| A | B |
|---|
| asteroid | small rocky objects that travel though space |
| comet | object made of gas, dust, and ice that moves around the Sun in an oval-shaped orbit |
| astronomical unit | average distance from Earth to the Sun—about 150 million km |
| Period of revolution | time it takes an object to travel once around the Sun |
| period of rotation | time it takes an object to complete one rotation |
| orbit | path an object follows as it moves around another object |
| greenhouse effect | when a planet’s atmosphere traps solar energy and causes the surface temperature to increase |
| terrestrial planet | inner planet that is made of rock and metallic materials and has a solid outer layer |
| Galilean moons | four largest moons of Jupiter |
| impact crater | round depression formed by the impact of a meteorite on the surface of a planet, moon, or other space object |
| meteoroid | small, rocky particle that moves through space |
| meterorite | meteoroid that strikes a planet or moon |
| meteor | streak of light in Earth’s atmosphere made by a glowing meteoroid |
| star | object in space made of gases in which nuclear fusion reactions occur that emit energy |
