If Sputnik 1 was the beep-beep-beep heard round the world, Explorer 1 announced itself 50 years ago this week by the collective sigh of relief from an anxious American public.

It was late in the evening, Jan. 31, 1958, almost four months after the Soviet Union stunned the world on Oct. 4 with the launching of Sputnik, the first artificial Earth satellite. A second, larger Sputnik soon followed, carrying a canine passenger.

The first American attempt, with the modest Vanguard 1 in December, was an embarrassing failure, immediately derided as “flopnik.”

Now it was up to a rocket assembled from German V-2 technology and American upper stages to boost the slender, bullet-shape Explorer into orbit. Liftoff from Cape Canaveral, Fla., looked good. That was progress. Vanguard had shut down a few feet above the launching pad, collapsing and exploding for all to see on live television.

But flight controllers waited and waited for a signal that Explorer had reached orbit. Tracking antennas were sparse in those days, communications unreliable. Only after the satellite had almost completely circled the globe was its signal finally received, by alert amateur radio operations near Los Angeles.