Small- and medium-class, PI-led Astrophysics missions, as well as Astrophysics missions of opportunity, are selected under the Astrophysics Explorers program, and are managed by the Astrophysics Division. Explorers are opportunities for focused scientific investigations, and fill the scientific gaps between the larger missions. For example, the Nuclear Spectroscopic Telescope Array (NuSTAR) is conducting a census of black holes for the Physics of the Cosmos program and studying the birth of elements for the Cosmic Origins program. The Transiting Exoplanet Survey Satellite (TESS) is now in operations identifying terrestrial planets in the habitable zones of nearby stars. The Neutron Star Interior Composition Explorer (NICER) mission of opportunity has been mounted on the space station and is measuring the variability of cosmic X-ray sources to explore the exotic states of matter within neutron stars and reveal their interior and surface compositions.
Several future missions are in development. The Imaging X-Ray Polarimetry Explore (IXPE) will use 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. Mission of Opportunity GUSTO (Galactic/extragalactic Ultra long duration balloon Spectroscopic Terahertz Observatory) will untangle the complexities of the interstellar medium and map out large sections of the plane of our Milky Way galaxy and the nearby galaxy known as the Large Magellanic Cloud. Finally, XRISM (X-Ray Imaging and Spectroscopy, previously named XARM) is a JAXA/NASA collaborative mission with ESA participation which will investigate X-ray celestial objects in the Universe with high-throughput, high-resolution spectroscopy.