Previous discussions: https://news.ycombinator.com/item?id=11132713 and https://news.ycombinator.com/item?id=11104124

The actual posters are at http://www.jpl.nasa.gov/visions-of-the-future/ — click on an image, scroll down and you can even download a PDF (non-vectorized, unfortunately) or a high-resolution TIFF.