The momentum of an object is the product of its mass and velocity. Like velocity, momentum is represented as a 3D vector in Universe Sandbox, so the momentum vector will point in the same direction as the velocity vector, and the magnitude of the momentum vector of an object will be
where M is the mass of the object and V is the magnitude of the object's velocity vector, also known as its speed. Momentum is sometimes referred to as linear momentum to distinguish it from angular momentum.
According to the law of conservation of momentum, momentum cannot be created or destroyed, so all of the momentum in a simulation must be accounted for. Occasionally, in Universe Sandbox, mass is lost from the simulation for performance reasons, which also removes momentum. The amount of lost mass and momentum can be seen in the Stats menu.