Space

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Across
  1. 5. the average distance between Earth and the Sun
  2. 7. As Earth passes through a comet's tail, the rocky debris collides with our atmosphere, creating the colorful streaks
  3. 10. Objects in space that range in size from dust grains to small asteroids
  4. 13. Compact, rocky surface
  5. 15. a rock-type object formed in the early solar system from collisions with other objects in the solar system
  6. 17. a rock that falls to Earth from space
  7. 18. upper ammonia clouds
Down
  1. 1. a streak of light in the sky
  2. 2. steep cliff
  3. 3. lighter color is believed to result from ammonia ice
  4. 4. it is too hot to stay a liquid but under too much pressure to float freely as a gas
  5. 6. large objects made of dust and ice that orbit the Sun
  6. 8. backward motion
  7. 9. a celestial body that -orbits the sun, has enough mass to assume a nearly round shape, has not cleared the neighborhood around its orbit and is not a moon
  8. 11. a doughnut-shaped region of icy objects beyond the orbit of Neptune
  9. 12. large planet mostly composed of helium and/or hydrogen
  10. 14. a measure of how far a planet's orbit about the Sun (or the Moon's orbit about the Earth) is from being circular compared to Earth
  11. 16. a circle which has been squashed or flattened a bi