The Explorer program is a United States space exploration program that provides flight opportunities for physics, heliophysics, and astrophysics investigations from space. Over 90 space missions have been launched from 1958 to 2011, and it is still active. Starting with Explorer 6, it has been a NASA program, and they have worked with a variety of other institutions and business, including many international collaborations. Currently, the major subprograms are Small Explorer program (SMEX) and Medium Explorer Program (MIDEX).
Currently running programs as of 2012 include THEMIS/Artemis, Swift, GALEX, RHESSI, ACE, IBEX. Some additional missions that may be in hibernation or reduced function include HETE-2, ISEE-3/ICE, SWAS, and WISE. RXTE competed its mission in 2012.
Some of the latest planned missions include IRIS, GEMS (SMEX), and NuSTAR.
Launchers have included Jupiter C/Juno I, Juno II, various Thor rockets such as Thor-Able, Scout, various Delta and Delta II rockets (see Delta rocket family), and Pegasus.