The planets are the largest, most famous moving sky objects. But when our solar system formed, not all the matter combined to form planets. If a planet tried to form near Jupiter, Jupiter’s enormous gravity would rip it apart. So the Solar System has a lot of leftover debris, mostly between the orbits of Mars and Jupiter.
The first asteroid was discovered in 1801 by Italian astronomer Giuseppe Piazzi. At first, he thought he had discovered a comet. But after he calculated its orbit, he realized that he had found a totally new type of object between Mars and Jupiter.
Most asteroids are small pieces of rock a few kilometers across. The largest is named Ceres and is about 900 km across. But large asteroids are rare: there are only 26 known asteroids larger than 200 km across. The total mass of all the asteroids is probably about the mass of the Earth’s moon.
Asteroids are so close that they move across the sky quickly. The SDSS telescope takes pictures through 5 filters, one after the other. If the asteroid is close enough, it appears to move between filters. You can see a very fast moving asteroid in the picture at the right. The asteroid moved as the picture was taken in each filter.
Notice how the red and green streaks are close together and the blue streak is farther away. Although the SDSS takes images through five filters, astronomers use only three to make the color image you see. Color computer images are made by combining red, green, and blue pictures. To make SDSS pictures, the i filter makes the red picture, r filter makes the green picture, and the g filter makes the blue picture. (Note: the assignment of picture colors to filters is not related to the color of light the filters see.)
Look at the picture below, which shows the SDSS camera. Look at the labels in the upper left corners of the center squares. You can see that the r and i filters are close together and the g filter is at the other end. Therefore, more time passes between the red and blue images than between the green and red images. The asteroid moves farther in this time. This is why the blue streak is so far from the other streaks in the picture above.
Most asteroids move much more slowly than the one above. Most do not move much as a picture is being taken, and most do not even move significantly between the r and i filters. But since the g filter is at the other end of the camera, the asteroid will move much more by the time the g filter takes a picture of that area of sky. For such a slow-moving asteroid, you will see what appears to be a yellow dot (the combination of red and green) next to a blue dot!
The animation above shows what happens when the SDSS’s filters scan a part of sky with a slow-moving asteroid. The asteroid is the brown dot moving across the animation. The animation shows the camera’s r, i, and g filters sweeping across the sky. (In reality, it works the opposite – the cameras stay still and the sky moves during the night.) The camera takes a picture of the asteroid through the r and i filters (which are next to one another), leaving a yellow dot. When the g filter scans the asteroid, the asteroid has moved; it shows up as a blue dot in its new place.
In the last frame of the animation, the asteroid is removed, leaving only the image that would be seen by the SDSS. So this animation tells you how to find asteroids – just look for a yellow dot and a blue dot close together!