Touching the Moon with Radio

On January 10, 1946, engineers at the U.S. Army Signal Corps lab in New Jersey pointed a radar antenna at the Moon and waited. They transmitted a pulse at 111.5 megahertz -- powerful enough to travel a quarter million miles, bounce off the lunar surface, and return. The round trip took 2.5 seconds. When the echo appeared on their oscilloscope, it proved humans could touch another world with electromagnetic radiation.

The project was called Diana, after the Roman Moon goddess. Led by John DeWitt, it grew from wartime radar work. Radar had been decisive in World War II, but no one had tried bouncing a signal off something 238,000 miles away.

It worked. The return signal was weak, buried in noise, requiring hundreds of averaged pulses to extract. But the timing was perfect. Light travels at a fixed speed, and the delay matched the distance.

The experiment had no immediate practical use. But if you could bounce a signal off the Moon, you could measure its distance with precision, map its surface with radio, communicate with spacecraft behind it. Within a decade, radar became the primary tool for studying the solar system. Just as Morse code compressed communication into dots and dashes, radar compressed distance into time, turning the speed of light into a measuring tool.

the full moon — the target that reflected the first radar signal sent from earth in 1946. source: wikimedia commons

Project Diana also laid the groundwork for satellite communication. If radio could reach the Moon, it could certainly reach satellites in low orbit. By the 1960s, communications satellites were bouncing signals around the planet.

Today, radar guides planes, tracks weather, maps terrain, watches for asteroids. The principle is always the same: send a signal, listen for the echo, measure the delay. That first faint blip from the Moon said hello to another world. That was enough.