Editing Explorer 11 (section)

{{Short description|NASA satellite of the Explorer program}}
{{confused|Explorer Two (disambiguation){{!}}Explorer II}}
{{Use American English|date=November 2021}}
{{Use dmy dates|date=November 2021}}
{{Infobox spaceflight
| name                  = Explorer 11
| names_list            = S-15<br/>NASA S-15
| image                 = Explorer 11 ground.gif
| image_caption         = Explorer-11 satellite
| image_size            = 300px

| mission_type          = [[Gamma-ray astronomy]]
| operator              = [[NASA]]
| Harvard_designation   = 1961 Nu 1
| COSPAR_ID             = 1961-013A
| SATCAT                = 00107
| website               = [http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/explorer11.html Explorer 11 at GSFC]
| mission_duration      = {{time interval|April 27, 1961, 14:16:38|November 17, 1961}}
| spacecraft            = Explorer XI
| spacecraft_type       = Science Explorer
| spacecraft_bus        = S-15
| manufacturer          = [[Goddard Space Flight Center]]
| launch_mass           = {{cvt|37.2|kg}} <ref name=NASA>{{cite web|title=Explorer 11|publisher=NASA|url=https://heasarc.gsfc.nasa.gov/docs/heasarc/missions/explorer11.html}}</ref>
| dimensions            = 
| power                 =

| launch_date           = 27 April 1961, 14:16:38 [[Greenwich Mean Time|GMT]]
| launch_rocket         = [[Juno II]] (AM-19E)
| launch_site           = [[Cape Canaveral Space Force Station|Cape Canaveral]], [[Cape Canaveral Launch Complex 26|LC-26B]]
| launch_contractor     = [[Army Ballistic Missile Agency]]

| entered_service       = 27 April 1961
| last_contact          = 17 November 1961
| destroyed             = 
| decay_date            =

| orbit_reference       = [[Geocentric orbit]]<ref name="Trajectory">{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1961-013A|title=Explorer 11|publisher=NASA|date=28 October 2021|access-date=4 November 2021}} {{PD-notice}}</ref>
| orbit_regime          = [[Low Earth orbit]]
| orbit_periapsis       = {{cvt|486|km}}
| orbit_apoapsis        = {{cvt|1786|km}}
| orbit_inclination     = 28.90°
| orbit_period          = 108.10 minutes
| apsis                 = gee

| instruments           = Crystal Sandwich / Cherenkov Counter<br/>Phoswich-Cherenkov Counter Telescope

| programme             = '''Explorer program'''
| previous_mission      = [[Explorer 10]]
| next_mission          = [[Explorer S-45A (satellite)|Explorer S-45A]]
}}

'''Explorer 11''' (also known as '''S-15''') was a [[NASA]] satellite that carried the first space-borne [[Gamma-ray astronomy|gamma-ray telescope]]. This marked the beginning of space gamma-ray astronomy. Launched on 27 April 1961 by a [[Juno II]], the satellite returned data until 17 November 1961, when power supply problems ended the science mission. During the spacecraft's seven-month lifespan it detected twenty-two events from gamma-rays and approximately 22,000 events from [[Cosmic ray|cosmic radiation]].