Complex sentence: He explains that the object is measured to be moving at 30,000 kilometers per second-enough to destroy nearly all life on Earth-however as it nears the planet, the object slows down just before impact.
Split into simpler sentences: He explains that the object is measured to be moving at 30,000 kilometers per second-enough to destroy nearly all life on Earth. However, as it nears the planet, the object slows down just before impact.