There are a few questions that we get all the time at The Planetary Society. Look up at space at night from a dark location and you can see innumerable stars. Why, then, do photos of things in space not contain stars? How come the black skies of the Moon contain no stars in Chang’e photos?

The answer: The stars are there, they're just too faint to show up.

I can illustrate with an example from everyday life. I'm sure that everyone reading this article has made the mistake of shooting a photo of a loved one standing in front of a brightly lit window. In your photo, all you can see is a silhouette; your subject's face is a nearly featureless shadow. Their face still exists, of course! It's just not brightly lit enough to show up in the photo.

The same issues that can make your casual snapshots look bad affect space images, too. Let's talk about three things that affect what details you can see in any photo, whether it's of a star, a planet, or a person: the sensitivity of the camera, the time your camera had to collect light, and the dynamic range of your camera.

How Sensitive Is Your Camera?

How much light does your camera need to see by? Fancy cameras can adjust sensitivity by opening and closing the aperture that lets in the light. Human eyes do the same thing, automatically, all the time, by dilating and contracting their pupils. If you're a sighted person walking from a brightly lit to a dark outdoor area, you won't see stars in the sky either, at least not right away. As your eyes dial up their sensitivity by opening up your pupils, you slowly notice fainter and fainter stars.

Most space cameras actually can't adjust their aperture in this way. Instead, scientists predict the light levels that a camera will encounter through its mission, and design their instruments to have an aperture that's an appropriate size for the range of targets they expect to encounter. This can be a challenge if your spacecraft will encounter a wide range of target brightnesses, but you make your camera to work on the intended science targets and don't worry if it isn't ideal for any fun extras you may photograph along the way. OSIRIS-REx, whose MapCam was designed to study the colors of a very dark-toned asteroid, couldn’t look at Earth without getting overwhelmed by the brilliant light reflecting off of bright clouds, causing the artifacts you see in the top of this image.