[2] It was a reflight of the failed Explorer S-56 mission, and consisted of a 7 kg (15 lb), 3.66 m (12.0 ft) balloon which was deployed into a medium Earth orbit.

[4] Because of its symmetrical shape, Explorer 9 was selected for use in determining upper atmospheric densities as a function of altitude, latitude, season, and solar activity.

Density values near perigee were deduced from sequential observations of the spacecraft position, using optical (Baker-Nunn camera network) and radar tracking techniques.

[10] With three years of data, scientists were able to discern a seasonal variation to the exosphere's density, with an increase of 25% observed in winter over summer at a reference latitude of 39°.

[1] A replica of the spacecraft, possibly a flight backup, is currently located in the Smithsonian Institution's National Air and Space Museum, although it is not on display.