How many space rocks hit the moon every year?

One of the numerous risks that NASA had to foresee before sending humans to the moon in 1969 was space pebbles piercing astronauts' spacesuits or other equipment. The moon is susceptible to whatever rocks, or even particles, are flying around in space, unlike Earth, which has a protective atmosphere in which meteoroids often disintegrate.

Thankfully, Bill Cooke, director of NASA's Meteoroid Environment Office at NASA's Marshall Space Flight Center in Alabama, said that the crew weren't in too much danger. According to Cooke, the likelihood of an astronaut being struck by a millimeter-sized item occurs around once in a million hours per person. The maximum size of a meteoroid that can enter a spacesuit for an astronaut is one millimeter.

Understanding how often our natural satellite is struck by objects is crucial since NASA is gearing up to return people to the moon by 2025 and eventually build a base there or in orbit around it.

How many things thus strike the moon daily? What about annually?

According to Cooke, the answer is based on the object's size. Cooke is extremely familiar with what is striking the moon every day since NASA's Meteoroid Environment Office examines the space environment around Earth and the moon to comprehend the flow of meteoroids, space rocks ranging in size from dust to tiny asteroids approximately 3 feet, or 1 meter, wide.

Cooke calculates that 11 to 1,100 tons (10 to 1,000 metric tons) of dust, or around 5.5 automobiles' worth of mass, crash with the moon per day for impactors smaller than a millimeter. This figure cannot be exactly calculated because of the size of the impactors. The estimations are more definite for bigger rocks.

About 100 pingpong-ball-sized meteoroids strike the moon every day, according to Cooke. That amounts to about 33,000 meteoroids annually. These tiny boulders, about the size of ping pong balls, each strike the ground with the power of 3.2 kilograms (7 pounds) of dynamite.

Less frequently, but with larger meteoroids, the moon is struck. About every four years, according to Cooke, bigger meteoroids that are 8 feet (2.5 meters) across collide with the moon. The impact of the items on the moon was equivalent to 1,000 tons (900 metric tons) of TNT, or one kiloton. Given that the moon is around 4.5 billion years old, it is not surprising that these collisions have left their imprint on its surface in the form of many craters.

There are several methods used by scientists to investigate lunar impacts. Scientists aim telescopes at the moon from the surface of the Earth to study impacts. According to NASA(opens in new tab), meteoroids can strike the surface at speeds between 45,000 and 160,000 mph (20 and 72 kilometers per second); the collision results in a flash of light that can be seen from Earth.

The craters left behind by impacts may also be observed by scientists using spacecraft circling the moon itself, such as NASA's Lunar Reconnaissance Orbiter (LRO). Because meteoroids travel so quickly, even an 11-pound (5 kilogram) meteoroid may throw 165,000 pounds (75,000 kg) of lunar dirt and rocks from the moon's surface and leave behind a crater that is 30 feet (9 meters) big, according to NASA. After they form, the LRO can quickly locate these craters.

Although the moon is struck frequently each year, this does not rule out the possibility of human existence. The moon has a surface area of around 14.6 million square miles (38 million square kilometers), which means that "if you choose a square kilometer patch of terrain, it will be struck by one of those pingpong-sized meteoroids once or twice in about a thousand years," according to Cooke.

Therefore, the chances are in our future moon explorers' and their spacecraft's favor.