| A | B |
|---|
| asteroid belt | region of space between Mars and Jupiter in which asteroids are found |
| comet | object made of ice, gas, and dust that travels through space |
| escape velocity | velocity needed to escape the Earth's gravitational pull |
| greenhouse effect | carbon dioxide and other gases in the atmosphere absorb infrared radiation from the sun, forming a heat blanket around the Earth |
| meteor | streak of light produced by a meteoroid as it burns up in the Earth's atmosphere |
| meteorite | meteor that strikes the Earth's surface |
| meteoroid | chunk of metal or stone that orbits the sun |
| nebular theory | the solar system began as a huge cloud of dust and gas called a nebula, which later condensed to form the sun and its nine planets |
| orbit | path an object takes when moving around another object in space |
| period of revolution | time it takes a planet to make one revolution around the sun |
| period of rotation | time it takes a planet to make one rotation on its axis |
| reaction engine | engine, such as a rocket, in which the rearward blast of exploding gases causes the rocket to shoot forward |
| retrograde rotation | reverse motion in which a planet rotates from east to west, instead of from west to east |
| solar system | sun, planets, and all the other objects that revolve around the sun |
| solar | relating to the sun |
| velocity | quickness of motion or speed, rate of motion or action |
