If you haven't used Celestia, the free open source astronomy simulation, you're definitely missing out. Not only can you fly all around the solar system and beyond, you can also change the dates and view the universe at any time in the past or future. Wanna see Halley's Comet but you're worried you won't make it to 2061 for its next arrival? Fire up Celestia and move forward to the date in question and see its tail flare in the night sky just as it would in the future.
However, Celestia is very mod-dependent with add-ons for planets, moons and nebulae, etc., which greatly enhance the experience and also greatly add to the system load. There's one tweak, though, you can do which can speed up certain add-ons. Say for example you install a Jupiter add-on that looks fantastic but slows your frame rates to a crawl. Well, if the add-on is a .jpg file, you're in luck. Try converting the .jpg to a .png file and you may find that your frame rates go back up to a non-stuttering rate. This may have something to do with the .png format being uncompressed and therefore less processor-intensive in rendering, making for a much smoother animation.
Either that or my computer's messing with me. And judging by previous behavior, I don't preclude that possibility for a second.