Planetary protection refers to the measures taken to prevent contamination of other planets, moons, or celestial bodies with Earth's microbes or other biological materials, and to protect Earth from potential harmful substances that may be brought back from these places. The goal of planetary protection is to preserve the pristine nature of other worlds and to prevent the inadvertent introduction of life from Earth that could interfere with the search for native life forms, or even result in false positives for signs of life.
On the other hand, planetary defense refers to the measures taken to protect Earth from potential impact by asteroids, comets, or other celestial objects that could cause significant damage to our planet. This includes identifying and tracking potentially hazardous objects, developing methods to deflect or destroy them, and preparing emergency response plans in case of an impact.
In summary, while both planetary protection and planetary defense involve protecting celestial bodies, the former focuses on preserving their natural state and preventing contamination, while the latter is concerned with protecting Earth from potential harm caused by cosmic impacts.