US Army in 1946 sent radar pulse towards moon and something astonishing happened 2.5 seconds later

What happens when you point a wartime radar at the Moon? In 1946, the US Army decided to find out. Just 2.5 seconds after sending a radio pulse towards the lunar surface, engineers detected something astonishing: the signal had come back.

At Camp Evans in Fort Monmouth, New Jersey, a team of military and civilian personnel waited as the modified radar equipment sent a series of radio pulses towards the rising Moon. The target was nearly 384,000 kilometres away, far beyond anything conventional wartime radar had been designed to reach.

Then, about 2.5 seconds after transmission, a faint signal appeared on an oscilloscope.


It was the echo of the pulse bouncing off the lunar surface and making its way back to Earth.

The experiment, known as Project Diana, was led by Lieutenant Colonel John DeWitt. Historical records maintained by the InfoAge Science & History Museums identify it as the first successful attempt to bounce an artificial signal off another celestial body.

The timing was crucial. Travelling at the speed of light, the radio pulse had covered roughly 800,000 kilometres on its journey to the Moon and back. The delay recorded by the team matched what their calculations had predicted, providing evidence that the signal had actually made the extraordinary round trip.

A wartime radar system takes aim at the Moon

Getting that faint echo was anything but straightforward.

The team had heavily modified an SCR-271 radar set, a system developed during the Second World War, and installed a large 8-by-8 antenna array on a tower. The arrangement was designed to concentrate the radio energy towards the Moon.

But there was a fundamental problem: by the time the signal travelled to the Moon and returned, only an extremely small fraction of the original energy was available to detect.

Engineers therefore had to build an exceptionally sensitive receiving system capable of separating the weak lunar echo from atmospheric interference and background radio noise. They also accounted for the Moon’s movement relative to the radar station, making precise Doppler adjustments to ensure they knew what signal to look for.

When the delayed blip finally appeared, it was more than a curious reading on a screen. It meant that radio waves had crossed the Earth's ionosphere, reached the Moon and returned to the same spot on Earth.

The signal that opened a new route into space

Project Diana transformed a piece of wartime detection technology into something with an entirely different purpose.

Until then, there had been uncertainty over whether radio signals sent at high frequencies could make it through the ionosphere and travel into space. The successful lunar echo provided a practical answer: they could.

The Moon was an ideal first target because its position and distance could be calculated with considerable precision. By comparing the predicted travel time with the measured delay, the team could establish that the returning signal had made the journey to the lunar surface and back.

The experiment became an early foundation for radar astronomy, demonstrating that radio technology could be used not merely to detect objects around Earth but to probe targets hundreds of thousands of kilometres away.

Decades before spacecraft routinely travelled beyond Earth, Project Diana offered a remarkable glimpse of what was possible from the ground: send a signal into space, wait, and listen for the universe to answer back.