Thousands of years ago, humans believed that Earth was the center of the universe, that the world they lived on was all there was. Truthfully, the solar system extends almost halfway to the nearest star. And it is composed of not only planets, asteroids, and comets, but also powerful forces and vast fields of energy. This is our solar system’s big backyard. The cold, dark world that lies at the farthest reaches of our solar system holds a vast collection of secrets, and for most of human history, we had no idea anything was out there. But, driven by curiosity and equipped with new technology, astronomers have determined that beyond the orbit of Neptune are countless icy comets, strange particles that dance under the influence of the sun, and signs of undiscovered planets. To learn more about these far-flung objects, scientists have finally begun to explore the distant solar system, finding answers to age-old questions at the same time that they encounter new mysteries.