NASA scientists and astronauts say that moonquakes – some with magnitudes as high as five on the Richter scale – may be caused by the moon shrinking as its interior cools.

“Objects expand when they heat up and shrink as they cool,” said Bill Steigerwald, science writer at NASA’s Goddard Space Flight Centre, in an email to CTV News. “The moon’s formation generated a lot of heat, and it has been gradually cooling off ever since.”

Using data generated by the Apollo missions and scans by the Lunar Reconnaissance Orbiter (LRO) spacecraft, NASA has been able to monitor the faults that form due to the moon’s brittle crust breaking as it shrinks.

“Our analysis gives the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink,” Thomas Watters, senior scientist at the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington, said in a press release.

Although the Apollo missions occurred decades ago, “the Apollo seismometers are the only ones deployed on the Moon,” Watters said in an email to CTV News.

Those seismometers (instruments used to measure the shaking produced by quakes) recorded the moonquakes that occurred between 1969 and 1977, which allowed Watters and his team to analyze their data using an algorithm to pinpoint the quake locations.

Their study found that the locations of the quakes were close enough to the faults “to tentatively attribute them” as the cause.

Neither the moonquakes nor the moon shrinking is dangerous to us on Earth, said Steigerwald.

In fact, Watters and his team attribute the moon’s interaction with the Earth as part of the reasons the lunar crust is compressing.

“Tidal forces exerted on the Moon by the Earth, those from the flow recessions of the Moon away from the Earth and diurnal (daily) forces that arise because of the Moon is…in orbit around Earth, contribute to the global contraction forces from interior cooling of the Moon’s still hot interior,” said Watters.

Future space projects, such as the Lunar Gateway (in which Canada is a major partner), “won’t be affected” as the space station is to be in orbit around the moon and not on its surface.

However, NASA believes that “establishing a new network of seismometers on the lunar surface should be a priority for human exploration of the Moon,” which will allow scientists “to learn more about the Moon’s interior and to determine how much of a hazard moonquakes present,” said Renee Weber, a planetary seismologist at NASA's Marshall Space Flight Center in Huntsville, Ala., in the release.

NASA plans on sending “the first woman” and “next man” to the moon by 2024, via the Lunar Gateway.

The space agency wants to establish “sustainable missions by 2028” – taking what they learn from the moon and applying it to reaching Mars.