Sputnik 1

On 4 October 1957 a new sound arrived that anyone could hear.

It came in on 20.005 megahertz, a frequency ordinary people already owned receivers for. That was no accident.

This is all of it. A sealed aluminium sphere about the width of a beach ball, four trailing whips, three batteries and a radio transmitter.

There was nothing else on board. No camera, no instruments, nothing to measure what it was flying through.

The satellite the Soviet Union initially meant to launch weighed well over a tonne and carried up to 300 kilograms of instruments. It was an orbital laboratory called Object D, and by the end of 1956 it was late.

So they threw it out. Object PS, the elementary satellite, was approved in February 1957 and built in under a month.

One of the most consequential spacecraft ever flown was built when the original plan collapsed.

Four whips in two matched pairs, 2.4 and 2.9 metres. Two lengths because two frequencies.

The 20 megahertz signal sat inside the shortwave band that ham radio operators already tuned across every night, anywhere in the world. The 40 megahertz signal was for the military, and for measuring how the ionosphere bent a radio wave on its way down.

One of those choices was engineering. The other meant that anybody with a receiver could listen for themselves, which made the proof impossible to ignore.

Three silver zinc batteries, 51 kilograms of a total 83.6.

Sixty percent of its mass was the power to keep broadcasting.

Two hemispheres two millimetres thick, closed with 36 bolts and a rubber seal, filled with dry nitrogen at 1.3 atmospheres.

The length of each pulse encoded the temperature inside the sphere. A barometric switch would have altered the signal if the shell was ever punctured.

It was not just a beep, it was transmitting its own vital signs, and most on the ground had no idea.

Sputnik was sixth magnitude, right at the limit of what an unaided eye can pick out.

The bright object people stood in gardens to watch was the spent core stage of the rocket that put it there, 26 metres of it, first magnitude and unmistakable.

Millions of people looked up at the wrong object. Sputnik was never really a sight. It was a sound.

A junior engineer lieutenant named Borisov caught the first signal about two minutes after separation.

Korolev, who was in overall command of the launch campaign, did not celebrate. He drove out to the mobile radio station and waited one full cycle, 96 minutes, to hear it come round again.

Anything can go up once, but completing an orbit is another matter. Only then did he telephone Khrushchev.