Forty years ago, the first rocket detected x-rays from space. In his Perspective, [Gursky][1] charts the history of this rocket flight and explains how the detection of astronomical x-rays has changed astronomy. Today, x-ray space missions are helping to resolve one of the last frontiers of astronomy by providing insights into the vicinity of neutron stars and black holes. [1]: http://www.sciencemag.org/cgi/content/full/297/5586/1485