Sputnik had no speaker, which is unsurprising for a metal ball in a vacuum. The stranger detail is that it did not put an audible tone into its radio signal either.
On October 4, 1957, the first artificial satellite began switching radio transmitters on and off as it circled Earth.[3][4][5] The famous recording sounds like a steady electronic beep, beep, beep, but the pitch was made in the receiving equipment below.[2][5] Sputnik supplied the rhythm. The radio supplied the note.
A signal you could not hear directly
Sputnik transmitted at 20.005 and 40.002 megahertz, frequencies far above human hearing.[2][3][5] Its transmitters sent an unmodulated carrier, roughly three-tenths of a second on and three-tenths off under normal conditions.[3][5]
A radio operator could turn that invisible pulse train into sound with a beat frequency oscillator.[5] The receiver mixed Sputnik's carrier with a nearby reference frequency, and the difference landed in the audible range.[5] Mix 20.005 MHz with 20.004 MHz, for example, and the result is a 1,000-hertz tone.
This means Sputnik recordings do not all have to sound alike. The St. Joseph High School Radio Club, which recorded the satellite in 1957, explains that the operator's oscillator setting determined the pitch.[5] Its surviving recording has a noticeable drop in tone as Sputnik passes overhead, but that is not the spacecraft changing melody.[5] It is Doppler shift: the received radio frequency changes as the satellite approaches and then recedes.[5]
There was another convenient reference already on the air. The U.S. time and frequency station WWV broadcast at 20.000 MHz, only 5 kilohertz below Sputnik's 20.005 MHz signal.[5][6] The two signals could produce an audible difference frequency in a receiver, while WWV also gave observers a stable standard for measuring how Sputnik's frequency moved.[5]
The first satellite was therefore public in an unusually practical way. Its one-watt signal sat within reach of amateur equipment, and radio operators around the world could listen without access to a Soviet ground station.[1][6] They were not receiving an audio program from space. They were making the radio-frequency pulses audible at home.
The pauses carried the message
Calling the sound a "beep" is still useful. It describes what people heard, and the timing was not arbitrary.
Sputnik's two transmitters alternated.[3] One sent while the other paused, with each burst lasting about 0.2 to 0.6 seconds.[3] Pressure and temperature switches inside the nitrogen-filled sphere could change the relationship between the bursts and pauses if conditions crossed set limits.[3] A different rhythm could warn that the sphere had lost pressure or become too hot or cold.[3] It was telemetry with very little vocabulary, but the satellite named Simplest Satellite-1 was not pretending to be chatty.
The carrier revealed more after it left the antennas. Observers used its Doppler shift to help track the orbit, and scientists studied how the signal propagated through the ionosphere.[3][6] Orbital drag also helped estimate the density of the upper atmosphere.[3][6] A transmitter designed to prove that an object was overhead became both a beacon and a measuring tool.
Most accounts understandably keep the beep. It is a better sound bite than "intermittent unmodulated carrier," and it made an invisible orbit immediate. The Soviet Union had placed an object over people's heads, and with ordinary radio equipment they could hear evidence arrive from it every 96 minutes.[1][2]
The technical correction makes the moment better, not smaller. The sound associated with the beginning of the Space Age was completed on Earth. Sputnik supplied the timing; each receiver supplied the pitch.
Sources
- National Air and Space Museum: What was Sputnik and why was it important?
- Engineering and Technology History Wiki: Sputnik
- Asif A. Siddiqi: Sputnik remembered, part 1
- NASA History: Korolev, Sputnik, and the International Geophysical Year
- St. Joseph High School Radio Club: Sputnik recording with Doppler shift
- ARRL: Remembering the launch of Sputnik 1