Impact strikes are a rare but dangerous threat to spacecraft. The highest threat is for spacecraft in near-Earth orbit, where decades of satellite debris have accumulated. Given the tremendous speeds of orbiting spacecraft, even a fleck of paint can pose a threat. Beyond Earth orbit the threat is less, but it isn’t zero, as demonstrated by a rare impact with the Lunar Reconnaissance Orbiter (LRO) currently orbiting the Moon.
The impact was small, and the spacecraft survives. The effect was so subtle that it would likely not have been noticed if the orbiter wasn’t taking images at the time. You can see the effect of the impact in the image above. It was taken in 2014 by one of the LRO’s Narrow Angle Cameras. These cameras take high resolution black and white images of the lunar surface. To take these images, the camera scans the surface line by line, Taking a narrow line image of the surface one after the other to create a complete picture. As you can see in the image, the sharp resolution near the top shifts suddenly to a wiggly image. This means the camera was jostled suddenly as it gathered lines of images. The LRO has two Narrow Angle Cameras, as well as a wide angle one, and only one Narrow Angle Camera showed this wiggly effect. This means the satellite itself was not jostled in a significant way, but only one camera.
This can be explained by a meteoroid impact with the camera. Based upon computer recreations of the image, the meteoroid was only about 0.8 millimeters in diameter, or about the size of the ball at the tip of a ball point pen. But the meteoroid was moving at more than 15,000 miles per hour relative to the spacecraft, which is about ten times faster than a speeding bullet. This gave it enough energy to jostle the camera in a noticeable way.
It did not, however, have enough energy to seriously damage the spacecraft. The impact occurred in 2014, and since that time the Lunar Reconnaissance Orbiter has worked perfectly well. It continues to gather data on the lunar surface, and will help us determine future landing sites for crewed missions to the Moon.