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Planetary Systems

Asteroid Belt

Asteroid Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Although it seems to be a very dense belt in the schematic diagram of Figure 07-10a, spacecraft that have flown through this zone have found that it is really quite empty and that asteroids are separated by very large distances. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that

Figure 07-10a Asteroid Belt
[view large image]

exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is the Barringer Meteor Crater near Winslow, Arizona (Figure 07-10b).
Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a
Barringer Crater Asteroid Orbits planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers across -- less than half the diameter of our Moon. Figure 07-10c shows some asteroid orbits, all of which are close to the planetary plane, in the same direction as the planets. Asteroids in the Main Belt take about 3 - 6

Figure 07-10b Meteor Crater
[view large image]

Figure 07-10c Orbits [view large image]

years to complete a revolution. They spin as they revolve in just hours.

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of the Earth. Asteroids that are on a collision course with the Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite. Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials (Figure 07-10a). Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

Asteroid Itokawa On September 2005, the Japanese spacecraft Hayabusa arrived at asteroid Itokawa and stationed itself only 20 kilometers away (see Figure 07-10d, not in proportion). Although a long term goal is to find out how much ice, rock and trace elements reside on the asteroid's surface, a shorter term goal is to determine the mass of the asteroid by measuring the attraction of the drifting spacecraft. In November, a small coffee-can sized robot is scheduled for release and is expected to hop around the asteroid taking pictures. Also in November, the spacecraft will fire pellets into asteroid Itokawa and collect some of the debris in a capsule. In December, the spacecraft will make its journey back to Earth and will deliver the capsule in 2007 June.

Figure 07-10d An Asteroid [view large image]

The return trip has been delayed because of communication problem (resolved in January 2006), and other issues. The spacecraft was seriously injured. Nevertheless, the project team is trying its best to bring it back by 2010.

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