With the advent of high resolution digital video projection and high quality spatial sound systems in modern planetariums, the planetarium can become the basis for a unique set of virtual musical instrument capabilities that go well beyond packaged multimedia shows. The dome, circular speaker and circular seating arrangements provide means for skilled composers and performers to create a virtual reality in which attendees are immersed in the composite instrument. This initial foray into designing an audio-visual computerbased instrument for improvisational performance in a planetarium builds on prior, successful work in mapping the rules and state of two-dimensional computer board games to improvised computer music. The unique visual and audio geometries of the planetarium present challenges and opportunities. The game tessellates the dome in mobile, colored hexagons that emulate both atoms and musical scale intervals in an expanding universe. Spatial activity in the game maps to spatial locale and instrument voices in the speakers, in essence creating a virtual orchestra with a string section, percussion section, etc. on the dome. Future work includes distribution of game play via mobile devices to permit attendees to participate in a performance. This environment is open-ended, with great educational and aesthetic potential.