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. It was a reflight of the failed S-56 mission, and consisted of a 7-kilogram (15 lb), 3.7-metre (12 ft) balloon which was deployed into a medium Earth orbit. The mission was conducted by NASA's Langley Research Center.
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 a 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. It was assigned the Harvard designation 1961 Delta 1.
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. It decayed from orbit on 9 April 1964.