In the 1950s, Earth’s closest neighbor, Venus, was a mystery. The dense, featureless clouds shrouding the planet made it impossible to determine the rate of its rotation, much less its geography. Even its distance was in question: Though many had estimated, nobody could say exactly how far away Venus was. A brand-new field, radar astronomy, promised a more exact measurement of the distance—by bouncing radio waves off the planet—than had been possible through optical observations. An accurate measurement was crucial to determining the orbital positions of the planets as well as the mean distance between Earth and the sun, known as the Astronomical Unit. Both measures are important in learning to navigate interplanetary spacecraft.

The first to make the attempt at a precise measurement was the MIT Lincoln Laboratory in Lexington, Massachusetts. In February 1958, just four months after Sputnik orbited, the university’s Millstone antenna, equipped with the latest technology, shot radio waves toward Venus. The results were ambiguous, and could have been interpreted as either weak radar returns or simply random “noise.”

The U.K.’s Jodrell Bank made its own try at Venus in 1959, which at first seemed to replicate the Lincoln Lab results. But not everyone was convinced, especially when Lincoln proved unable to repeat the original results, even with a more powerful radar. Both Lincoln and Jodrell Bank were looking in the wrong place.

Solomon Golomb was a young engineer at NASA’s Jet Propulsion Laboratory in Pasadena, California, where he was helping to build the first U.S. interplanetary spacecraft. “JPL was about to launch space probes toward Venus, and for that purpose it would be very useful to know where Venus actually was,” Golomb told me in a 2016 interview, shortly before his death. A Venus radar experiment would also be a good test run for JPL’s new Deep Space Instrumentation Facility in California’s Mojave Desert, built to provide reliable communications with the upcoming Ranger and Mariner missions.

On March 10, 1961, JPL turned the 85-foot-diameter dishes at DSIF’s Goldstone Pioneer Tracking Station toward Venus. Six and a half minutes later, they began receiving strong radar returns. Over the following two months, hundreds of hours of observation confirmed those results: JPL had the first radar observations of Venus.

The measurements proved as critical as expected. As Golomb explained, “The problem with all those other measurements was that Venus wasn’t where they said it was. The astronomers had adopted a value based on 19th century observations of the transit of Venus across the sun, and that’s not a very accurate way to determine the [Astronomical Unit]….” When Venus’ proper position was charted, he said, “we found that the conventional value adopted by the astronomical community was off by considerably more than [one] part in a thousand.” He noted in a 2006 talk that if NASA had relied on the AU value commonly accepted before JPL’s experiment, the Mariner 2 probe—which in 1962 became Earth’s first successful interplanetary mission—might have missed Venus by more than 100,000 miles.

JPL’s Venus experiment solidified radar astronomy as a legitimate field, and astronomers went on to use radars to study the moon, Mercury, Mars, and even the moons of Jupiter and Saturn. But for practical purposes, radars can be made only so powerful; today they are mainly used to study near-Earth asteroids.