Bookmark the official NASA GMAT download page and the ESA Orekit library. Forget the shady "free download hot" link farms—the real heat is in open-source science.
This isn't one toolkit, but a combination. Epic Games just released their project for free. It includes volumetric nebulae, LOD (Level of Detail) planet rendering, and a gravity simulation component.
: Support for languages like Python or C++, allowing researchers to script custom maneuvers or physical constants. Conclusion