|Launch vehicle.||Thor DM-18 Able I rocket.|
|Launch date.||August 17, 1958, 12:18 UTC.|
|Launch site.||Cape Canaveral LC-17A.|
|Ceased operations.||Failed to orbit after 73.6 seconds.|
|Owner(s).||The United States Air Force (the USAF).|
|Major contractor(s) .||TRW Space Technology Laboratories.|
|Is it still in orbit.||The Rocket's upper stages fell in to the Atlantic Ocean and the satellite probably fell with it.|
|Launch mass,||83.8 pounds (38.0 kg).|
|Satellite type.||Lunar orbiter.|
The failure was suspected to be due to a turbopump bearing that came loose, causing the liquid oxygen pump to stop. The abrupt loss of thrust caused the Thor to lose attitude control and pitch downward, which caused the LOX tank to rupture from aerodynamic loads and resulting in complete destruction of the launch vehicle. Erratic telemetry signals were received from the payload and upper stages for 123 seconds after the explosion, and the upper stages were tracked to impact in the ocean.
The original plan was for the spacecraft to travel for 2.6 days to the Moon at which time a TX-8-6 solid propellant motor would fire to put it into a 29,000 kilometres (18,000 mi) lunar orbit which was to nominally last for about two weeks. Air Force officials stated that they were not surprised at the failure, adding that "it would have been more of a shock had the mission succeeded".
It was the only mission in the Pioneer program carried out by the United States Air Force, as subsequent missions were conducted by NASA.