 Four follow-up satellites of the Explorer series were launched by the Juno 1 rocket in 1958, of these, Explorer 3 and 4 were successful, while Explorers 2 and 5 failed to reach orbit.The final flight of the Juno-1 booster, the satellite Beacon-1, also failed.The Juno-1 vehicle was replaced by the Juno-2 in 1959.