I’m building an $N$-body simulator, and I have everything ready to begin simulating. But my issue is is that I have no idea how to get all the starting positions and velocities for the celestial bodies in the Solar System, such as the planets and their moons. I suppose that if I knew their periods and their positions, I might be able to find out their velocities, but I don’t know how. Is there a source or database that can give me all this data?
Also, the point of this simulator is to simulate the trajectory of the Asteroid Apophis. If you know of any sources to find the data for its position and velocity vector, that would be very helpful. Otherwise, I’ll have to figure out some other way to find the velocity vectors/positions.