Space

3. The biggest planet of the Solar system
5. The biggest galaxy of the Local group
7. A power that keeps planets orbiting the sun
8. The smallest planet of the Solar system
9. The fourth biggest moon in the solar system. Has a lot of volcanoes.
11. Second planet from the sun
12. Number of the Apollo flight that landed to the Moon
14. A star system consisting of two stars orbiting around their common barycenter
16. Number of moons orbiting planet Mars
17. A satellite orbiting the Sun, may go really far away from the Sun and return after hundreds or thousands of year. Some contain water ice.
18. A natural satellite orbiting Earth

1. The biggest moon orbiting Saturn
2. Eighth planet from the sun
4. The center of the Solar system
6. A circumstellar disc between planets Mars and Jupiter that contains asteroids, minor planets and debris
7. A gravitationally bound system of starts, stellar remnants, interstellar gas, dust and dark matter
8. Second biggest galaxy of the Local group
10. Last name of the man who did the first spacewalk
13. Third planet from the Sun
15. The planet with least density on the Solar system