Astronomy, the oldest of the sciences, originated in the human urge to understand the mysterious lights we see in the sky above us—the Sun, the Moon, the planets and the stars. Over the centuries, new tools have become available to study these cosmic icons—telescopes that allow us to see further and fainter, detectors that are sensitive to electromagnetic signals at non-visible wavelengths, and satellites that can observe from outside the confines of the Earth’s atmosphere. These tools have answered many questions, but raised even more.