Here again, astronomers can make some educated guesses. There is more planet-building “stuff” at Saturn’s orbital distance than at Earth’s orbital distance. This is because Saturn’s orbit is so far from the Sun that ice becomes a substantial source of planet-building material. The more planet-building material, the more material for forming moons around the planet. Many of Saturn’s moons and rings are composed largely of ice. Saturn may also have moons that are captured asteroids. This is the most likely origin for Saturn’s outermost moon, Phoebe, and the Cassini spacecraft will make an investigation of this on its way into the Saturn system. Earth’s Moon is unusually large relative to the size of its parent planet. The diameter of Earth is less than 4 times larger than the Moon’s diameter. By contrast, Saturn’s diameter is nearly 25 times greater than Titan’s diameter. Thus, Earth’s Moon is far too large, compared with the size of Earth, to have been formed as an original moon from the spinning disk of gas and dust that formed the planet. The present Moon was most likely formed as a result of a tremendous collision between Earth and a huge asteroid the size of Mars or larger, which broke apart Earth and created the Moon. This impact may have actually created multiple moons around Earth, which later collided with each other or Earth, and now we are left with one large moon.