Your project manager has a Ph.D. in Physics. So, he is an expert in gravity, speed, acceleration, impulses, torques, moments of inertia, friction coefficients, and so on. The UFO and the spaceship will be fighting near the Earth. Therefore, you must consider gravitational force. Speed cannot be linear. The UFO and the spaceship should have impulses. The UFO should rotate using an angular velocity and considering a rotational drag coefficient. You have to improve the game applying the laws of 3D physics.
You have great difficulties with the laws of physics. Your project manager knows this and he suggests using a high-level physics engine—wait, do not run away! You will not need to learn complex physics laws in order to apply them in your games.
Farseer Physics Engine (http://farseerphysics.codeplex.com/) is an easy-to-use, open source 2D physics engine designed for XNA Framework and Silverlight. It focuses on simplicity and useful features...