whAt ArE thE ChAllEngEs oF spACE ExplorAtion
A．thE ChAllEngE wAs DEvEloping roCkEts powErFul Enough AnD rEliABlE Enough to Boost A sAtEllitE into orBit.
B．thE ChAllEngE wAs BuilDing thE sAtEllitEs thEmsElvEs.
C．EnginEErs Also hAD to BuilD trACking stAtions to mAintAin rADio CommuniCAtions with thEsE ArtiFiCiAl "moons" As thEy CirClED thE plAnEt.
D．thE DEvElopmEnt oF roCkEts AnD sAtEllitEs.