The distribution of asteroids is shown in Fig. 26.13. Most of the asteroids lie in a band between the orbits of Mars and Jupiter. This band is called the asteroid belt. Over 3000 asteroids have been cataloged to date, but there are many more. The ones that have been cataloged are the brightest, and presumably the largest. We would expect there to be many more small ones. The combined mass of the asteroids is less than that of the Moon.

The sizes of the asteroids are determined from stellar occultations, as shown in Fig. 26.14. The orbits of many asteroids are well known. When we talk about the orbit of an asteroid (or any other object), we are talking about the path of its center. When the center comes close to passing in front of a star, the asteroid can only occult the star if the asteroid is large enough. When an occultation is expected, astronomers from various parts of the Earth watch. If they all see the occultation, then the asteroid is larger than some size. If none sees the occultation, the asteroid is smaller than some size. If some see it and others don't, the size of the asteroid can be determined quite accurately.

Of the asteroids that have been studied in this way, only six are larger than 300 km, 200 are larger than 100 km, and there are many smaller than 1 km. If we know the size and how bright they appear, we can estimate the albedo of the r e

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