TERRIERS (Tomographic Experiment using Radiative Recombinative Ionospheric EUV and Radio Sources) was a spacecraft designed to survey the upper atmosphere by tomography, measuring ultraviolet light emissions to build up a global three-dimensional map of electron density in Earth's ionosphere. As secondary goals, TERRIERS was to have examined related upper atmospheric phenomena, and tested the utility of long term solar irradiance measurements. It carried five imaging spectrometers of which four could operate in the night portion of the orbit, two photometers, and a Gas Ionization Solar Spectral Monitor (GISSMO). Riding piggyback at the base of the spacecraft bus was a small payload built by Cleveland Heights High School. TERRIERS was the second mission in NASA's STEDI (Student Explorer Demonstration Initiative, a precursor program to the UnEX (University-class Explorer) series, and was to be operated by the space physics group at Boston University for ionosphere studies. However, although the spacecraft was placed in the correct orbit, it failed to orient its solar panel toward the Sun and ran out of battery power by May 20. All attempts to revive the mission failed.