When NASA sent humans to the moon in 1969, one of the many hazards that the agency had to look out for was space rocks penetrating astronauts’ spacesuits or equipment. Unlike Earth, which has a protective atmosphere where meteoroids usually disintegrate, the moon is vulnerable to whatever rocks or specks are whizzing around in space.
According to Bill Cooke, head of NASA’s Meteoroid Environment Office at NASA’s Marshall Space Flight Center in Alabama, the crew weren’t in too much danger. “An astronaut getting hit by a millimetre-sized object has a 1 in 1 million chance of being hit each hour per person.” (A meteoroid must be a millimetre in diameter to puncture an astronaut’s spacesuit.)
NASA is planning to send humans back to the moon by 2025, with the goal of eventually establishing a base either orbiting or on the moon’s surface, so it’s more vital than ever to understand how often our natural satellite is hit.
So, on a daily basis, how many objects collide with the moon? What if you did it every year?
Cooke explained that the answer is dependent on the object’s size. Cooke is very familiar with what hits the moon every day because he works for NASA’s Meteoroid Environment Office, which studies the space environment around Earth and the moon to understand the flux of meteoroids (space rocks ranging in size from dust to small asteroids about 3 feet, or 1 metre, across).
Although the number of impactors smaller than a millimetre cannot be exactly calculated, Cooke thinks that 11 to 1,100 tonnes (10 to 1,000 metric tonnes) of dust crash with the moon per day, equivalent to the mass of around 5.5 vehicles. Estimates for larger rocks are more precise.
“On a daily basis, around 100 ping pong-ball-sized meteoroids strike the moon,” Cooke said. This equates to approximately 33,000 meteoroids every year. Despite their modest size, each of these ping pong-ball-sized boulders has a force of 7 pounds (3.2 kilogrammes) of dynamite when it hits the surface.
Larger meteoroids strike the moon as well, but they do so less frequently. Larger meteoroids, such as those 8 feet (2.5 metres) broad, bang into the moon every four years, according to Cooke. Those objects had the force of a kiloton, or 1,000 tonnes (900 metric tonnes) of TNT when they collided with the moon. Because the moon is nearly 4.5 billion years old, it’s no surprise that its surface is littered with craters from past hits.
Lunar impacts are studied in a variety of methods by scientists. Scientists use telescopes to observe collisions on the moon from the Earth’s surface. According to NASA(opens in new tab), meteoroids can hit the surface at speeds ranging from 45,000 to 160,000 mph (20 to 72 kilometres per second), causing a burst of light that can be seen from Earth.
Scientists can also observe the craters left behind by impacts using spacecraft circling the moon, such as NASA’s Lunar Reconnaissance Orbiter (LRO). According to NASA, an 11-pound (5 kilogrammes) meteoroid can leave a crater 30 feet (9 metres) across and fling 165,000 pounds (75,000 kg) of lunar soil and rocks off the moon’s surface because meteoroids move so quickly. After these craters form, the LRO can easily see them.
Although the moon receives numerous hits each year, this does not rule out the possibility of human existence. “If you take a square kilometre patch of terrain, it will be hit by one of those ping pong-sized meteoroids about per thousand years or so,” Cooke said, citing the moon’s surface area of 14.6 million square miles (38 million square kilometres).