The first Explorer satellites (Explorer I-V) were part of a US Army program. When NASA came into being in 1958, it took over the Explorer name; the first successful NASA Explorer was Explorer VI. In the early days, the satellites were given names in the S (for Science) series before launch, and only assigned Explorer names if the launch was successful. This system was superseded by pre-launch names indicating specific sub-programs within Explorer; for instance, AE-A was Atmosphere Explorer A. Letters A,B,C.. were assigned to each spacecraft within a subprogram. Again, Explorer names were assigned and used after the launch was successful, so AE-A was known as Explorer XVII after launch. The nomenclature becomes more confusing because some satellites were also given sub-program names after launch; thus, IMP A became both Explorer XVIII and IMP I when it reached orbit. In the 1960s these sub-program names were not often used, but by the early 1970s, the sub-program names began to take over and the Explorer numbers were rarely used (Explorer 48 is almost always called SAS 2). Finally in 1975, NASA stopped assigning overall Explorer numbers; IUE, although an Explorer, is not `Explorer 57' or any other number.
Explorer launches in the 1960s and 1970s can be divided roughly on the basis of launch vehicle - the small Scout-launched Explorers and the somewhat more ambitious Delta-launched missions. In the 1980s, elephantiasis slowed Explorer science launches almost to a halt; the Delta Explorers had grown to match the increased power of the Delta 2 launch vehicle. With the early 1990s, the Explorer program was given renewed vigor and two main lines of Explorer satellite were begun with competed selection. The SMEX (Small Explorer) and the MIDEX (Mid-sized Explorer) were both smaller than the Delta Explorers which were phased out with FUSE. In addition, a number of tiny 'Missions of Opportunity' were added on an occasional basis, and there was a short-lived line of small 'University Explorers' (UNEX).
In the table below the Explorer program is summarized, grouped by broad scientific discipline. The table gives the mission class, and for astronomy missions also the mass in kg and the operational lifetime in years. Note that very few payload failures have occurred in the program, and after the early 1960s, almost no launch failures.
For the astronomical missions, I have also noted which ones had significant Guest Observer participation (GO missions). IUE (together with the Observatory-class Einstein mission) pioneered the GO mission, allowing observers around the world to use spaceborne telescopes as general-user facilities, in contrast to the more usual PI (Principal Investigator) missions where the instrument team decides all the targets and gets first crack at all the data.
Astronomy Explorer missions
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Mass/kg Class Life/yr
S-15 (Explorer 11) 1961 40 Juno 0.7 LEO - Upper limit to gamma ray background
Radio Astronomy Explorer A (Explorer 38/RAE 1) 1968 189 Delta 4.5 MEO - 228m antenna, 0.4-10 MHz radio background
Small Astron. Sat. A (Explorer 42/SAS-1/Uhuru) 1970 143 Scout 2.5 LEO - first X-ray all-sky catalog
Radio Astronomy Explorer B (Explorer 49/RAE 2) 1973 200 Delta 3.8 Lunar orbit - 0.2-13 Mhz radio sky (Joe Alexander)
Small Astron. Sat. B (Explorer 48/SAS-2) 1972 174 Scout 0.5 LEO - first real gamma ray studies
Small Astron. Sat. C (Explorer 53/SAS-3) 1975 181 Scout 3.9 LEO - X-ray source positions, X-ray binary discovery
International Ultraviolet Explorer (IUE) 1978 470 Delta 18.7 GEO - UV spectra (GO mission)
Cosmic Background Explorer (COBE) 1989 2206 Delta 4.0 polar - confirmed Big Bang model, Nobel Prize
Extreme Ultraviolet Explorer (EUVE) 1992 3280 Delta 8.5 LEO - EUV source catalog (GO mission)
Rossi X-ray Timing Explorer (RXTE) 1995 2955 Delta >13.0 LEO - Studied time-variable X-ray sources (GO mission)
High Energy Transient Expt. (HETE) 1996 114 M/Oppy 0.0 LEO Gamma ray bursts, LAUNCH FAILURE
Submillimeter Wave Astronomy Sat. (SWAS) 1998 282 SMEX 6.5 LEO - searched for water and oxygen in Galaxy
Far Ultraviolet Spectroscopic Explorer (FUSE) 1999 1130 Delta 8.3 LEO - high resolution UV spectra (GO mission)
Wide Field Infrared Explorer (WIRE) 1999 187 SMEX 0.0 polar - infrared survey, PAYLOAD FAILURE
HETE 2 2000 124 M/Oppy 6.5? LEO Gamma ray bursts
Wilkinson Microwave Anisotropy Probe (WMAP) 2001 840 MIDEX >7.5 L2 - solved cosmology
Cosmic Hot Interstellar Plasma Spec. (CHIPS) 2003 60 M/Oppy 5.3 polar- EUV background spectrometer
Galaxy Evolution Explorer (GALEX) 2003 280 SMEX >5.7 LEO - UV observatory (GO mission)
Swift Gamma Ray Burst Explorer 2004 1470 MIDEX >4.1 LEO - Gamma ray bursts (GO mission)
Interstellar Boundary Explorer (IBEX) 2008 107 SMEX >0.2 HEO - neutral particles from interstellar shock
Heliophysics and Space Physics Explorers
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Grouped as: solar, deep space, earth orbit
Solar Explorer A (Explorer 30/NRL Solrad 8) 1965 57 Scout 2.0 LEO UV/X-ray photometers
Solar Explorer B (Explorer 37/NRL Solrad 9) 1968 119 Scout 6.4 LEO UV/X-ray photometers
Solar Explorer C (Explorer 44/NRL Solrad 10) 1971 119 Scout 7.0 LEO UV/X flux, Hard X, X-ray stellar sky survey
Transition Region and Coronal Explorer (TRACE) 1998 250 SMEX >10.5 polar - EUV high res solar imaging
Ramaty High Energy Solar Spectroscopic Explorer
(RHESSI) 2002 304 SMEX >7.0 LEO -solar flares
Anchored Interplanetary Monitoring Platform D 1966 57 Delta 4.5 PARTIAL LAUNCH FAILURE, in deep Earth-Moon orbit
(AIMP-D, Explorer 33) but good science results
Anchored Interplanetary Monitoring Platform E 1967 67 Delta 6.0 Lunar orbit; solar wind studies
(AIMP-E, Explorer 35)
International Sun-Earth Explorer 3 1978 479 Delta 18.5 L1 First L1 mission, later to comet
Advanced Composition Explorer (ACE) 1997 752 Delta >11.5 L1 Solar wind
S-1 1959 41 Juno LEO space physics, LAUNCH FAILURE
S-2 (Explorer 6) 1959 40 Able HEO studied magnetosphere
S-1A (Explorer 7) 1959 42 Juno LEO space physics
S-46 1960 10 Juno HEO space physics, LAUNCH FAILURE
S-30A (Explorer 8) 1960 41 Juno LEO ionosphere
S-45 1961 34 Delta LEO ionosphere, LAUNCH FAILURE
P-14 (Explorer 10) 1961 36 Delta HEO magnetosheric B, first direct solar wind detection
S-45A 1961 33 Juno LEO ionosphere, LAUNCH FAILURE
Energetic Particles Explorer A (Explorer 12) 1961 38 Delta HEO space physics, magnetosphere
Energetic Particles Explorer B (Explorer 14) 1962 38 Delta HEO space physics
Energetic Particles Explorer C (Explorer 15) 1962 44 Delta HEO space physics/STARFISH belt
Interplanetary Monitoring Platform A (Ex. 18) 1963 63 Delta HEO magnetosphere/solar wind
Beacon Explorer A 1964 54 Scout LEO ionosphere, LAUNCH FAILURE
Ionosphere Explorer A (Explorer 20) 1964 44 Scout LEO ionosphere
Interplanetary Monitoring Platform B (Ex. 21) 1964 62 Delta HEO magnetosphere/solar wind
Beacon Explorer B (Explorer 22) 1964 52 Scout LEO ionosphere
Injun Explorer A (Injun 4/Explorer 25) 1964 41 Scout LEO auroral particles
Energetic Particles Explorer D (Explorer 26) 1964 46 Delta HEO space physics
Beacon Explorer C (Explorer 27) 1965 55 Scout LEO ionosphere
Interplanetary Monitoring Platform C (Ex. 28) 1965 70 Delta HEO magnetosphere/solar wind
Direct Measurements Explorer A (Explorer 31) 1965 98 Delta MEO ionosphere
Interplanetary Monitoring Platform F (Ex. 34) 1967 74 Delta HEO magnetosphere/solar wind
Injun Explorer B (Injun 5/Explorer 40) 1968 70 Scout LEO radiation belts
Interplanetary Monitoring Platform G (Ex. 41) 1969 79 Delta HEO magnetosphere/solar wind
Interplanetary Monitoring Platform I (Ex. 43) 1971 288 Delta HEO magnetosphere/solar wind
Small Scientific Satellite A (Explorer 45) 1971 52 Scout HEO magnetosphere
Interplanetary Monitoring Platform H (Ex. 47) 1972 266 Delta HEO magnetosphere/solar wind
Interplanetary Monitoring Platform J (Ex. 50) 1973 272 Delta HEO magnetosphere/solar wind
Neutral Point Explorer (Hawkeye, Explorer 52) 1974 52 Scout HEO magnetosphere
International Sun-Earth Explorer 1 1977 340 Delta HEO magnetosphere
Dynamics Explorer 1 1981 424 Delta HEO solar-terrestrial
Dynamics Explorer 2 1981 403 Delta HEO solar-terrestrial
Charge Composition Explorer (AMPTE-CCE) 1984 240 Delta HEO magnetosphere
Solar and Anomalous Magnetospheric Particles
Explorer (SAMPEX) 1992 158 SMEX polar studied radiation belts
Fast Auroral Snapshot Explorer (FAST) 1996 162 SMEX polar auroral
Terriers 1999 125 UNEX polar ionosphere, PAYLOAD FAILURE
IMAGE 2000 494 MIDEX polar HEO, aurora and magnetosphere
THEMIS 1/2/3/4/5 2007 125*5 MIDEX HEO, magnetosphere constellation
Atmosphere and Earth Sciences
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S-56 1960 Scout MEO air density, LAUNCH FAILURE
S-55 1961 Scout LEO micrometeors, LAUNCH FAILURE
S-56A (Explorer 9) 1961 Scout MEO air density
S-55A (Explorer 13) 1961 Scout LEO micrometeors
S-55B (Explorer 16) 1962 Scout LEO micrometeors
Atmosphere Explorer A (Explorer 17) 1963 Delta LEO atmosphere
Air Density Explorer A (Explorer 19) 1963 Scout MEO air density
S-55C (Explorer 23) 1964 Scout LEO micrometeors
Air Density Explorer B (Explorer 24) 1964 Scout MEO air density
Geodetic Explorer A (Explorer 29) 1965 Delta MEO geodesy
Atmosphere Explorer B (Explorer 32) 1966 Delta LEO atmosphere
Geodetic Explorer B (Explorer 36) 1968 Delta polar MEO geodesy
Air Density Explorer C (Explorer 39) 1968 Scout MEO air density
Atmosphere Explorer C (Explorer 51) 1971 Delta LEO atmosphere
Meteoroid Technology Satellite (Explorer 46) 1972 Scout LEO micrometeors
Atmosphere Explorer D (Explorer 54) 1975 Delta Polar LEO atmosphere
Atmosphere Explorer E (Explorer 55) 1975 Delta LEO atmosphere
Dual Air Density Explorer (DAD-A/B) 1975 Scout LEO air density, LAUNCH FAILURE
Applications Explorer Mission A (HCMM) 1978 Scout LEO heat capacity mapping
Applications Explorer Mission B (SAGE) 1979 Scout LEO stratospheric aerosol and gas
Applications Explorer Mission C (MAGSAT) 1979 Scout LEO magnetic field mapping
Solar Mesosphere Explorer (SME) 1981 Delta LEO atmospheric studies
Student Nitric Oxide Explorer (SNOE) 1998 UNEX polar atmospheric studies
Aeronomy of Ice in the Mesosphere (AIM) 2007 SMEX LEO atmospheric studies