There are no traffic signals in lunar orbit. And for the most part, they’re not needed – at least not yet. Only about a dozen spacecraft are circling the Moon. But every once in a while, they can pass dangerously close to each other. And that triggers a “red alert” – a warning to the operators of both craft.

It might sound surprising that there’s ever any kind of problem – there’s a lot of space around the Moon. But many of the spacecraft follow similar orbits.

Many of them orbit from pole to pole, allowing them to study the entire lunar surface. And there’s no system for tracking the second-by-second locations of the orbiters.

Instead, NASA engineers with a project called MADCAP track the “orbital elements” of every craft in orbit around the Moon and Mars – a set of details that includes altitude, the angle of the orbit, and much more. Computers constantly plot the motions of every craft for which they have those details. If it looks like there might be a dangerously close encounter, the alert goes out. Operators of the conflicting missions then get together to find a solution.

Overall, the number of alerts is small. But in December 2024 it jumped to more than 20. And with more lunar missions scheduled, we can expect more “red alerts” in the years ahead.

The crescent Moon is in the west as darkness falls. The brilliant planet Venus – the “evening star” – is close to the right.

Script by Damond Benningfield