Of course, the Earth's atmosphere is completely opaque to X-rays from space -- thus, X-ray astronomy must be conducted from the upper atmosphere or, preferably, space. Early X-ray astronomy utilized detectors on sub-orbital rockets and high-flying balloons. Over the past 30 years, however, X-ray astronomy has been conducted primarily via earth-orbiting satellites, launched by a wide range of nations and international collaborations. Countries participating in these missions have included the United States of America, the United Kingdom, Japan, the European Union, Germany, the Soviet Union and Russia, Italy, the Netherlands, and India.
Below we discuss some of those satellites that have made the greatest contribution to the study of accretion disk features in compact object systems. The list is not meant to be exhaustive or exclusive, but it does provide a fair sampling of the observational capabilities at the disposal of X-ray astronomers in the past, present, and near future. We also discuss some of the future missions being planned by US National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA).