Space

5. the average distance between Earth and the Sun
7. As Earth passes through a comet's tail, the rocky debris collides with our atmosphere, creating the colorful streaks
10. Objects in space that range in size from dust grains to small asteroids
13. Compact, rocky surface
15. a rock-type object formed in the early solar system from collisions with other objects in the solar system
17. a rock that falls to Earth from space
18. upper ammonia clouds

1. a streak of light in the sky
2. steep cliff
3. lighter color is believed to result from ammonia ice
4. it is too hot to stay a liquid but under too much pressure to float freely as a gas
6. large objects made of dust and ice that orbit the Sun
8. backward motion
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
11. a doughnut-shaped region of icy objects beyond the orbit of Neptune
12. large planet mostly composed of helium and/or hydrogen
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
16. a circle which has been squashed or flattened a bi