Explorers Program

Two Wide-Angle Imaging Neutral-Atom Spectrometers (TWINS) are a pair of NASA instruments aboard two United States National Reconnaissance Office (NRO) satellites in Molniya orbits. TWINS was designed to provide stereo images of the Earth's ring current. The first instrument, TWINS-1, was launched aboard USA-184 on 28 June 2006. TWINS-2 followed aboard USA-200 on 13 March 2008.

AMPTE-UKS, also called AMPTE-United Kingdom Subsatellite, was a United Kingdom satellite designed and tasked to study the magnetosphere of Earth, being launched as part of the Explorer program. The AMPTE (Active Magnetospheric Particle Tracer Explorers) mission was designed to study the access of solar wind ions to the magnetosphere, the convective-diffusive transport and energization of magnetospheric particles, and the interactions of plasmas in space.

The Compton Spectrometer and Imager (COSI) is a NASA SMEX astrophysics mission that will launch a soft gamma-ray telescope (0.2–5 MeV) in 2027. It is a wide-field compact Compton telescope (CCT) that is uniquely suited to investigate the "MeV gap" (0.1–10 MeV). It provides imaging, spectroscopy, and polarimetry of astrophysical sources, and its germanium detectors provide excellent energy resolution for emission line measurements.

The Small Astronomy Satellite 2, also known also as SAS-2, SAS B or Explorer 48, was a NASA gamma ray telescope. It was launched on 15 November 1972 into the low Earth orbit with a periapsis of 443 km and an apoapsis of 632 km. It completed its observations on 8 June 1973.

The Explorers Program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United States to achieve orbit. Over 90 space missions have been launched since. Starting with Explorer 6, it has been operated by NASA, with regular collaboration with a variety of other institutions, including many international partners.

Explorer 46, (also Meteoroid Technology Satellite-A or MTS-A), was a NASA satellite launched as part of Explorer program.

Explorer 24 (also called AD-B, Air Density experiment-B and S-56C) was a NASA satellite designed for atmospheric studies. Explorer 24 was launched on 21 November 1964 from Vandenberg Air Force Base, Lompoc, California, with a Scout X-4. Explorer 24 was launched along with its successor satellite, Explorer 25.

SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) is a near-infrared space observatory that will perform an all-sky survey to measure the near-infrared spectra of approximately 450 million galaxies. In February 2019, SPHEREx was selected by NASA for its next Medium-Class Explorers mission, beating out two competing mission concepts: Arcus and FINESSE. SPHEREx launched on 11 March, 2025 on a Falcon 9 Block 5 rocket alongside the PUNCH microsatellites from Vandenberg Space Force Base. The principal investigator is James Bock at California Institute of Technology (Caltech) in Pasadena, California.

Imaging X-ray Polarimetry Explorer, commonly known as IXPE or SMEX-14, is a space observatory with three identical telescopes designed to measure the polarization of cosmic X-rays of black holes, neutron stars, and pulsars. The observatory, which was launched on 9 December 2021, is an international collaboration between NASA and the Italian Space Agency (ASI). It is part of NASA's Explorers program, which designs low-cost spacecraft to study heliophysics and astrophysics.

The X-Ray Imaging and Spectroscopy Mission (XRISM, pronounced 'crism' or 'krizz-em', as if the X was a chi), is an X-ray space telescope. It is a mission of the Japan Aerospace Exploration Agency (JAXA) in partnership with NASA and ESA, intended to study galaxy clusters, outflows from galaxy nuclei, and dark matter.