Editing Explorer 4 (section)

{{Short description|Satellite launched by the United States (1958)}}
{{Use American English|date=November 2021}}
{{Use dmy dates|date=November 2021}}
{{Infobox spaceflight
| name                = Explorer 4
| names_list          = Explorer IV<br/>1958 Epsilon 1
| image               = Explorer4 instruments.png
| image_caption       = Explorer 4 instruments
| image_size          = 300px

| mission_type        = [[Earth science]]
| operator            = [[Jet Propulsion Laboratory|JPL]] / [[Army Ballistic Missile Agency]]
| Harvard_designation = 1958 Epsilon 1
| COSPAR_ID           = 1958-005A
| SATCAT              = 00009
| mission_duration    = 71 days (achieved)

| spacecraft          = Explorer IV
| spacecraft_type     = Science Explorer
| spacecraft_bus      = [[Explorer 1]]
| manufacturer        = [[Jet Propulsion Laboratory]]
| launch_mass         = {{cvt|25.50|kg}}
| dimensions          = Cylinder: {{cvt|94.6|cm}} long,<br/>{{cvt|16.5|cm}} in diameter,<br/>Total length with attached rocket motor: {{cvt|2.05|m}}
| power               =

| launch_date         = 26 July 1958, 15:00:57 [[Greenwich Mean Time|GMT]]
| launch_rocket       = [[Juno I]] (RS-24)
| launch_site         = [[Cape Canaveral Space Force Station|Cape Canaveral]], [[Cape Canaveral Launch Complex 5|LC-5]]
| launch_contractor   = [[Army Ballistic Missile Agency]]

| entered_service     = 26 July 1958
| last_contact        = 5 October 1958
| destroyed           = 
| decay_date          = 23 October 1959

| orbit_reference     = [[Geocentric orbit]]<ref name="Trajectory"/>
| orbit_regime        = [[Medium Earth orbit]]
| orbit_periapsis     = {{cvt|263|km}} 
| orbit_apoapsis      = {{cvt|2213|km}} 
| orbit_inclination   = 50.30°
| orbit_period        = 110.20 minutes
| apsis               = gee

| instruments         = Charged Particle Detector

| programme           = '''Explorer program'''
| previous_mission    = [[Explorer 3]]
| next_mission        = [[Explorer 5]]
}}

'''Explorer 4''' was an American satellite launched on 26 July 1958. It was instrumented by Dr. [[James van Allen]]'s group. The [[United States Department of Defense|Department of Defense]]'s [[Advanced Research Projects Agency]] (ARPA) had initially planned two satellites for the purposes of studying the [[Van Allen radiation belt]]s and the effects of [[nuclear weapon|nuclear explosions]] upon these belts (and the [[Earth]]'s [[magnetosphere]] in general), however Explorer 4 was the only such satellite launched as the other, [[Explorer 5]], suffered launch failure.

Explorer 4 was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (protons and electrons) trapped in the terrestrial radiation belts.