If spacecraft sent to other planets don't come back to Earth, how do we get pictures from them?

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Neptune, as seen by Voyager 2

Our friends at the Lafayette Natural History Museum and Planetarium in Lafayette, Louisiana, asked, if spacecraft sent to other planets don't come back to Earth, how do we get pictures from them?

I'm pleased to hear that question, because many space scientists and engineers have worked very hard to develop and perfect the answer. You have probably seen some of the breathtaking images on TV, in magazines, or in books of many distant and fascinating world, including Mars, Saturn, Jupiter, and some of their very strange-looking moons. These were taken by spacecraft such as Mars Global Surveyor, Voyager, and Galileo. And, you may have seen images of our own world taken by Earth-orbiting spacecraft. Once spacecraft like all of these are launched, we never see them again. But pictures and other information they have sent back from their journeys have revealed more amazing things about our solar system and the universe than we could have ever imagined.

Neptune's moon Triton, as seen by Voyager 2 DSN antenna in Madrid, Spain

Well, you are already familiar with some other technologies that transfer pictures from one place to another without wires or any other obvious means of delivery. What about television? Although some TVs receive their picture and sound information over a cable, many receive this information from a transmitting tower some distance away, or even from a satellite thousands of kilometers out in space. Now it is becoming common even for cell phones to transfer photos across long distances. Images and other information from very distant spacecraft travel basically the same way, using radio waves. The main differences are the radio waves are much weaker and require a much bigger, more powerful antenna to receive them.But what about the pictures themselves? How do we turn a picture taken by a camera into the kind of information that can be sent as a signal through space?

Dr. Marc in pixels.

Like digital cameras (including cell phone cameras), spacecraft cameras do not have film. Instead, they contain a small device about the size of a postage stamp that's made up of thousands or even millions of tiny light-sensitive cells called "pixels" (from the words "picture element"). Each pixel is smaller than the width of a human hair. When the camera takes a picture, say of Jupiter, each pixel measures the brightness of a tiny part of the scene, much the way each little section of a film would. But these pixels can translate their measurement of the brightness into a number that is sent to a computer. For example, no light at all could be recorded as a 0, and a very bright light could be recorded as 100. Everything between deep black and brilliant white would be different shades of gray and would get a number between 0 and 100, based on how dark or bright the shade of gray.

All these numbers together, along with numbers that tell the location of each pixel in the scene, are all the information needed for another computer to recreate the picture. This kind of data can then be radioed through space to Earth, where the giant antennas of NASA's Deep Space Network receive the signal. Computers on Earth then turn the numbers back into the pixels that make up a picture from space, showing what the spacecraft's camera saw.

But, now you may be wondering how we get color pictures from space. For each color picture, different pictures are taken through different colored filters. Each colored filter lets through only a certain color of light. For example, a red filter lets through only red light. So the red-filtered pixel data will show the brightness of the red in the picture. If you put together the pixel data from three pictures, one taken through a red filter, one through a blue filter, and one through a green filter, you can recreate the original colors in the scene—and all from shades of gray!

Diagram, how we get images from space.

Play "Pixel This" and learn more about how pictures can be sent through space. And while you're at it, watch the movie "Yelling Across the Solar System" and learn how numbers are turned into radio signals and beamed across space.