Explorer 9

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Explorer 9
Explorer 9.jpg
Explorer 9 before launch
Mission type Air density
Operator NASA
Harvard designation 1961 Delta 1
Start of mission
Launch date 16 February 1961, 13:05:00 (1961-02-16UTC13:05Z) UTC
Rocket Scout X-1 ST-4
Launch site Wallops LA-3
End of mission
Decay date April 9, 1964 (1964-04-10)
Orbital parameters
Reference system Geocentric
Regime Medium Earth
Perigee 635 kilometers (395 mi)
Apogee 2,581 kilometers (1,604 mi)
Inclination 38.8 degrees
Period 118.4 minutes

Explorer 9, known as S-56A before launch, was an American satellite which was launched in 1961 to study the density and composition of the upper thermosphere and lower exosphere.[1] It was a reflight of the failed S-56 mission, and consisted of a 7-kilogram (15 lb), 3.7-meter (12 ft) balloon which was deployed into a medium Earth orbit.[2] The mission was conducted by NASA's Langley Research Center.

The launch of Explorer 9

Explorer 9 was launched from Launch Area 3 at the Wallops Flight Center, atop a Scout X-1 rocket with the serial number ST-4. It was the first spacecraft launched from Wallops Island to achieve orbit, with one previous attempt having failed. The launch occurred at 13:05:00 UTC on 16 February 1961, and resulted in Explorer 9 being deployed into an orbit with an apogee of 2,581 kilometres (1,604 mi), a perigee of 635 kilometres (395 mi), 38.8 degrees of inclination and a period of 118.4 minutes.[3] It was assigned the Harvard designation 1961 Delta 1.[4]

The second of six identical air density research satellites to be launched, Explorer 9 was the first to successfully reach orbit. It was still operational when the next satellite, Explorer 19, was launched, allowing simultaneous readings to be taken and compared.[5] It decayed from orbit on 9 April 1964.[3]


  1. ^ Smith, Woody. "Explorer Spacecraft Series". NASA History Division. Retrieved 17 June 2010. 
  2. ^ Wade, Mark. "S-56". Encyclopedia Astronautica. Retrieved 17 June 2010. 
  3. ^ a b McDowell, Jonathan. "Satellite Catalog". Jonathan's Space Page. Retrieved 17 June 2010. 
  4. ^ McDowell, Jonathan. "Launch Log". Jonathan's Space Page. Retrieved 17 June 2010. 
  5. ^ Krebs, Gunter. "Explorer: AD". Gunter's Space Page. Retrieved 17 June 2010.