Thousands of rocks called asteroids or minor planets orbit the Sun. The biggest is Ceres, (625 miles long-1,000 kilometers) which is almost as wide as Texas. More than 5,000 smaller asteroids of different sizes have been discovered in Space between Mars and Jupiter. Millions more tiny ones probably exist there. Together all of these aste roids probably weigh less than our moon. (0.012 x 5,976 million*3 tonnes) Several small, icy objects such as Chiron, discovered first in 1977, orbit beyond Neptune. They may be the first asteroids found in outer Zone, or they may be comets. Asteroids look like stars through a telescope.
The word 'asteroid' means 'like a star'. The large asteroids mostly formed at the same time as the Sun - about 5 billion years ago. They are clumps of material that never grew big enough to form planets. Some of the aster oids contain carbon compounds and water. These same chemicals are necessary for all plants and animals on earth. Apparently the raw materials for the living things have been in space ever since our solar system began.A group of asteroids called Apollo objects come close enough to earth for possible visits in the future. A robot spacecraft could photograph them first. Then robot spacecraft could make round-trip flights to the larger asteroids. They could land and collect and bring back samples for study i n the laboratory. Asteroids could be a rich source of metals and water for future space colonists.
In the twenty-first century (2,000) some asteroids might be towed into orbits around earth and their resources used by space colonists or transported back to earth to be used here. Not all asteroids orbit within the main belt between Mars and Jupiter. So many asteroids crowd the belt that there are inevitable collisions that send newly formed fragments on orbits that take them virtually anywhere. Some fifty thousand years ago, one of these asteroids, as big as a large room, slammed into an area now part of northern Arizona. This danger of asteroids hitting Earth will be devastating.
The Apollo asteroids cross Earth's orbit and sometimes penetrate it. Eros, the largest Apollo asteroid, is 10 miles (16 kilometers) long, practically the size of New York City. Eros last came close to Earth at 15.5 million miles (22.5 million kilometers) in January 1975.
Most Apollo asteroids are less than a mile wide. If one crashed into Earth, it could destroy our biggest cities. But there's no need to worry- the chances of such a crash are extremely small.
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