Science Office for Mission Assessments: Explorers Acquisition
The mission of the Explorers Program is to provide frequent flight opportunities for world-class scientific investigations from space utilizing innovative, streamlined and efficient management approaches within the heliophysics and astrophysics science areas. The program seeks to enhance public awareness of, and appreciation for, space science and to incorporate educational and public outreach activities as integral parts of space science investigations.
Small Explorer (SMEX):
NASA's Small Explorers carry out the most highly focused space science investigations. Intended to be the lowest cost missions, SMEX missions are, on average, 200 to 300 kg in mass. Access to space utilizes the smallest expendable launch vehicle available.
Medium-Class Explorer (MIDEX):
The Medium-Class Explorers are the most capable scientific investigations that can be built within the streamlined, frequent flight model of the Explorer Program. MIDEX missions are, on average, approximately 500 to 1500 kg in mass. MIDEX missions typically utilize mid-range expendable launch vehicles (historically, these have been of the Delta II class).
Earth System Explorers (ESE):
The ESE Program conducts Principal Investigator (PI)-led space science investigations as recommended by The National Academies of Sciences, Engineering, and Medicine 2017 Decadal Survey for Earth Science and Applications from Space
. The Program is designed to accomplish high-quality Earth system science investigations addressing one or more observables identified as ESE Target Observables (TO) in the Decadal Survey. These missions will conduct scientific investigations of modest and focused programmatic scope and can be developed relatively quickly.
The Explorer missions were introduced with the 2010 Announcement of Opportunity for new missions. EX missions were conceived in response to the temporary unavailability of mid-range expendable launch vehicles. EX missions will accomplish world-class science via spacecraft whose capabilities are expected to fall between the SMEX and MIDEX classes. Access to space will utilize one of the several, lower-cost expendable launch vehicles currently available.