BACKGROUND: NASA is launching a spacecraft to the asteroid Vesta. In a few years, astronomers will be able to see this asteroid not as a small point of light, but as a small body from which the larger planets grew. Vesta could tell astronomers more about the state of the early solar system 4.5 billion years ago.
WHAT ARE ASTEROIDS? Asteroids are not the same as comets. Comets are essentially large, dirty snowballs. They are made of ice and frozen gases, mixed with rocky material and dust, and travel in large oval orbits, sometimes leaving our solar system. In contrast, asteroids are metallic, rocky bodies, without atmospheres, that orbit the Sun, just like planets, but are too small to be considered planets themselves. There are tens of thousands of them gathered into the main asteroid belt, a donut-shaped ring located between the orbits of Mars and Jupiter. Astronomers conclude they are made from primordial rock. Jupiter's strong gravity prevented these small bodies from forming into a planet when the solar system began 4.6 billion years ago.
ABOUT VESTA: Vesta is very different from many other known asteroids -- it's the only one with distinctive light and dark areas. It's much more evenly-colored, like the surface of the Moon. Images show evidence that lava once flowed on its surface, which has a geology resembling Earth or Mars. This means that the asteroid once had a molten interior, which contradicts the previous scientific idea that asteroids are the cold rocky fragments left over from the early days of planet formation. One idea is that Vesta is made up in part of radioactive "shrapnel" from a supernova explosion some four billion years ago, which may have melted the core to cause heavier elements to sink to the center while the lighter rock rose to the surface -- common for terrestrial planets.
DEEP IMPACT: Asteroids and comets have routinely collided with Earth since our planet was formed four billion years ago, although these are rarely catastrophic, because most aren't large enough. The total mass of all the asteroids combined is less than that of the Moon. But occasionally a very large asteroid -- a quarter mile wide or more -- will strike with devastating effects, such as the extinction of the dinosaurs. When an asteroid, or part of it, crashes into Earth, it is called a meteorite. There are currently two programs in the US to actively search for these near-earth objects: NASA's Near Earth Asteroid Tracking program and Spacewatch at the University of Arizona.
The American Astronomical Society contributed to the information contained in the TV portion of this report.
ON THE WEB: http://www.solarviews.com/eng/vesta.htm http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html