If you want to do Space Missions there are a few tools out there to help you. The most advanced tool is the Systems Tool Kit (STK) from AGI. They give you a free version if you register on their website.
Another option is the General Mission Analysis Tool (GMAT) from NASA. This is an open source project and they could use some help to make a better tool. It’s great to have open source tools like these available for anyone to play with or even for students.
There is also FreeFlyer from AI solutions but this is a tool with much more restricted use. Have never been able to work with it.
Let me know if you have more tools to add to this list.