Solar Walk is the Free App of the Week on the iTunes App Store. Solar Walk features a 3D Solar System model and lets you navigate through space and time, see all the planets in close-up, learn their trajectories, inner structures, history of their exploration, points of interest and more. It even has detailed 3D models of the most interesting man-made satellites.
The app normally goes for $0.99; however, it is free through Thursday, October 18, 2012. Get it here on the iTunes App Store.