[6][7] The first launch of a Juno II, Pioneer 3 on 6 December 1958, suffered a premature first-stage cutoff, preventing the upper stages from achieving sufficient velocity.

[6][7] After Pioneer 4, NASA shifted their lunar efforts to the bigger Atlas-Able booster and decided instead to utilize the Juno II for Earth orbital launches.

The attempted launch of an Explorer satellite on 16 July 1959 failed dramatically when the Juno II lost control almost immediately at liftoff, performing a cartwheel before the range safety officer sent the destruct command.

[6][7][9] The almost fully fueled booster crashed a few hundred feet from the pad, blockhouse crews watching in stunned surprise at the upper stage motors burning on the ground.

[6][7][10] One intended experiment on this mission was the ejection of four flares stowed in the interstage section, which would be tracked and photographed during the launch.

The JPL team who developed the Juno II had originally only intended it for the Pioneer lunar probes and their interest started waning as soon as NASA began Earth orbital launches with the vehicle.

Even worse, most of the design team had been disbanded and its members reassigned to other projects, making it difficult to obtain technical information for the Juno II.

The conversion of the booster for LEO launches also threw off the calibration of the spinning tub third stage which was designed for the tiny Pioneer probes and not the larger Explorer satellites.

[6][7] A control cable came loose during ascent and wrapped itself around the spinning third-stage tub, damaging the upper stages and payload.

[6][7] The instrument unit lost power following first-stage separation, resulting in no second-stage ignition and the payload falling into the ocean instead of reaching orbit.