Interstellar Boundary Explorer (IBEX or Explorer 91 or SMEX-10) is a NASA satellite in Earth orbit that uses energetic neutral atoms (ENAs) to image the interaction region between the Solar System and interstellar space. The mission is part of NASA's Small Explorer program and was launched with a Pegasus-XL launch vehicle on 19 October 2008.

The mission is led by Dr. David J. McComas (IBEX principal investigator), formerly of the Southwest Research Institute (SwRI) and now with Princeton University. The Los Alamos National Laboratory and the Lockheed Martin Advanced Technology Center built the IBEX-Hi and IBEX-Lo sensors respectively. The Orbital Sciences Corporation manufactured the satellite bus and was the location for spacecraft environmental testing. The nominal mission baseline duration was two years after commissioning, and the prime ended in early 2011. The spacecraft and sensors are still healthy and the mission is continuing in its extended mission.

IBEX is in a Sun-oriented spin-stabilized orbit around the Earth. In June 2011, IBEX was shifted to a new, more efficient, much more stable orbit. It does not come as close to the Moon in the new orbit, and expends less fuel to maintain its position.

The spacecraft is equipped with two large aperture imagers which detect ENAs with energies from 10 eV to 2 keV (IBEX-Lo) and 300 eV to 6 keV (IBEX-Hi). The mission was originally planned for a 24 month operations period. The mission has since been extended, with the spacecraft still in operation as of March 2023.