There is a great variety of sizes ranging from several meters across to more than one thousand kilometers for the largest one named Ceres. The second largest is Pallas at 608 kilometers and the third largest is Vesta at 538 kilometers. As a general rule an object needs to be one kilometer or larger in diameter to be called an asteroid, although there is no formal agreement on that size. Some astronomers feel that we should include all objects 20 meters or larger in size since an object just 20 meters across travelling at cosmic speeds can invoke serious damage on the earth. A more complete list of the names and sizes of the largest asteroids can be found in the RASC Observer's Handbook.
Our solar system contains many thousands of rocky bodies large enough to be called asteroids. When we include the number of objects greater than 20 meters, the number is most certainly hundreds of thousands to millions, although no count has formally been made. There are about 220 asteroids that are larger than one hundred kilometers across. Asteroids in the one kilometer to one hundred kilometer range number in the thousands. The smaller asteroids are far more numerous than the larger ones and are therefore more likely to impact the earth. Larger bodies have more stable orbits and are less of a threat. Objects smaller than asteroids are called meteoroids and are thought to number in the "billions and billions" as the late Carl Sagan would have said.
The area of the solar system between the fourth planet Mars and the fifth planet Jupiter is where the greatest population of asteroids can be found. This area is known as the Main Belt and contains an enormous number of asteroids. The largest asteroid, named Ceres, is located in the Main Belt. There are also a significant number of asteroids that inhabit the inner solar system where the four planets Mercury, Venus, Earth and Mars reside. Asteroids that travel near the earth are known as Near Earth Objects and are of great interest to astronomers because these ones could impact the earth someday. Jupiter, the largest planet in the solar system has two groups of asteroids, called the Trojans, that are in orbital unison with it. One Trojan group orbits ahead of Jupiter by sixty degrees and the other Trojan group follows Jupiter by sixty degrees. These two areas near the giant planet Jupiter are known as the Lagrangian points and are a safe haven for asteroids. Beyond Jupiter, the number of asteroids drops off dramatically. Many of the smaller moons of the outer four giant planets Jupiter, Saturn, Uranus and Neptune are thought to be captured asteroids. Mars also has two small moons named Phobos and Deimos that are likely captured asteroids.
Yes, the brightest asteroids can be observed through good quality binoculars or small telescopes and a large number are visible through medium sized or larger telescopes. Occasionally a very bright asteroid at or near opposition can be glimpsed from a dark rural location with unaided eyes. The annual RASC Observer's Handbook contains a list, with coordinates at ten day intervals, of asteroids that will be magnitude ten or brighter each year. Information about asteroids fainter than magnitude ten can be obtained from the Minor Planet Center in Cambridge, Mass.
Yes, asteroids can collide with the earth, although the chances of a major impact occuring during an average human lifetime is very low. The largest asteroids have enough mass to maintain a stable orbit and are therefore less of a threat. The smaller asteroids are much more dangerous because they are more easily influenced by gravational perturbations that can cause them to drift into a collision course with the earth. Asteroids one kilometer or larger impacting the earth at cosmic speeds could have a devastating global effect on the earth's atmosphere. Even asteroids as small a twenty meters across, if they were to impact a populated area, could kill thousands of people. Evidence of major impacts during earth's long history has been identified and an excellent list of them, including a map of several crater locations, can be found in the RASC Observer's Handbook. Meteor crater in Arizona, USA is a fine example.
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