Imagine you’re on the side of a racetrack trying to get a good photo of a car as it zips past you. One way to help is to track the camera as quickly as you can in the same direction the car is moving, to reduce the speed blur. Background objects like trees will look like streaks but the car will look sharp.

Astronomers can do the same thing, kinda. They can take a lot of images of some small region of the sky, then after the fact shift the images relative to one another and add them together. We know, for example, how rapidly asteroids in the main belt move across the sky, so the astronomers can shift the images to mimic that motion, then add them up. Stars will appear as streaks or a series of tightly aligned dots, but asteroids moving at the virtually tracked rate will appear as dots.