The final chapter of this book provides an overview of the exciting field of planetary geology – the study of geological processes and geologic evolution of other planets and our Solar System. Earth is presented in context relative to its companions in space, and you will learn that our understanding of geologic processes on Earth can be extended to help understand the long-term history of other planets and satellites. Throughout this book, we hope you have been impressed with the scale of geologic time. Now you will be impressed with the scale of space – Earth is but a small part of a vast Universe in which there may be many other solar systems (planets revolving around central star). Though humans have walked on only one other body in the Solar System (the Moon), we have visited every other planet except Pluto with remote satellites. Further, we have successfully landed on the surfaces of Venus, the Moon, Mars, and an asteroid. Probes have descended through the atmosphere of Jupiter, and several satellites have departed the Solar System altogether. Scientists are planning a number of space missions to send unmanned vehicles to the surface of nearby planets and asteroids, collect geological samples from their surfaces, and return these samples safely to Earth for study. All of the celestial bodies in our Solar System originated from the same solar nebula and thus share a common Solar System history. Geologists and space scientists are busy studying this history and how it has influenced the geologic evolution of different planets. They are discovering how various processes have directed planetary development, and why those processes varied from planet to planet, giving each a somewhat unique history despite their common origin from the solar nebula. Finally, the chapter concludes with a brief discussion of the effects of collisions between Earth and other Solar System objects, notably asteroids. There is obvious geologic evidence throughout the Solar System of episodic impacts, and Earth has experienced its share of large-scale impacts throughout its history. There is excellent evidence for a very large impact event at the end of the Cretaceous Period involving an extraterrestrial body estimated to have been about 10 kilometers (6.2 miles) in diameter. Geologists continue to search Earth’s geologic repository to provide details of the planetary effects of this impact and to understand the probabilities of potential collisions in the future.