Addressing critical astrophysical problems with NASAʹs small explorer (SMEX) missions Original Research Article

The Small Explorer (SMEX) program of NASA was initiated during the late 1980s to provide frequent, cost-effective opportunities to carry out sharply focused and relatively space science missions. The SMEX program together with the Mid-size Explorer (MIDEX) program were timely reactions to the increasingly long development times and cost over-runs for a number of large space missions during the previous decades. SMEX spacecrafts are typically 180 to 250 kg with orbit-averaged power consumptions of 50–200 watts. Frequently innovative or novel technologies and instrumentation are employed to achieve important results. The current cost cap (which includes cost of launch vehicle, development and operations) is $75M (Fiscal Year 2000 $US). For comparison, the cost cap of the MIDEX mission is $135M. Both SMEX and MIDEX are solicited Principal Investigator (PI) missions that are peer-reviewed before selection. The SMEX program is supported by the NASAʹs Office of Space Research (OSS) and a wide spectrum of science is addressed. The themes include Origin and Evolution of Stars and Planets, the Structure and Evolution of the Universe, and the Sun-Earth Connection. Within these major themes, the missions may also focus on fundamental laws of physics as they relate to astrophysics and cosmology. So far, seven SMEX missions have been flown or are approved for flight within one or two years. In this paper, the past, current, and possible future SMEX missions are discussed and evaluated. Some highlights and important scientific returns from the current and approved missions are also included.