Editing Explorer 3 (section)

{{Short description|Satellite launched by the United States (1958)}}
{{For|International Sun/Earth Explorer 3 (ISEE-3) satellite|International Cometary Explorer}}
{{Use American English|date=February 2021}}
{{Use dmy dates|date=February 2021}}
{{Infobox spaceflight
| name               = Explorer 3
| names_list         = Explorer III<br/>1958 Gamma
| image              = Explorer III.jpg
| image_caption      = Juno I rocket with Explorer 3 satellite
| image_size         = 
| mission_type       = [[Earth science]]
| operator           = [[Jet Propulsion Laboratory|JPL]] / [[United States Army Ordnance Corps|U.S. Army Ordnance]]
| Harvard_designation = 1958 Gamma
| COSPAR_ID          = 1958-003A
| SATCAT             = 00006
| mission_duration   = 93 days (achieved)<br/>120 days (planned)
| spacecraft         = Explorer III
| spacecraft_type    = Science Explorer
| spacecraft_bus     = [[Explorer 1]]
| manufacturer       = [[Jet Propulsion Laboratory]]
| launch_mass        = {{cvt|14.1|kg}}
| payload_mass       = {{cvt|8.4|kg}}
| dimensions         = {{cvt|203|cm}} length<br/>{{cvt|15.2|cm}} diameter
| power              = 60 [[watt]]s
| launch_date        = 26 March 1958, 17:38:01 [[Greenwich Mean Time|GMT]]
| launch_rocket      = [[Juno I]] (RS-24)
| launch_site        = [[Eastern Range|Atlantic Missile Range]], [[Cape Canaveral Launch Complex 5|LC-5]]
| launch_contractor  = [[Army Ballistic Missile Agency]]
| entered_service    = 26 March 1958
| last_contact       = 28 June 1958<ref name="Trajectory"/>
| destroyed          = 
| decay_date         = 28 June 1958<ref name="Trajectory"/>
| orbit_reference    = [[Geocentric orbit]]<ref name="Trajectory"/>
| orbit_regime       = [[Medium Earth orbit]]
| orbit_periapsis    = {{cvt|186|km}}
| orbit_apoapsis     = {{cvt|2799|km}}
| orbit_inclination  = 33.38°
| orbit_period       = 115.70 minutes
| apsis              = gee
| instruments        = Cosmic Ray counter<br/>Micrometeorite detector
| programme          = '''Explorer Program'''
| previous_mission   = [[Explorer 2]]
| next_mission       = [[Explorer 4]]
}}

'''Explorer 3''' (Harvard designation '''1958 Gamma''') was an American artificial satellite launched into [[medium Earth orbit]] in 1958. It was the second successful launch in the [[Explorer program]], and was nearly identical to the first U.S. satellite [[Explorer 1]] in its design and mission.