Like Google Earth, for space. Celesita overlays modern telescope and spacecraft photography on a surprisingly detailed working model of the known universe. You can fly around like a flight simulator and change the flow of time — watching cities light up as the night sweeps across Earth one minute and zooming out past the Milky Way the next.
There’s also a lively community that makes and shares expansions to the program, allowing you to add new objects and trajectories, and switch out the images rendered on existing objects. You can also load up and track space telescopes, probes, satellites, and stations; or even the Millennium Falcon and Deep Space Nine.