The NASA Explorer program of scientific satellites

NASA's Explorer program covers a continuing, heterogeneous series of small and medium size scientific satellites which form the backbone of the US space science program. Explorer satellites are to be contrasted with the large 'Observatory' class missions such as the astronomical Great Observatories and the EOS Terra/Aqua/Aura remote sensing spacecraft.

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 This system was superseded by pre-launch names indicating specific sub-programs within Explorer; for instance, AE-A was Atmosphere Explorer A. Letters A,B,C..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

                                                    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

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

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