On the Cover: A love affair with space exploration began early in life for many of us, when we first gazed at a sparkling night sky and wondered what kinds of worlds might fill the universe. Many Earthlings—denizens of cities and densely populated areas—have never seen a truly clear sky, however. Excess, unshielded light at night creates light pollution. Not only do runaway photons cloud our view of the heavens, but they also can disrupt ecosystems and the well-being of animals such as birds, reptiles, and humans. Maximilian Stock, Ltd./Science Photo Library
6 In Memoriam: Louis Friedmanreflects on the death of a longtime friend.