National Aeronautics and Space Administration
NASA History Division
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 --------------------------------------------------------------------- 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 equatorLink to Explorer Program Office at NASA GSFC
Updated February 22, 2006
Woody Smith, Author
Steve Garber, NASA History Web Curator
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