How does a fast moving satellite or a probe take those clear pictures of space objects like asteroids, Jupiter, etc? Doesn't the picture get blurry when objects move?

(from Il Hwan Lee via Quora)


Space is really REALLY big and that has an amazing impact on angular momentum. Think about the sun in the sky. We're on a ball of rock that is moving at 67,000 mph and spinning at around 1000 miles per hour, but yet the sun crawls across the sky.

The reason is because that while the speeds may be very high, the distance is even higher. At very long distances, the difference that a given amount of movement makes is insignificant in comparison to the distance. It moves a very small amount of angle (thus angular momentum) if you were to make a triangle going from where the object or camera is at the start of the shot compared to where it is at the end of the shot.

Additionally, even if the angular momentum was high enough to cause an issue with resolving the image clearly, you can actually use motion control to track the movement if it's a known type of movement. This is frequently used with start tracking land based telescopes for long exposure photography. (Try pointing a camera for 60+ seconds at the sky on a dark clear night and you'll see star trails.) A motion control rig can adjust for the movement and stay focused on the moving subject despite the fact it is moving.

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  • Interestingly, the answer to this question is really more astronomical than photographic. I think the best example is that an ant may think it's jetting along at its top speed, but if you're standing five feet away from it, it takes it a good long while to occlude a tiny piece of gravel on the ground because its relative angular velocity is so small. Good answer, AJ Henderson. – ilinamorato Jul 15 '13 at 13:55

When the apparent angular velocity is actually nontrivial, such as when imaging a planet or another celestial body from a low orbit, line-scan cameras are sometimes used. These consist of a thin strip-like sensor that's continuously read out while the satellite orbits.

The very successful HiRISE telescope aboard the Mars Reconnaissance Orbiter is a good example - the camera contains 14 CCD sensors, each with 2048 x 128 resolution, and captures images circa 20000 (red channel) or ~4000 (green-blue and near-IR) pixels wide. The height of the images is only limited by the available storage space.

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