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Celestia is a freeware planetarium software for Windows, Linux, and Mac OS X.

Travel now throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy. Explore space across a huge range of scales, from galaxy clusters down to spacecraft only a few meters across, using an exponential zoom feature. A 'point-and-goto' interface makes it simple to navigate through the universe to the object you want to visit. Celestia is expandable! You can download dozens of easy to install add-ons adding more objects.

Portable download link Celestia Portable version