Asteroids, sometimes called minor planets, are small, rocky fragments
left over from the formation of our solar systemabout 4.6 billion years ago.
Asteroids range in size from Ceres, about 952 kilometers (592 miles)
in diameter, to bodies that are less than 1 kilometer (0.6 mile) across. The total mass of all the asteroids is less than
that of Earth's Moon.
This region called the asteroid
belt or simply the main belt, may contain millions of asteroids. Because asteroids have remained mostly unchanged for billions
of years, studies of them could tell us a great deal about the early solar system.
Nearly all asteroids are irregularly
shaped, though a few are nearly spherical, and are often pitted or cratered. As they revolve around the Sun in elliptical
orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go.
More than 150 asteroids are known
to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies
of roughly equal size orbit each other, as well as triple asteroid systems.
The three broad composition classes
of asteroids are C-, S-, and M-types.
- The C-type asteroids
are most common, probably consist of clay and silicate rocks and are dark in appearance. They are among the most ancient objects
in our solar system.
- The S-types (stony)
are made up of silicate materials and nickel-iron.
- M-types are metallic
(nickel-iron). The asteroids' compositional differences are related to how far from the Sun they formed. Some experienced
high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava
to the surface. One such asteroid, Vesta, survives to this day.
Jupiter's massive gravity and occasional
close encounters with Mars or another object change the asteroids' orbits, knocking them out of the main belt and hurling
them into space in both directions across the orbits of the planets. Stray asteroids and asteroid fragments slammed into Earth and the other planets in the
past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth. Scientists
continuously monitor Earth-crossing asteroids, whose paths intersect Earth's orbit, including near-Earth asteroids that may
pose an impact danger.
Radar is a valuable tool in detecting
and monitoring potential impact hazards. By bouncing transmitted signals off objects, images and information can be derived
from the echoes. Scientists can learn a great deal about an asteroid's orbit, rotation, size, shape, and metal concentration.
The U.S. is the only country that has an operating survey and detection program for discovering near-Earth objects.
In 2005, the Japanese spacecraft
Hayabusa landed on the near-Earth asteroid Itokawa and attempted to collect samples. When Hayabusa returns to Earth in June
2010, we will find out if it was successful.
NASA's Dawn mission, launched in September
2007 on a 3-billion-kilometer (1.7-billion-mile) journey to the asteroid belt, is planned to orbit the asteroids Vesta and
Ceres. Vesta and Ceres are sometimes called baby planets - their growth was interrupted by the formation of Jupiter, and they
followed different evolutionary paths. Scientists hope to characterize the conditions and processes of the solar system's
earliest epoch by studying these two very different large asteroids.
How Asteroids Got Their
Names
The International Astronomical Union's Committee on Small Body Nomenclature.is a little less
strict when it comes to naming asteroids. So out there orbiting the Sun we have giant space rocks named for Mr. Spock (of
Star Trek fame), rock musician Frank Zappa, regular guys like Phil Davis and more somber tributes such
as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Asteroids are also named for places
and a variety of other things. Pet named are not allowed.
Asteroids are also given a number,
for example 99942 Apophis.
