In the early 1960s, during the space race, neither American nor Soviet scientists really knew where planets like Mars or Venus were—especially at the accuracy and precision essential for spacecraft navigation. That may sound faintly ludicrous. They of course knew roughly where a target like Venus would be when a spacecraft got there. But “roughly” in this context might be an offset of 10,000 or 100,000 kilometers. Planetary positions, their ephemerides, rely on the calibration of their orbits to extremely high precision over time. But the only way to do that properly is to make direct measurements, just as the mariners of old would need to sail right by an island or shoreline in order to nail down its latitude and longitude.

An infamous example of the problem came in early 1961. Plans were afoot to send a probe to Venus, starting with the Soviet launch of the Venera 1 probe. Soviet and American scientists were racing to pin down Venus’ position and use it to also refine the astronomical unit—then defined as the