For as long as humans have looked up, the Moon has refused to be just one thing.
To some it was a god. To others, a calendar. To sailors, a guide. To farmers, a clock. To astronomers, a world. And to NASA, just before Apollo, it became something even stranger, a place close enough to reach, but still full of things no one could quite explain.
Strange lights had been reported on its surface. Spacecraft refused to orbit it exactly as expected. The Moon looked dead, but it did not behave as like a dead world should.
By the 1960s, the Moon had been mapped, photographed and studied more than ever before. But it was still not yet a place that humans had visited. And that meant NASA had to answer a question that sounds simple, but was not.
Was it truly dead? Was its surface stable? Was its gravity predictable? Were reports of flashes and glowing patches just mistakes, or signs of something still active below the surface?
And if astronauts were going to land there, orbit above it, take off again, and rendezvous in lunar orbit, then “probably” was not good enough. NASA needed to know answers.
For centuries, people had reported temporary changes on the Moon. They described bright flashes, hazy patches, reddish colourations and strange glows that appeared, changed, and disappeared.
Some reports were almost certainly mistakes. The Moon is a difficult object to observe. Its surface is full of harsh contrast. Turbulence in the Earth’s atmosphere causes the image to shimmer. The angle of sunlight changes dramatically from night to night. And human colour perception through early telescopes is not always reliable because of chromatic aberration.
But some reports were harder to dismiss. Again and again, observers pointed to the same regions. Aristarchus, one of the brightest craters on the Moon. Schröter’s Valley, a winding channel nearby. Alphonsus, a crater that had already attracted attention because some believed it might show signs of activity.
If the Moon was completely dead, why did certain places seem to repeated catch peoples attention. In the mid-1960s, with Apollo approaching, NASA backed a project to look for these events more systematically.
It was called Project Moon-Blink and it would be done with network of 12 professional and 4 amateur observatories.
The name sounds almost like science fiction, but the idea was simple and clever. If a part of the Moon temporarily changed colour, especially if it became redder or bluer than the surrounding surface, then a normal telescope observer might miss it.
Moon-Blink used a rotating colour filter wheel split between red and blue filters. If the lunar surface was normal, the image would remain steady. But if a small patch became unusually red, it would appear bright through the red filter and dark through the blue filter. To an observer looking at a section of the lunar surface, a light anomaly would show as a blinking area in that part of the Moon as the filter wheel rotated
The plan was not just to rely on one telescope, but to build a network of observers who could confirm events quickly. If one station saw something, others could be alerted to confirm it something had been seen.
This was In effect a lunar surveillance network before modern automated astronomy existed.
But while some scientists were looking for strange lights on the Moon, another team was finding something stranger still and that was that the Moon’s gravity was not behaving.
Before astronauts could land on the Moon, NASA had to map it. The Lunar Orbiter spacecraft were sent ahead to photograph potential Apollo landing sites. But their cameras were not the only instruments they used.
Every spacecraft in lunar orbit sent radio signals back to Earth. And by measuring tiny Doppler shifts in those signals, engineers could tell how the spacecraft was moving.
This is where NASA found something unusual and potential serious for orbiting spacecraft. If the Moon’s gravity had been even, the orbits should have been predictable. Not perfectly super smooth, but close enough. A spacecraft should have followed a clean mathematically predictable path around the Moon.
But the tracking data they revealed said otherwise. The orbiters were being tugged, accelerated and slowed in ways that did not match an even gravity field. At first, this was just an orbital problem. A mismatch in the numbers. But when scientists looked closer, the pattern began to reveal something hidden beneath the surface. The Moon had Mascons.
The term stands for “mass concentration.” These were regions where the Moon’s gravitational pull was stronger than expected. They were not mountains or visible cliffs. A camera could not show them on a photograph.
They were invisible lumps in the Moon’s gravitational field. And many of the largest ones sat beneath the great lunar maria, the dark “seas” visible from Earth, such as Imbrium, Serenitatis, Crisium, Humorum and Nectaris.
To the eye, these looked were smooth dark plains, but to a spacecraft, they were gravity traps. For the mascon researchers, the question was more fundamental. Why was the Moon’s mass distributed so unevenly? And what did that mean for a spacecraft flying just above it?
These were not fringe concerns. They mattered directly to Apollo. Those brief flashes of colour might reveal outgassing from the lunar interior. That would mean the Moon was not completely inert. It might have pockets of trapped gas, fractures, or regions where material could escape.
If the flashes were impacts, that mattered too. The Moon has almost no atmosphere. Meteoroids that burn up harmlessly on Earth can hit the lunar surface directly intact and cause severe damage to a lunar lander. Understanding the rate of impacts matters for spacecraft, surface equipment, astronauts and future bases was vitally important.
However, for Project Moon-Blink it was turning out to be harder than expected. The light flashes were transient and random and having human observers sitting around and waiting meant the network was difficult to sustain. Volunteers lost interest. Equipment was limited. The hoped-for automatic detection system was harder to build than planned.
This left NASA with a frustrating answer, yes some things were probably were happening, but the evidence was not clean enough to say exactly what they all were.
Mascons were different. They were not fleeting. They did not depend on a lucky observer at a telescope. They showed up in tracking data again and again.
The Apollo missions were not simply a matter of flying to the Moon and dropping straight down. The spacecraft had to enter lunar orbit. The Lunar Module had to detach, descend to the surface, land, then take off again and meet the Command Module still orbiting above.
That rendezvous depended on knowing where the Command Module would be. If the Moon’s gravity field was wrong in the models, the orbit prediction would drift. Maybe not by a catastrophic amount in a single orbit, but enough that engineers had to understand it, correct for it and build it into mission planning.
However, the lower the orbit, the more important the local gravity became. A mascon could tug a spacecraft forward, backwards, sideways, or downward depending on the spacecraft’s path. Over time, those tiny pulls could reshape an orbit.
Around the Earth, low satellites are dragged down by atmosphere, around the Moon, a low satellite can be pulled into trouble by gravity itself.
Lunar Orbiter tracking improved the gravity model. Apollo 8 tracking added more. Later missions refined it further. This mattered because every move to correct an orbit required fuel and there was limited capacity.
Engineers learned that low lunar orbit was not automatically stable. Mission planners had to account for gravitational anomalies, carry fuel for correction burns, and choose orbits carefully.
Some lunar orbits are naturally more stable than others. These are sometimes called frozen orbits, where the gravitational perturbations balance out well enough that a spacecraft can survive much longer without constant correction.
The Apollo 16 subsatellite, PFS-2, was a small, 80-pound scientific probe deployed into lunar orbit on April 24, 1972. Its primary mission was to map the Moon’s gravity field and study its plasma, particle, and magnetic environment. But it also showed what could happen when the orbit was less favourable.
Due to problems with the main engine the probe was released at a lower than expected orbit where it was subject to the Mascon pull more than it would have been if it had been higher. Its mission was meant to last about a year, but its low orbit was rapidly altered by gravitational perturbations, and it crashed into the Moon after just over a month.
The mascons were eventually understood as relics of giant ancient impacts. Billions of years ago, large asteroids and comets slammed into the young Moon. These impacts excavated huge basins, disturbed the crust, and in some cases allowed dense material to rise or melted material to pool. Later, basaltic lava filled many of these basins, creating the dark maria visible from Earth. The result was not just a visual scar but a gravitational one too.
Where the Moon looked like a flat dark sea, the mass below could be denser than the surrounding crust and this region had of stronger pull on spacecraft passing near by.
Modern missions, especially NASA’s GRAIL flew twin spacecraft, Ebb and Flow, in tandem around the Moon to map variations in the lunar gravitational field. The probes generated the highest resolution gravity map of any celestial body to date. At the end of the mission, the probes were purposely crashed on the Moon.
For Moon-Blink, the lesson was more complicated. Some strange lunar flashes are now known to be real impacts. Modern lunar impact monitoring uses telescopes and video cameras to watch the dark part of the Moon for brief flashes when meteoroids hit the surface.
This does not explain every historical report. Some reported events lasted too long to be simple impact flashes. Some may have been due to outgassing, dust, observational effects, atmospheric distortion, or simple observational mistakes.
But the modern view is not that all old observers were fools, nor that every strange report was a mystery beyond science.
The truth is that the Moon is mostly quiet, but not perfectly still. It is hit by space debris, it has moonquakes, it cools and shrinks. Its surface dust responds to extreme conditions. Its weak exosphere and possible outgassing events are still scientifically interesting.
And with our latest Artemis missions and the long term plan for bases we will need to know what moon is and what it does as well as we know the earth, you can’t just run outside in case of an unexpected event on the moon.
So, I hope you enjoyed the video and if you did then please thumb up, share and subscribe and thank you to our patreons for their ongoing support.
