I was just recently watching a documentary called the"voyage of curiosity".About three quarters of the way through they start explaining that curiosities experiments will be shut down do to the fact of the sun being blocked,a seasonal issue.During that time the cameras start taking panaramic photos of the surrounding landscape..If you look at the herizon during this segment it shows a black round object in the sky,could be something on the lens,but none the less unidentified.