solar system.
Fired in a Stellar Kiln Chondrules—glassy, silicon-rich spheres abundant in some meteorites—formed from dust that heated quickly, melted into droplets, and then cooled in just a matter of hours. Simulations by Japanese astrophysicists suggest that shock waves (perhaps from high-energy collisions in the infant solar system) flash-heated the chondrules, which then became part of larger asteroids.
Crash Landing Air resistance typically slows stony meteorites to about 200 miles per hour, so they land with a relatively soft thud. But a meteorite that smashed into the Andes in 2007 generated shock waves that toppled a half-ton bull 500 feet away. Analysis of the crater suggests the hurtling object was traveling at hypersonic speed when it hit; scientists are debating how it maintained such velocity.
Life Rafts in Space? Meteorites could theoretically transport primitive life from planet to planet, but the question is whether anything could survive the journey of thousands or millions of years in the harsh vacuum of space. In August, researchers reported that bacteria survived for more than 18 months on the unprotected exterior of the International Space Station. Other experiments have shown that lichens, along with microscopic invertebrates called tardigrades, can also survive long stretches in the vacuum.
Ancient Ingredients Flakes rich in calcium and aluminum, as old as the solar system itself, probably originated in the hot center of the flattened, spinning disk of gas and dust that gave rise to our sun and its planets. The flakes were incorporated into asteroids that later collided and crumbled. By modeling how the disk evolved over time, researchers at the University of Chicago hope to determine how different kinds of material spread out through the solar system.
