The lack of consistency stems in part from the fact that the first explorer missions predated the formation of NASA. As a consequence, Explorers 2 and 5 got counted in the sequence even though they failed to achieve orbit. Following the creation of NASA on October 1, 1958, the agency established the practice of no longer counting such launches, but the problem of definition remained a real one.

This was so because even the early Explorers performed a large variety of scientific missions ranging from energy particle exploration through atmospheric and ionospheric studies to investigations of micrometeroids, air density, radio astronomy, geodesy, and gamma ray astronomy--not to mention interplanetary and solar monitoring. While Langley Research Center and Goddard Space Flight Center designed and built many of the early "Explorer" satellites, contractors and universities provided some experiments, components, and even entire spacecraft. The one constant amidst this diversity was that the early "Explorers" were smaller, simpler, and less costly than the orbiting observatories also used in scientific exploration of physical and astronomical phenomena.

Unfortunately for even this single piece of consistency in the midst of diversity, it did not apply solely to what may be called the "Explorer" series of spacecraft proper; there were numerous other explorer-class satellites that did not bear the name "Explorer." These included Vanguard 1-3, Pioneer 5, Ariel 1-2, Alouette 1, and a San Marco series of spacecraft launched from the site of that name off the coast of Kenya, Africa. All of these smaller, simpler satellites carried out missions analogous to those of the "Explorers," but they bore different names and were not counted in the explorer series.

To confuse the issue further, other similar missions involving explorer-class satellites, launched jointly with international partners, sometimes bore the "Explorer" name but not a mission number in the "Explorer" series. These included the International Sun-Earth Explorer missions (ISEE 1-3) as well as other missions with names like Aeros, Ariel, and Boreas. There have also been a few larger spacecraft of the observatory class that have borne the name "Explorer" (for example, the Cosmic Background Explorer launched in 1989), further underlining the complexity of the issue regarding which spacecraft fit into what category. The listing at the end of this narrative shows the satellites that clearly belong in the "Explorer" series because they were relatively small and uncomplicated, performed a scientific mission, and -- until quite recently -- appeared in satellite situation reports and post-launch reports under the name "Explorer," accompanied by a mission number. (This last practice ended with Explorer 55, however.)

Mission                     Purpose                     Launch Date

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Explorer 1   Energy particles studies (discovered      Jan. 31, 1958

                  Van Allen radiation belt)



Explorer 2   Failed to achieve orbit                   Mar. 5, 1958



Explorer 3   Provided Van Allen belt data              Mar. 26, 1958



Explorer 4   Spatial study of Argus radiation          Jul. 26, 1958



Explorer 5   Failed to achieve orbit                   Aug. 24, 1958



Explorer 6   Magnetosphere studies--radiation          Aug. 7, 1959

                    belt meteorology



Explorer 7   Studied energetic particles; micro-       Oct. 13, 1959

                    meteoroids



Explorer 8   Ionosphere; atmospheric composition       Nov. 3, 1960



Explorer 9   Measured characteristics and composi-     Feb. 16, 1961

                    tion of the upper thermosphere &

                    lower exosphere over entire globe



Explorer 10   Studied interplanetary magnetic          Feb. 25, 1961

                    field near Earth;

                    particle radiations



Explorer 11   Astronomical studies: gamma rays         Apr. 27, 1961

                    from space



Explorer 12   Magnetospheric studies: how the ra-      Aug. 16, 1961

                    diation belts around the Earth

                    receive, trap, and lose their

                    charged particles



Explorer 13   Technology satellite; studied            Aug. 25, 1961

                    micrometeoroids



Explorer 14   Studied charged particles and mag-       Oct. 2, 1962

                    netic fields in magnetosphere



Explorer 15   Study of enhanced radiation belt         Oct. 27, 1962



Explorer 16   Technology satellite; studied            Dec. 16, 1962

                    micrometeoroids



Explorer 17   Collected data on temperature, com-      Apr. 3, 1963

                    position, density, and pressure

                    for studies of atmospheric physics

                    on a global basis



Explorer 18   Studied charged particles and mag-       Nov. 26, 1963

                    netic fields in cislunar

                    space (Planetary Monitoring 

                    Platform--IMP) 



Explorer 19   Determined air density of upper at-      Dec. 19, 1963

                    mosphere; studied heating effects

                    from energetic particles and

                    ultra-violet radiation



Explorer 20   Probe of topside of ionosphere           Aug. 25, 1964



Explorer 21   Studied magnetic fields and their        Oct. 4, 1964

 (IMP)              interactions with solar plasma,

                    solar wind, cosmic rays,

                    intensities and distribution 

                    of space radiation



Explorer 22   Ionospheric and geodetic research        Oct. 9, 1964

                    (Beacon-Explorer--BE)



Explorer 23   Measured micrometeoric penetration       Nov. 6, 1964



Explorer 24   Provided information on radiation-       Nov. 21, 1964

                    air density relationships in

                    upper atmosphere



Explorer 25   Same as Explorer 24 except for use       Nov. 21, 1964

                    of radiation sensors instead

                    of radio beacon to collect data



Explorer 26   Studied how high-energy particles        Dec. 21, 1964

                    are injected, trapped, and lost

                    in the Van Allen Belt and the 

                    depth to which high-energy solar

                    protons penetrate into the 

                    geomagnetic field



Explorer 27   Studied variations of electron den-      Apr. 29, 1965

 (BE)               sity and orbital perturbations

                    in order to deduce the size and

                    shape of the Earth and the nature

                    of its gravitational field



Explorer 28   Provided data on Earth's magneto-        May 29, 1965

 (IMP)              sphere



Explorer 29   Part of the U.S. Geodetic Satellite      Nov. 6, 1965

 (GEOS 1)           Program, it compared tracking 

                    system accuracies; studied the 

                    gravitational field; and improved 

                    geodetic datum accuracies



Explorer 30   Monitored solar x-rays during the        Nov. 19, 1965

 (Solrad-1)         International Quiet Sun Year



Explorer 31   Sounded the topside of the iono-         Nov. 29, 1965

 (Direct Mea-       sphere using topside sounder and

  surements         measurement techniques

  Satellite--

  DME)



Explorer 32   Studied the structure and physics of     May 25, 1966

                         the upper atmosphere



Explorer 33   Provided data on solar plasma,           Jul. 1, 1966

 (IMP)              energetic particles, and

                    magnetic fields



Explorer 34   Studied solar and galactic cosmic ra-    May 24, 1967

 (IMP)              diation, solar plasma, and

                    energetic particles within the

                    magnetosphere and interplanetary

                    magnetic field



Explorer 35   Orbited Moon to measure solar plasma     Jul. 19, 1967

 (IMP)              flux, energetic particles,

                    magnetic fields, and cosmic

                    dust; no detectable lunar 

                    magnetic field discovered



Explorer 36   Same as Explorer 29                      Jan. 11, 1968

 (GEOS 2)



Explorer 37   Monitored solar x-ray emissions          Mar. 5, 1968

 (Solrad-2)



Explorer 38   Monitored low-frequency cosmic radio     Jul. 4, 1968

 (Radio As-         noise

  tronomy

  Explorer

  -RAE)



Explorer 39   Used radio beacon to study the inter-    Aug. 8, 1968

                    action of solar radiation with

                    the upper atmosphere during the

                    solar maximum



Explorer 40   Same as Explorer 39 but used a variety   Aug. 8, 1968

                    of instruments including 

                    spherical partical analyzers



Explorer 41   Studied environment within and beyond    Jun. 21, 1969

 (IMP)               Earth's magnetosphere



Explorer 42   Mapped the universe in x-ray wave-       Dec. 12, 1970

 (Small              lengths; discovered x-ray 

  Astronomy          pulsars and evidence 

  Satellite          black holes.  Also called

  SAS)               Uhuru because launched from

                     San Marco off the coast of

                     Kenya on Kenya's Independence

                     Day)



Explorer 43   Studied cislunar environment during a    Mar. 13, 1971

 (IMP)              period of decreasing solar 

                    activity



Explorer 44   Monitored solar flux                     Jul. 8, 1971

 (Solrad-3)



Explorer 45   Measured meteoroid penetration and       Nov. 15, 1971

 (Small             impact velocity

  Scientific

  Satellite)



Explorer 46   Same as Explorer 45                      Aug. 13, 1972

 (Meteoroid

  Technology

  Satellite)



Explorer 47   Same as Explorer 43                      Sep. 22, 1972

 (IMP)



Explorer 48   Studied gamma radiation                  Nov. 16, 1972

 (SAS, also

  called

  Gamma Ray

  Explorer)



Explorer 49   From lunar orbit, measured the inten-    Jun. 10, 1973

 (Radio             sity of radio signals from

  Astronomy         celestial sources and provided

  Explorer)         data on lunar gravity



Explorer 50   Studied particle and field interac-      Oct. 25, 1973

 (IMP)              tions in the distant magnetotail



Explorer 51   Collected data on the relationship of    Dec. 15, 1973

                    solar ultraviolet activity to

                    atmospheric physics around the

                    world



Explorer 52   Studied the interaction of the solar     Jun. 3, 1974

 (Hawkeye)          wind with the geomagnetosphere

                    over the Earth's polar caps



Explorer 53   Measured the x-ray emissions of dis-     May 7, 1975

 (SAS)              crete extragalactic sources but

                    also investigated other regions

                    of the electromagnetic spectrum



Explorer 54   Obtained data relating ultraviolet ac-   Oct. 6, 1975

 (Atmospheric       tivity to atmospheric composition

  Explorer)         in the lower atmosphere



Explorer 55   Investigated the chemical processes      Nov. 20, 1975

 (Atmospheric       and energy transfer mechanisms

  Explorer)         controlling the structure and

                    behavior of the Earth's

                    atmosphere and ionosphere



Explorer 56   Obtained a global data base for strat-   Feb. 18, 1979

 (Strato-           ospheric aerosols and ozone to

  spheric           promote a better understanding 

  Aerosol           of the Earth's environmental 

  and Gas           quality and radiation budget

  Experiment

  -- SAGE)



Explorer 57   Studied solar-terrestrial interactions   Aug. 3, 1981

 (Dynamics          and their effects on such issues

  Explorer-1        as the polar wind and the inter-

  -- DE-1)          change of particles between the 

                    magnetosphere and the ionosphere



Explorer 58   Same as Explorer 57, except that Ex-     Aug. 3, 1981

 (DE-2)             plorer 58 operated in a lower

                    polar orbit than its companion

                    satellite



Explorer 59   Studied reactions between sunlight,      Oct. 6, 1981

 (Solar             ozone, and other constituents

  Mesosphere        of the atmosphere -- in some 

  Explorer)         0sense an extension of Explorer 

                    56's mission to the polar regions

                    it could not examine from its

                    orbit around the equator