The planets have been the subject of careful observations and myth for millennia and the subject of telescopic studies for centuries. Our remote ancestors looked into the night sky, and wondered why the celestial wanderers or planets moved across the stellar background. They saw the planets as powerful gods, whose Greek and Roman names are still in use today. Then progressively larger telescopes enabled the detection of faint moons and remote planets that cannot be discerned with the unaided eye, and resolved fine details that otherwise remain blurred.

Only in the past half century have we been able to send spacecraft to the planets and their moons, changing many of them from moving points of light to fascinating real worlds that are stranger and more diverse than we could have imagined. Humans have visited the Moon, and robot spacecraft have landed on Venus and Mars. We have sent vehicles to the very edge of the planetary system, capturing previously unseen details of the remote giant planets, dropping a probe into Jupiter's stormy atmosphere, and perceiving the distant satellites as unique objects whose complex and richly disparate surfaces rival those of the planets. Probes have also been sent to peer into the icy heart of two comets, and robotic eyes have scrutinized the battered and broken asteroids.