The Chandra X-ray Observatory, a NASA Great Observatory, provides the most detailed view to date of the X-ray universe. With its exquisite imaging capabilities and high spectral resolution scientists have investigated phenomena as diverse as the spectra of Jupiter's aurora, the effects of dark energy on the growth of galaxy clusters, and the properties of faint x-ray sources in deep fields.

The Fermi Gamma-Ray Space Telescope (formerly GLAST) is providing our deepest and most detailed map of the gamma-ray sky. Fermi has recorded high-energy gamma rays produced by supernovae, pulsars, extreme flows of energy from systems powered by black holes, and gamma-ray bursts.

XMM-Newton, the X-ray Multi-Mirror Mission, is the second cornerstone of the ESA Horizon 2000 program. With high collecting area in the x-ray band, XMM provides vital information for studies of fundamental and relativistic processes from neutron stars and active galactic nuclei, the creation and dispersal of the elements in supernovae, the distribution of dark matter in clusters, groups, and elliptical galaxies, and young active stars to constrain models of the early solar system and star forming regions.

The Imaging X-ray Polarimetry Explorer (IXPE) will exploit the polarization state of light from astrophysical sources to provide insight into our understanding of X-ray production in objects such as neutron stars and pulsar wind nebulae, as well as stellar and supermassive black holes. IXPE will improve sensitivity over OSO-8, the only previous X-ray polarimeter, by two orders of magnitude in required exposure time. IXPE also will introduce the capability for X-ray polarimetric imaging, uniquely enabling the measurement of X-ray polarization with scientifically meaningful spatial, spectral, and temporal resolution, to address NASA's Science Mission Directorate's science goal "to probe the origin and destiny of our universe, including the nature of black holes, dark energy, dark matter, and gravity."

The Neutron star Interior Composition Explorer (NICER) is an International Space Station (ISS) payload devoted to the study of neutron stars through soft X-ray timing. Neutron stars are unique environments in which all four fundamental forces of nature are simultaneously important. The nature of matter under these conditions is a decades-old unsolved problem, one most directly addressed with measurements of the masses and, especially, radii of neutron stars to high precision (i.e., better than 10 percent uncertainty). NICER will enable rotation-resolved spectroscopy of the thermal and non-thermal emissions of neutron stars in the soft (0.2-12 keV) X-ray band with unprecedented sensitivity, probing interior structure, the origins of dynamic phenomena, and the mechanisms that underlie the most powerful cosmic particle accelerators known.