Click and Ye Shall Identify (Martian Craters)
                      Explore Mars by clicking on Martian craters at NASA's
                        web site for Clickworkers
                        and make circularities on rims of craters. You have an
                        interactive training session that lets you know how well
                        you are doing. Then you can graduate to the real thing.
                        You will be presented with images from the Viking
                        Orbiter. The site keeps track of how many craters you
                        have done. If you are more venturesome, you can go for
                        the new images of Mars from the  mission, Mars
                        Global Surveyor. Further along is the Mars Reconnaissance Orbiter. 
                      The NASA clickworkers site is an experiment in public
                        involvement for doing routine scientific work that
                        require months of a scientist's time. So far, since
                        November 17, 2000, clickworkers have done 679,824 crater
                        markings and 138,263 crater classifications. 
                      I have done some clicking. I find that for the smaller
                        craters approaching the size limit they show on the
                        screen, a magnification aid is helpful. The Opera 5.0
                        browser has magnification of the browser screen by means
                        of a drop-down box selection in the upper right. 
                      I have made use of screen magnifiers from Magnifiers.org
                          where can be found freeware and shareware for
                        perusal. Magnification, in some cases can go to 32X. I
                        find that 2 to 3X is plenty. Some magnify wherever the
                        mouse goes. Some also impose a grid on your view. 
                      You can click away for a few minutes or longer. Their
                        server is sometimes slow. Give it a go.