NASA makes a new Image Of The Day available every day in their gallery, complete with a fairly detailed description of what you see. This plugin grabs a thumbnail of the current image along with an extract of the accompanying description, and makes it available as a WordPress sidebar widget. Clicking on the thumbnail will take you to the NASA Gallery where you can view the full sized image and read the rest of the text. No local storage is used, the image and text is fetched on every view.
A friendly user let me know that the Image Of The Day is also available as an RSS feed. That greatly simplified this plugin and should make it impervious to cosmetic changes at NASA. In addition to this, the image width is now customizable. The height is calculated per image so that it stays proportional when scaled (down) to width.