The solar system is the Sun and a collection of objects, including planets, their moons and asteroids, held in orbit around the Sun by its gravitational pull. Anything with mass has gravity. More mass means more pull, and the pull gets weaker with distance. The Moon stays in orbit because it keeps falling around Earth instead of flying off in a straight line. The same pull, joined with the pull of billions of other stars, holds our Milky Way galaxy together. This week the crew rolls a marble inside a bowl and tosses a ball to model an orbit.