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Grade Level: Middle School

Description: How is the data from the Galileo spacecraft sent to scientists on Earth?

Objective: Students learn how digital signals are created and returned to Earth, and then apply these concepts to the interpretation of images from the Galileo Orbiter.

Materials: Digital images (To print images, click here); pencils

Vocabulary: Digital signals, charge-coupled device (CCD), pixel, resolution

Except for some journeys to the Moon, all spacecraft sent to explore the solar system have been one-way trips. This means that the information collected, including pictures, has to be sent back to Earth electronically. At first, this would seem to be very easy; after all, television stations beam billions of pictures into homes every day. However, the energy required for TV broadcast is very high and the signal gets very weak after a few tens of kilometers. Not only is the distance in space much greater, but the energy available on the spacecraft is tiny; the total is less than the light bulb in your reading lamp. To solve this problem, cameras on the spacecraft convert pictures into digital signals that can be sent as a series of ones and zeros using very little electrical power. This exercise will show you how these digital images ("pictures by the numbers") work.

Because film is not returned from the spacecraft, we must use some other means to record a picture. Most spacecraft today use cameras somewhat similar to personal video cameras, based on charge-coupled devices, or CCDs. CCDs are electronic "chips" divided into a grid (see Figure 1). Each cell in the grid is called a picture element, or pixel. To keep track of the pixels, the grid is divided into lines and samples. The area to be "pictured" is focused by a lens onto the CCD, which is sensitive to light -- the more light, the higher the electrical charge. The electrical charge of each pixel (identified by line and sample) is measured and recorded in computer memory, then sent back to Earth.

Figure 2 shows an image returned from the Galileo spacecraft's CCD camera. The image is of Europa, one of Jupiter's moons. The area in the box is enlarged to show the pixels of different brightness levels (shades of gray). For simplicity, let us say that only two possible brightness values for each pixel can be returned to Earth, one value for bright pixels and the other value for dark pixels. In computer language, this would be called 1-bit data, meaning 2¹ (2 raised to the first power), which equals 2 possible values (bright or dark). The computer attached to the camera keeps track of the value for each pixel by line and sample. Figure 3 shows a line and sample grid and the value for each pixel (blank square = white, or bright; b = black, or dark). Take a pencil and fill in each pixel labeled with a b. What do you see as the result? What type of landform might this be?

Although this is a crude picture, you should get the idea!

Now, how could we improve the result shown in Figure 3? Instead of having only black or white pixels, how about having some shades of gray? Remember that we still have to send the information on each pixel back to Earth electronically in computer form. Instead of 1-bit coding (2¹ = 2 values), we will use 2-bit coding, or 2² (2 x 2 = 4 values). Now we can have white, black, and two shades of gray. Figure 4 shows a grid "map" on which b = black, g= dark gray, 1 = light gray, and blank = white. Use a pencil and shade in these tones. Now what do you see? Do you see anything that you were unable to detect in the first picture?

We could continue improving the picture by increasing the number of gray levels with 3-bit data (2³, or 2 x 2 x 2 = 8 values), 4-bit data (2⁴, or 2 x 2 x 2 x 2 = 16 values), etc. Most of the pictures you have seen for Jupiter and its moons used 8-bit encoding (2⁸). How many shades of gray does this represent? _________________

In fact, the human eye can separate less than two dozen shades of gray. This means that 8-bit pictures contain much more information than you could detect if you were looking directly at the area. Computer processing of the pictures enables all of the data to be used for analysis.

Can you think of some other ways of improving the picture shown in Figure 4? The grid is rather coarse, meaning there are not many pixels. Figure 5 shows the same scene but with more lines and samples of pixels. The encodement is still the same (2² = 4 levels of brightness). Shade the pixels using the same method as in Figure 4. Now what are you able to see in the picture?

This improvement refers to resolution, meaning the size of the area shown by one pixel. Resolution depends on the size and number of pixels on the CCD chip, the camera lens (telephoto, normal, or wide-angle lens), and the distance to the scene.

Now when you look at a picture from space, see if you can recognize the pixels and remember how the image was returned to Earth!

1. What is meant by the term bit rate? How fast can Galileo send information back to Earth? Why is this figure lower than the anticipated value for the mission?

Teacher notes are available here.

Galileo's CCD Camera

Sky and Telescope - CCD Imaging

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Top Level "Bringing Jupiter to Earth"

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This module was written by Brian Exton (National Optical Astronomy Observatories, Tucson AZ).

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Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton

The SSI Education and Public Outreach webpages were originally created and managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan, and Nalin Samarasinha.

Last updated: September 17, 1999, by Matthew Fishburn

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