The Virtual Astronaut (VA) is an interactive 3D environment created using multi-source and multi-instrument data from orbital and landed missions. The VA allows you to explore and view the Martian landscape as an astronaut cooperatively working with a rover, and to better understand and visualize the geomorphic and geologic contexts of Mars. As a 3D visualization tool, the VA supports navigation through the virtual environment with a mouse, keyboard shortcuts, or a gamepad. A virtual astronaut can walk across the Martian surface or control a rover driving along a path taken by Opportunity. The scene is created with multiple image mosaics overlain on a digital elevation model. You may adjust the contrast of the scene, change the terrain, make measurements, and visit targets where Opportunity performed in-situ measurements. The first release of the Virtual Astronaut is a prototype study of Mars Exploration Rover (MER) Opportunity's Santa Maria campaign. Santa Maria is an impact crater on Mars that is about 90 meters in diameter and is located at 2.172° S, 5.445° W in Meridiani Planum. The Virtual Astronaut runs within web browsers on Windows and Mac OS X computers with at least 2 GB of RAM, preferably with dedicated video memory. The Unity Web Player plug-in is required (freely available online). Additional information on recommended system specifications is in the User's Manual.