The 1924 Mars radio silence

In August 1924, when Mars passed close to Earth, the US Navy ordered the country's radio stations quiet for a five-minute pause every hour from 21 to 24 August, so listeners could pick up any signal from Mars.

In August 1924 Mars came within 55.7 million kilometres of Earth, the closest approach since 1804, according to radio historian Thomas H. White. Centauri Dreams reports that the US Navy ordered a country-wide radio quiet, a five-minute pause every hour from 21 to 24 August. The plan was to let observatories worldwide listen for Martians. The Henry Ford gives the dates as 22 to 24 August and says the military asked for the quiet periods each day.

The Henry Ford describes the listening equipment on the ground as an SE-950 NESCO radio and Jenkins's picture-transmission machine, the Radio Camera. Any stray signal bleeding through the equipment of Jenkins and Todd could, in theory, have shown that life existed on Mars.

The silence was a planned scientific and military exercise. White adds that any Martians would, for once, have found Earth listening.