Mascons

Anyone old enough to remember the first manned landing on the moon will probably remember the term “mascon.” The moon is full of them, and landing the Apollo 11 spacecraft required an emergency maneuver by Neil Armstrong to avoid ending up in a pile of rocks. The landing spot in the Sea of Tranquility had been very carefully researched, and it was programmed into the automatic system responsible for getting them to a nice flat place. But for some reason the lander was headed for a very inhospitable place strewn with boulders as big as the lander! Armstrong’s last-minute diversion saved the mission.

It turned out the problem was partly caused by mass concentrations, an odd feature of the moon’s gravity. Also inputs from the two landing radars overwhelmed the lander’s computer. There are places in some of the large craters where the upwelling lava is much denser than the surrounding terrain. As you orbit the moon, these anomalies can turn your carefully calculated orbit into hash. Lunar scientists call this “bumpy gravitation” and have figured ways to overcome its effects on anything that orbits close to the surface (mostly they don’t orbit close to the surface!). In the illustration, mascons are shown in red. An exploration of mascons is here.

Although not exactly the same thing, mass concentrations are also found in places like Vatican City.

Illustration: NASA