How to Find Asteroids
It is important to get across the point that very distant objects appear to move very slowly. You may try discussing how a car passing you when you are standing on the sidewalk appears to be moving faster than a plane flying overhead, even though the plane is moving much faster! The Moon is moving faster than the plane, yet appears to be moving even slower due to its greater distance.
The stars are so far away that they appear to move very little over the course of a human lifespan. Even the fastest moving star, Barnard’s Star, takes over 200 years just to move across an length of sky equal to the diameter of the full Moon!
Objects close to Earth appear to move much more rapidly. For example, you can have students go out and look at a satellite (see Heavens Above for when and where to see satellites in your area). You might also describe how American astronomer Clyde Tombaugh discovered Pluto by looking at photos of the same part of the sky taken on different nights and seeing an object that had moved.
If an object is moving fast enough, it will actually leave a streak on the film during a time exposure. The SDSS telescope takes pictures through five different filters over the course of approximately six minutes. A fast moving object will leave three streaks on the image: one blue, one red, and one green. An extremely fast moving object, such as a meteor or satellite, may leave a streak in only one color (see the meteor picture on the opening page).
If an object moves slowly, it not move significantly between two filters. The CCD camera has five filters. If the asteroid does not move significantly between the filters used for red and green in the image (the i and r filters), these two colors will combine to form yellow. (For more information about how the CCD camera works, see the SDSS Instruments page).The asteroid may have moved enough by the time it gets to the blue filter that you get a blue dot next to the yellow dot. This is the telltale sign of an asteroid in the SDSS data.
The actual image formation is probably the most confusing part of the lesson. The final SDSS image is formed by combining a red, a green and a blue image. Your television works the same way. Each pixel is composed of a red, a green and a blue element that can be used to produce different colors.
The SDSS camera does not have a red, green and blue filter, however. It has a red and a green filter. In order to do make tri-color image, a third filter must be used. The third filter is the infrared filter, i. This leads to a rather confusing convention. The green filter is used for the blue part of the image, the red filter is used for the green part of the image, and the infrared filter is used for the red part of the image. You can choose how in-depth you wish to discuss the camera. It is only essential the students understand red and green filters can combine to make yellow.
Find Those Asteroids
This page shows three images with easily visible asteroids in them. The students simply need to identify the asteroid in each image. They then compare with another group to be sure they have found the asteroid. If the groups disagree, they need to come to a consensus. There may be more than one asteroid in some images, so students may have found a faint one that is not in the solution guide!
Give the students a reasonable time limit to find as many asteroids as they can. They can look at as many different images as they wish. They will find it easier if they download the highest resolution images.
When time is up, have the groups with the most asteroids show them to the class, so the class may verify their findings. If they cannot reproduce their results and find the asteroids again, they should not get credit for their discovery! Emphasize that scientific results must be reproducible.