Early Explorer satellites The Explorers Program began as a
U.S. Army proposal (
Project Orbiter) to place a "civilian"
artificial satellite into orbit during the
International Geophysical Year (IGY). Although that proposal was rejected in favor of the
U.S. Navy's
Project Vanguard, which made the first sub-orbital flight
Vanguard TV0 in December 1956, the
Soviet Union's launch of
Sputnik 1 on 4 October 1957 (and the resulting "
Sputnik crisis") and the failure of the
Vanguard 1 launch attempt resulted in the Army program being funded to match the Soviet space achievements.
Explorer 1 was launched on the Juno I on 1 February 1958, becoming the first U.S. satellite, as well as discovering the
Van Allen radiation belt. Four follow-up satellites of the Explorer series were launched by the Juno I launch vehicle in 1958, of which
Explorer 3 and
Explorer 4 were successful, while
Explorer 2 and
Explorer 5 failed to reach orbit. The Juno I vehicle was replaced by the
Juno II in 1959.
Continuation of the Explorers Program With the establishment of NASA in 1958, the Explorers Program was transferred to NASA from the
U.S. Army. NASA continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus.
Explorer 6 in 1959 was the first scientific satellite under the project direction of NASA's
Goddard Space Flight Center (GSFC) in
Greenbelt, Maryland. The
Interplanetary Monitoring Platform (IMP) was launched in 1963 and involved a network of eleven Explorer satellites designed to collect data on space radiation in support of the
Apollo program. The IMP program was a major step forward in spacecraft
electronics design, as it was the first space program to use
integrated circuit (IC) chips and
MOSFETs (MOS transistors). The IMP-A (
Explorer 18) in 1963 was the first spacecraft to use IC chips, and the IMP-D (
Explorer 33) in 1966 was the first to use MOSFETs. The first three SMEX missions were chosen in April 1989 out of 51 candidates, and launched in 1992, 1996 and 1998 The second set of two missions were announced in September 1994 and launched in 1998 and 1999. The first announcement opportunity for MIDEX was issued in March 1995, and the first launch under this new class was
FUSE in 1999. The Explorer missions were at first managed by the Small Explorers Project Office at NASA's Goddard Space Flight Center (GSFC). In early 1999, that office was closed and with the announcement of opportunity for the third set of SMEX missions NASA converted the SMEX class so that each mission was managed by its
principal investigator, with oversight by the GSFC Explorer Project. The Explorers Program Office at Goddard Space Flight Center, provides management of the many operational scientific exploration missions that are characterized by relatively moderate costs and small to medium-sized missions that are capable of being built, tested, and launched in a short time interval compared to larger observatories like NASA's
Great Observatories. Excluding the launches, the MIDEX class has a current mission cap cost of US$250 million in 2018, with future MIDEX missions being capped at US$350 million. The cost cap for SMEX missions in 2017 was US$165 million. UNEX missions are capped at US$15 million. A sub-project called
Missions of Opportunity (MO) has funded science instruments or hardware components of onboard non-NASA space missions, and have a total NASA cost cap of US$70 million. == Classes ==