Final answer:
A planet's kinetic energy would decrease if struck head-on by an asteroid, but the sun-planet system's angular momentum would remain conserved. The collision could alter the planet's velocity and potentially its elliptical orbit, affecting the distances of its closest and farthest points from the sun.
Step-by-step explanation:
When a planet moving in an elliptical orbit around the sun is struck by a large asteroid, the kinetic energy and angular momentum of the system can change depending upon the details of the collision. If the asteroid hits the planet head-on in its counterclockwise orbit, the planet's speed could be decreased, causing a loss in kinetic energy. However, the angular momentum of the system is conserved assuming no external torques are acting on the system. The asteroid and the planet would effectively become a single system post-collision, and any momentum the asteroid had would be transferred to the planet, thereby altering its orbital velocity and possibly its orbital path.
Kepler's laws, specifically Kepler's First Law, inform us that the planets move in elliptical orbits with the sun at one focus. A direct collision such as described would likely perturb the orbit, potentially making it less elliptical depending on the characteristics of the impact. Such a change may lead to variations in the planet's perihelion and aphelion distances if the asteroid's momentum changed the direction or magnitude of the planet’s velocity significantly.