Science Office for Mission Assessments Explorer Program Acquisitions.

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).

 

Astrophysics Probe Explorer (APEX):  In 2021, the National Academies' 2020 Decadal Survey in Astronomy and Astrophysics, Pathways to Discovery in Astronomy and Astrophysics for the 2020s (https://www.nap.edu/catalog/26141/pathways-to-discovery-in-astronomy-and-astrophysics-for-the-2020s) (2020 Decadal Survey), recommended NASA Astrophysics Division should implement a line of probe missions with a mission cost cap of ~$1.5 billion fiscal year 2020 and a targeted launch rate of approximately one per decade. It also recommended that these missions should be competed, with solicitations calling for concepts in priority areas identified by decadal surveys. Thus, the Probe class of mission was added to the Explorers Program portfolio. For the coming decade, the 2020 Decadal Survey recommended a far-infrared mission or an X-ray mission as a Probe-class mission.

 

Explorer (EX):   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.