Explorer 1 was designed and built by the Jet Propulsion Laboratory (JPL), while a Jupiter-C rocket was modified by the Army Ballistic Missile Agency (ABMA) to accommodate a satellite payload; the resulting rocket known as the Juno I. The Jupiter-C design used for the launch had already been flight-tested in nose cone reentry tests for the Jupiter intermediate-range ballistic missile (IRBM) and was modified into Juno I. Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C and building Explorer 1 in 84 days. However, before work was completed, the Soviet Union launched a second satellite, Sputnik 2, on 3 November 1957. The U.S. Navy attempted to put the first U.S. satellite into orbit but failed with the launch of the Vanguard TV-3 on 6 December 1957.
exPLorER 1 waS desiGnED And builT By tHE jEt prOpulsion LaBORatorY (jPl), WhiLE A juPiTEr-c rockeT wAS MOdified By the ARMY BAllIStiC missIlE agency (Abma) to AcCommOdATe a saTelLiTe pAYLOad; The rEsulting roCKet kNown As The jUNO i. the juPiteR-C DesiGn used For tHe LAUncH had aLrEady been fliGHt-teSted in noSE cone reeNtry teStS For the juPIter IntERMedIate-ranGe bAllistIc MiSsile (irBm) anD was mODIFiEd into juNo I. WoRking closElY tOgETher, abmA aNd jpl comPleted the jOb of moDiFYiNg the JupIter-c and BUILdiNg ExpLOrEr 1 In 84 daYs. hoWeVeR, befoRe wORk Was ComPLeTEd, the sOViET Union LauNcHEd A seCoND sAtelliTE, SpUtNik 2, on 3 novembeR 1957. thE U.s. nAVy aTtEMPTed tO PUt the FirSt u.s. sAteLlite into OrBiT bUT fAILed with the LauNcH of tHE VanguArd tV-3 on 6 decEmber 1957.