Artifact, as a term in Image Processing and Analysis, covers a lot of territory. A single anomalous pixel can be easily made to appear as "normal," as you suggest, by performing a Linear Stretch of the Greytone values (stretching 0 and 1 to 0-256, as an example). This is very routine process, done with most images.<br /><br />Another very real possibility - already peripherally commented on by several of you - is that the images were not taken at the same time and from the exact same orbit. When your look angle changes, various objects can appear or disappear due to altered sun position and altered camera/sensor angle.<br /><br />In point of fact, it would likely take hours to work through all of the possible causes for artifact. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis: </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>