NASA’s Lunar Reconnaissance Orbiter (LRO) is a marvelous spacecraft that takes exquisite high-resolution images of the Moon. So it was a surprise when it produced a weird wavy image back in October 2014. Now astronomers think they know what caused it: The image was snapped as the probe was hit by a tiny meteoroid.

The LRO Camera (LROC) team analyzed the image and used a computer simulation to work out how it could have formed. The algorithm suggested that the impacting meteoroid was 0.8 millimeters, about half the size of a pin head with the density of aluminum. But don’t let the diminutive size fool you. It was moving at 7 kilometers (4.3 miles) per second, so it’s lucky it didn’t do any damage.

“The meteoroid was traveling much faster than a speeding bullet. In this case, LROC did not dodge a speeding bullet, but rather survived a speeding bullet,” LROC principal investigator Professor Mark Robinson from Arizona State University said in a statement. “LROC was struck and survived to keep exploring the Moon thanks to Malin Space Science Systems’ robust camera design.” 

The LROC is a system made of three cameras. A wide-angle moderate-resolution color camera and two narrow-angle cameras (NACs) that get high-resolution black and white images. Out of the three instruments, only the left NAC was hit by the meteoroid.

The team knew this because of how the pictures are taken. They are not snapped like a normal camera, instead, the NAC images are composed line by line, to account for the changes in position as the spacecraft orbits the Moon.

The team is certain that this wasn’t a probe-wide accident, or there was a problem with the craft as neither the solar panels nor the antennas were shaken by anything when the picture was taken.

“Even if there had been [a problem], the resulting jitter would have affected both cameras identically,” Robinson said. “The only logical explanation is that the NAC was hit by a meteoroid.”

Launched in June 2008, the LRO has released incredible images of our natural satellite, including a 681 Gigapixel map of the Moon’s North Pole. It has a suite of seven different instruments and has played a vital role in our understanding of the Moon, so it’s a good thing that it wasn’t damaged permanently.

“A meteoroid impact on the LROC NAC reminds us that LRO is constantly exposed to the hazards of space,” added Noah Petro, deputy project scientist at NASA Goddard. “And as we continue to explore the Moon, it reminds us of how precious are the data that is being returned.”