Sputnik 1 was almost empty: a 58 cm aluminum sphere weighing 83.6 kg, pressurized to about 1.3 atmospheres with dry nitrogen, and carrying nothing more than a 1-watt radio transmitter, three silver-zinc batteries, a small cooling fan, and a thermal regulation system — and yet the beeping signal it broadcast on 20.005 and 40.002 MHz for 22 days is what made the Space Race.
The transmitter broadcast on 20.005 and 40.002 MHz, frequencies chosen to be picked up by amateur radio operators. The signal was a series of 0.3-second beeps alternating between the two frequencies, repeating every 0.6 seconds. Amateurs worldwide could detect it with a standard communications receiver, and within hours of the launch, operators in the United States, the United Kingdom, and West Germany had independently confirmed it. The signal was the satellite’s entire purpose; the IGY’s amateur radio network was used as a free verification system.
The silver-zinc batteries powered the transmitter for 22 days, with intermittent signals continuing into late October. The final signal was received on November 26, 1957. The thermal regulation system, the most underappreciated part of the design, used a circulating fan and a heat exchanger to keep the interior between roughly 10°C and 40°C, which is why the satellite was built as a sphere (the lowest surface-area-to-volume ratio of any shape). The satellite re-entered the atmosphere over the Pacific Ocean on January 4, 1958, after about 1,440 orbits.
For the rest of the Sputnik article and the space race overview.