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| | Basics of Space Flight Section I. The Environment of Space |
 | | Eight spacecraft have crossed the main asteroid belt en route to their destinations, as of January 2007: Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, Galileo (crossed twice), Cassini, and New Horizons. |
 | | The "main belt" asteroids can actually be categorized as divided into two belts, according to confirming data from the current Sloan Digital Sky Survey. |
 | | The orbit of a Kuiper belt object is sometimes perturbed by gravitational interactions with the Jovian planets causing it to cross Neptune's orbit, where eventually it may have a close encounter with Neptune, either ejecting the comet or throwing it deeper into the solar system. |
| www2.jpl.nasa.gov /basics/bsf1-3.html (1658 words) |
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